Crystal Structure and Luminescence Properties of a Thermally Stable Single-Phase White Emitting Phosphor CaSr2(PO4)2: Dy3+, Li+

Crystal Structure and Luminescence Properties of a Thermally Stable Single-Phase White Emitting Phosphor CaSr2(PO4)2: Dy3+, Li+ | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Crystal Structure and Luminescence Properties of a Thermally Stable Single-Phase White Emitting Phosphor CaSr 2 (PO 4 ) 2 : Dy 3+ , Li + Qingfeng Guo, Yuying Chen, Qiang Hao, Stefan Lis, Haikun Liu, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-577287/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Single-phase CaSr 2 (PO 4 ) 2 :Dy 3+ ,Li + phosphors were prepared using the high-temperature solid-state method in the air. To characterize the luminescence properties of the synthesized phosphors, Powder X-ray diffraction patterns (XRD), scanning electron microscopy images (SEM), photoluminescence spectra, and concentration-dependent emission spectra were measured to characterize the luminescence properties of the synthesized phosphors. The results showed that the CaSr 2 (PO 4 ) 2 :Dy 3+ ,Li + phosphors exhibited white luminescence, and the emission spectra of the phosphors consisted of two sharp peaks at ≈486 and ≈578 nm (the most intense one). The optimum concentration of Dy 3+ doping was determined to 0.06 mol.%. On the basis of the Dexter's theory, the mechanism of energy transfer between the Dy 3+ ions was determined to dipole–dipole interactions. The results of the temperature-dependent luminescence confirmed that the as-prepared phosphors are proved to be promising UV-convertible material capable of white light emitting in UV-LEDs due to its excellent thermal stability and luminescence properties. Luminescence intensity and decay time of the CaSr 2 (PO 4 ) 2 :Dy 3+ ,Li + phosphors were improved remarkably with the addition of charge compensators (Li + ions), which would promote their applications in white light-emitting diodes based on the near-UV chip. Ceramics Optical Materials and Devices Electronic Materials and Devices Magnetics Materials and Devices CaSr2(PO4)2 Phosphor Dy3+ Thermal stability Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1. Introduction In recent years, white light-emitting diodes (LEDs) have been widely applied to various purposes due to their superior performance, like device indicators, automobile headlights and general illumination [1–3]. Commercial manufacture of white LEDs is typically achieved through coating a yellow-emitting phosphor on a blue LED chip, because of its lower cost and higher technical development [4,5]. However, these white LEDs exhibit a low color rendering index (CRI < 80) and high correlated color temperature (CCT) due to the deficiency of the red component [6]. Nowadays, many researchers have attempted to obtain the white LEDs by combining a near-UV LED chip with red, green and blue-emitting (RGB) phosphors [7,8]. These RGB systems, however, are expensive to manufacture, and may have a low efficiency of blue emission due to its reabsorption by the red and green-emitting phosphors [9–11]. Therefore, efficient, durable, and single-phase white light-emitting phosphors attract increasing research interest because of these disadvantages. Until now, various single-phase white phosphors have been explored, such as Eu 2+ -Mn 2+ , Mn 2+ -Tb 3+ , Ce 3+ -Tb 3+ , Eu 2+ -Eu 3+ co-doped silicates, phosphates and so on [12–16]. However, energy transfer between activators is needed for white light in these systems, which results in the serious loss of energy. Luminescence spectrum of trivalent dysprosium (Dy 3+ ) ions consists of two intense emission bands in the blue (470–500 nm) and yellow regions (560–600 nm), which correspond to their 4 F 9/2 – 6 H 13/2 and 4 F 9/2 – 6 H 15/2 electron transitions, respectively [17,18]. By adjusting the intensity ratio of yellow emission to blue emission, it is possible to obtain near-white light emission [19]. Particularly, the luminescence properties of Dy 3+ -doped phosphate phosphors have attracted a lot of attention due to their significant use in solid state light, especially in the case of white light emission. Therefore, doping Dy 3+ into the appropriate host is an important strategy to obtain single-phase white light-emitting phosphors. For instance, CdSiO 3 : Dy 3+ [20], M 2 Si 5 N 8 : Dy 3+ [21], Ca 3 (PO 4 ) 2 : Dy 3+ [22], MZr 4 (PO 4 ) 6 : Dy 3+ [23], GdVO 4 :Dy 3+ [24], SrAl 2 O 4 :Dy 3+ [25], GdScO 3 :Dy 3+ [26], and Y 2 O 3 :Dy 3+ [27] have been extensively studied as white-light emitting phosphors for UV chip-based white LEDs. On the basis of literature studies and the summary of knowledge, we found that similar to the M 3 (PO 4 ) 2 (M = Ca, Sr, Ba) structure [28-29], among which CaSr 2 (PO 4 ) 2 is also a suitable host for phosphors [30]. The effect of Li + , Na + and K + co-doping on the luminescence enhancement of CaSr 2 (PO 4 ) 2 :Dy 3+ phosphors for white light-emitting diodes has been reported [31]. However, the focus was on the effect of the phosphor reinforcing agent. Moreover, the concentration quenching and thermal stability of Dy 3+ were not studied in CaSr 2 (PO 4 ) 2 . Hence, to study in detail the luminescence properties and the concentration quenching, the thermal stability of Dy 3+ in CaSr 2 (PO 4 ) 2 is essential. Here, a series of CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + phosphors was synthesized by a high-temperature solid-state reaction method in an air atmosphere. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence excitation (PLE) and emission (PL) spectra, including time-resolved spectroscopy (luminescence decay curves). The XRD pattern of the CaSr 2 (PO 4 ) 2 phase is similar to the whitlockite mineral that crystallizes in the space group R3c. In addition, the as-prepared phosphors have good thermal stability, confirmed by temperature-dependent emission spectra. The obtained results indicate that the as-prepared CaSr 2 (PO 4 ) 2 :Dy 3+ ,Li + phosphor can act as a UV convertible, white phosphor for w -LEDs. 2. Experimental Section 2.1. Materials and synthesis Powder samples CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + ( x = 0–0.4) were synthesized by a high temperature solid-state method. The starting materials, CaCO 3 (99.9%), SrCO 3 (99.9%), Li 2 CO 3 (99.9%), (NH 4 ) 2 HPO 4 (99.9%), and Dy 2 O 3 (99.999%) were purchased from Aldrich. First, according to stoichiometric ratios, these starting materials were mixed and thoroughly ground in an agate mortar. The pre-sintered samples were then transferred evenly to a tube furnace and calcined at 800 ℃ for 1 h of decomposition of the calcium carbonate and strontium carbonate in air. Finally, the as-prepared powders were calcined again at 1250 ℃ for 5 h in an air atmosphere, and the furnace was cooled down to room temperature before their removal. 2.2. Characterization The obtained CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + phosphors were studied by XRD analysis (XD-3, PGENERAL, China) in the 2θ range of 10° to 70°, with Cu Kα radiation (λ= 0.15406 nm) operating at 40 kV and 40 mA. The PL and PLE spectra of the phosphors were measured on a F-4600 spectrofluorometer (HITACHI, Japan) with a photomultiplier tube operating at 500 V, and a 150 W Xe lamp used as an excitation source. A 400 nm cut-off filter was used to eliminate the second-order emission. Temperature-dependent PL spectra were also measured using the same spectrofluorometer with the home-made heating controller. The CaSr 2 (PO 4 ) 2 were observed by SEM using the HitachiS-520 instrument. The room-temperature luminescence decay curves were recorded with a spectrofluorometer (Horiba, JobinYvon TBXPS), using a tunable pulse laser radiation as the excitation source. 3. Results And Discussion 3.1. Crystal structure The valence charge is unbalanced when Mn + ions are substituted by M (n+1) . This is undesirable for the phosphor materials and decreases the luminescence intensity. To avoid the charge unbalance and the formation of vacancy in the sample, Li + ions were employed as charge compensators and added along with Dy 3+ ions. The mechanism of charge compensation is based on the fact that two Sr 2+ ions are replaced by one Dy 3+ ion and one Li + ion. Therefore, the crystallinity of CaSr 2 (PO 4 ) 2 :Dy 3+ ,Li + improves because the doping with Li + lowers the crystallization temperature. The XRD patterns of the synthesized CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + ( x = 0.01, 0.03, 0.06, 0.12, 0.18, 0.24, 0.27 and 0.3) are shown in Fig. 1. The XRD patterns of the synthesized CaSr 2-1.5x (PO 4 ) 2 :xDy 3+ , ( x = 0.01, 0.03, 0.05, 0.07, 0.09, 0.12, 0.15 and 0.18) and CaSr 2 (PO 4 ) 2 are shown in Fig. 2. It is clearly seen that all the XRD patterns can be well fitted with the standard patterns of the Ca 3 (PO 4 ) 2 (JCPDS no. 09-0169) belonging to the trigonal crystal system, with the space group of R3 c (161). The introduction of Li + did not have any significant influence on the structure of the as-prepared samples, which suggests that all samples were crystallized in a single phase. All diffraction peaks shifted to the smaller 2θ angle side (larger d-spacing, i.e. interplanar distances), which can be ascribed to the substitution of Ca 2+ by larger Sr 2+ . Dy 3+ and Li + ions have been successfully embedded into the crystal lattice, and also the Dy 3+ and Li + dopant ions had negligible influence on the structure of the host CaSr 2 (PO 4 ) 2 in varying amounts of doping. The crystal structure of CaSr 2 (PO 4 ) 2 and different coordination environments for Ca/Sr atoms have already been discussed in the literature. [32] The crystalline structure of CaSr 2 (PO 4 ) 2 is a result of the lattice deformation of Ca 3 (PO 4 ) 2 by partial substitution of Ca 2+ with Sr 2+ . In the crystal structure of CaSr 2 (PO 4 ) 2 , the Ca 2+ (Sr 2+ ) ions are distributed between five crystallographic sites, four of which can be occupied by both Ca 2+ and Sr 2+ ions in various ways defined by chemical composition, and one site only can be occupied by Ca 2+ ions, which are too small to be occupied by Sr 2+ ions. If the number of Sr 2+ ions continues to increase in the Ca 2 − x Sr 1+ x (PO 4 ) 2 structure, the replacement of Ca 2+ by Sr 2+ in all these compounds does not change the space group, and the crystal structure remains unchanged. If all crystallographic sites are occupied by Sr 2+ ions, the structure of CaSr 2 (PO 4 ) 2 would change to Sr 3 (PO 4 ) 2 , which is different from the former one. In the crystal structure of Sr 3 (PO 4 ) 2 there are two nonequivalent crystallographic sites for the Sr 2+ ions (Sr1 and Sr2). The Sr1 atoms are located on the threefold axis, and show 10 coordination sites with 6 oxygen atoms as nearest neighbors belonging to the PO 4 group and site symmetry C3v slightly closer than other coordination sites with 8 oxygen atoms. The other Sr2 atoms in the unit cell show an octahedral configuration, coordinated with eight oxygen atoms. The Sr2 site has 12 coordination numbers and is the largest site in this structure from the PO 4 tetrahedral on the six-fold axis. It was found that the formation of CaSr 2 (PO 4 ) 2 structure is more favorable than the formation of Sr 3 (PO 4 ) 2 [33]. This phenomenon can be explained by the fact that the formation energy of CaSr 2 (PO 4 ) 2 from Ca 2 Sr(PO 4 ) 2 is lower by 17.34 eV than the formation energy of CaSr 2 (PO 4 ) 2 from Sr 3 (PO 4 ) 2 [34]. In the crystal structure of CaSr 2 (PO 4 ) 2 , the Ca 2+ /Sr 2+ ions are distributed between five crystallographic sites - all of them are occupied by both Ca 2+ and Sr 2+ ions in various ways. The Sr1/ Ca1, Sr2/Ca2, Sr3/Ca3, Sr4/Ca4 and Sr5/Ca5 positions are coordinated with six, six, seven, three and six oxygen atoms, respectively. It is worth noting that the Ca4 is three0fold coordinated, suggesting weak bonding and the formation of crystal defects. To investigate the composition and morphology of the material, the CaSr 1.88 (PO 4 ) 2 : 0.06Dy 3+ , 0.06Li + phosphor was selected as a representative example for measurements. Fig. 3(a) displays the results elemental analysis of the sample measured by the EDS method, and the inset shows SEM images of the CaSr 1.88 (PO 4 ) 2 : 0.06Dy 3+ , 0.06Li + sample with elemental mapping of CaSr 1.88 (PO 4 ) 2 : 0.06Dy 3+ , 0.06Li + phosphor provided in Fig. 3(b). The EDS results indicate that the synthesized phosphor is composed of Ca, Sr, P, O, and Dy, which is consistent with the composition of CaSr 1.88 (PO 4 ) 2 : 0.06Dy 3+ , 0.06Li + material. Besides, the results revealed the contents of each element in Table 1. The SEM image reveals that the as-prepared samples are well-crystallized. The substances synthesized by solid-state method are usually agglomerated, but the sample obtained consist of irregular crystal sizes, which meet well the requirements of phosphor used in w -LEDs. Moreover, the mapping results showed that the distribution of the elements in this material is very uniform. 3.2. Photoluminescence properties Fig. 4 depicts the excitation spectrum (λ em = 578 nm) of various Dy 3+ -doped CaSr 2 (PO 4 ) 2 phosphors. The excitation spectrum of the Dy 3+ ion, monitored at 580 nm emission (corresponding to the 4 F 9/2 → 6 H 13/2 transition), consists of several sharp peaks centered at 299, 328, 351, 390, and 455 nm, which are assigned to the f-f transitions of Dy 3+ from its ground state 6 H 15/2 to the excited states 4 M 17/2 , 6 P 7/2 , 6 P 3/2 , 4 F 7/2 and 4 G 11/2 , respectively. Excitation peaks between 320 and 400 nm indicate that CaSr 2 (PO 4 ) 2 :Dy 3+ phosphors can be effectively excited by near-UV LED-chips. Fig.5 shows the emission spectra of the CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + ( x = 0.01–0.3) phosphors, measured at room temperature, wavelength with the excitation of 403 nm ( 6 H 15/2 → 6 P 3/2 ); the inset shows the dependence of the emission intensity at 486 and 578 nm on the Dy 3+ doping concentration. Different from other Dy 3+ - doped phosphors [35], as the Dy 3+ amount increases, the emission intensity increases and reaches a maximum at 6 mol.% Dy 3+ doping content, which is considered as the optimum concentration. A higher Dy 3+ ion concentration results in a reduction of the luminescence intensity associated with concentration quenching phenomenon. This is because when the concentration of Dy 3+ increases, the distance between the ions reduces and enhances the energy transfer cross-relaxation processes between the dopant ions. Fig. 6(a) shows the emission spectra of the CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + ( x = 0.01 and 0.03) and CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Na + ( x = 0.01 and 0.03) phosphors, under 351 nm excitation. The emission spectra of CaSr 2 (PO 4 ) 2 :0.03 Dy 3+ ,Li + / Na + samples include some peaks centered at 486 and 578 nm, which are similar to those mentioned in Fig. 5. This indicates that co-doping with Li + enhances the luminescence intensity of phosphors and it is more effective than co-doping with Na + . The maximum luminescence intensity is achieved with the content of 3% Li + doping. Fig. 6(b) illustrates the emission spectra of CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + and CaSr 2-1.5 x (PO 4 ) 2 : x Dy 3+ phosphors, showing the beneficial effect of Li + co-doping on the luminescence intensity of the samples. The phosphors need to produce ion defects to maintain the charge balance because the replacement of Sr 2+ with Dy 3+ is not equal. However, too many defects produce crystal lattice distortions that reduce the luminescence intensity. Instead, the introduction of Li + ions means that the charge compensation takes place according to the following formula: 2Sr 2+ = Dy 3+ + Li + . This limits the number of crystal defects and leads to the enhanced luminescence intensity. Moreover, Li + co-doping changes symmetry of the local coordination environment and the related crystal field strength. The reduced site symmetry may also lead to the enhanced luminescence intensity. 3.3. Energy transfer mechanism The quenching of the emission intensity depends on the critical distance Rc , which is the shortest average distance between the nearest dopant Dy 3+ ions at a critical concentration x c . The critical distance Rc is described by the following equation [36]: in which V stands for the volume of the unit cell, x c is the critical concentration of activator ion (Dy 3+ ) beyond the concentration quenching, and N represents the number of host cations in one unit cell. In our case, N = 6 V was estimated to be 3744.14 Å, and x c is 0.06 according to the above discussions. According to the eq. (1), Rc was changed to 27.08 Å ( x c = 0.06). It is well-known that exchange interactions play a crucial role in the energy transfer mechanism when the critical distance between the sensitizer and the activator ions is less than 4 Å. With a much higher Rc value, the energy transfer mechanism is considered to be an electric multipolar interaction. Based on the Dexter's theory, if the energy transfer occurs by electric multipolar interactions, then the relationship between the luminescent intensity ( I ) and the activator concentration ( x ) can be expressed by the following equation [37]: where x is the activator concentration, K and β are constants for each interaction at the same excitation. θ is a multipolar interaction constant equal to 3, 6, 8 or 10, corresponding to the nearest-neighbor ions, i.e., dipole–dipole ( d – d ), dipole–quadrupole ( d – q ) and quadrupole–quadrupole ( q – q ) interactions, respectively. We chose the CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + ( x = 0.06, 0.12, 0.18, 0.24, 0.27 and 0.3) samples for the constant emission intensity measurements at 578 nm exceeding the quenching concentration. The relation of log(I/ x ) vs. log( x ) for the CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + ( x = 0.06, 0.12, 0.18, 0.24, 0.27 and 0.3) peaks at 486 and 578 nm was plotted and depicted in Fig. 7. All q values are close to 6, hence we can conclude that the quenching mechanism between Dy 3+ ions in the CaSr 2 (PO 4 ) 2 samples conforms to the dipole–dipole ( d – d ) interactions. 3.4. Temperature-dependent luminescent properties It is well-known that the luminescence intensity for most phosphors decreases if the operating temperature exceeds a certain value (thermal quenching). [38] Thus, the thermal stability of the phosphor is a key issue for high-power w -LEDs. One of the key requirements for a good phosphor is to maintain the performance at the operating temperature of the device. Usually, the luminescence intensity of the phosphors at 423 K with respect to that at room temperature is used to assess the thermal stability. The reason for this has been reported as an increase in the non-radiative transition probability in the configurational coordinated diagram. [39] Fig. 8 shows the temperature dependent emission spectra of the CaSr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + phosphor from 298 to 523 K, at 351 nm excitation; the inset shows the relative emission intensities at 486 and 578 nm as a function of temperature. The shape of the emission bands remains unchanged with increasing temperature (Fig. 8), suggesting that the phosphor has excellent color stability, which is crucial in LEDs or high temperature LEDs. Upon heating the phosphor samples in the temperature range from 298K to 523K, the emission intensity decreased slightly, since the probability of nonradiation is increased and luminescent c enter is released through the crossing point between the excitation state and the ground state, causing the luminescence quenching. [40] Besides, Fig. 9 illustrated the PL intensity of CaSr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + phosphor with respect to time, monitored under 351 nm excitation continuously for 60 min at (a) 100˚C and (b) 150˚C for 30 min at each temperature with a time interval of 5 min, and the emission profile of phosphor maintained at 100˚C and 150˚C continuously for 30 min at each temperature showed that the emission intensity are very stable and remain unchanged, as well as without variation of the emission wavelength. In addition, the emission intensities of CaSr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + maintains 77% of the initial emission intensity corresponding to a temperature of 423K, revealing that the CaSr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + phosphors have good thermal stability, which confirms the stable chromaticity coordinates of CaSr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + phosphors. 3.5. Luminescence decay curves and chromaticity coordinates Fig. 10 shows the decay curves of Dy 3+ emission for CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + ( x = 0.03, 0.09, 0.15 and 0.18) samples excited at 351 nm and monitored at 578 nm. All decay curves can be well fitted to a bi-exponential decay equation as follows: I = A 1 exp(- x /τ 1 ) + A 2 exp(-x/τ 2 ) where I is the luminescence intensity at time t, A1 and A2 are amplitudes, and t1 and t2 are the luminescence lifetimes. The average emission lifetimes τ were calculated by following formula: The calculated average emission lifetimes decrease with increasing Dy 3+ concentration, i.e. 0.74, 0.47, 0.22 and 0.11 ms for the CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + samples with x = 0.03, 0.09, 0.15 and 0.18, respectively. This is due to the decreasing distance between Dy 3+ –Dy 3+ ions, resulting in the observed concentration quenching phenomenon (enhanced cross-relaxations processes), as well as enhanced probability of energy transfer to the luminescence killer sites. Thereby, the luminescence lifetimes of Dy 3+ ions are shortened due to the favorable nonradiative energy transfer processes when the Dy 3+ concentration increases. The observed bi-exponential character of the decay curves could be because of the non-equal occupation of the cation sites by the emitting activator Dy 3+ that has a concentration-dependent preferential occupation in one of the sites as well. The Commission International de I′Eclairage (CIE) chromaticity coordinates for the representative sample (Ca 2 Sr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + ) were calculated based on the corresponding emission spectra, and the results are shown in Fig. 11(a). It can be seen that the coordinates (x = 0.3450, y = 0.3787) are located in the near-white region. In addition, we also calculated the CIE chromaticity coordinates of Ca 2 Sr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + at different temperature values (Table 1), and we found that with the temperature rising the phosphor exhibits almost no change in the color of emission, which is showed in Fig. 11(b). Thus, the CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + material can be potentially used as a white phosphor for w -LEDs in solid-state lighting applications. 4. Conclusions In summary, a series of CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + , CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Na + and CaSr 2-1.5x (PO 4 ) 2 :xDy 3+ phosphors were prepared by a conventional solid-state reaction. The phase structure, luminescence properties, thermal quenching and emission decay curves were investigated. Under the excitation of 351 nm, the CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + phosphors showed two emission bands centered at 486 and 578 nm, characteristic of Dy 3+ . The excitation spectra showed narrow excitation bands from 250 to 550 nm with a maximum at 403 nm. This means that the phosphor can be effectively excited by UV chips for potential applications in w -LEDs. The optimum dopant concentration of Dy 3+ ions is 0.06 mol.%. Furthermore, the quenching mechanism between the Dy 3+ ions was recognized as dipole–dipole ( d – d ) interactions. The dependence of the emission spectra on temperature indicated that the phosphor has a good thermal stability in both emission color and intensity. The CIE chromaticity coordinates of the selected Ca 2 Sr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + sample were calculated ( x =0.3450, y = 0.3787), and they are located in the white-light region. These results indicate that CaSr 2-2 x (PO 4 ) 2 : x Dy 3+ , x Li + phosphors have a good potential for their use as white-emitting luminophores for the phosphor-converted w -LEDs. Thus, the results of this work indicate the potential applications of these effective phosphors in white light-emitting diodes, excited with a near-UV chip. Declarations Conflicts of interest There are no conflicts to declare. Acknowledgements The present work is supported by the National Natural Science Foundation of China (Grant No. 41802040) and the Fundamental Research Funds for China University of Geosciences, Beijing (Grant No. 2652019080). References M.M. Shang, G.G. 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Swart, Low voltage electron induced cathodoluminescence degradation and surface characterization of Sr 3 (PO 4 ) 2 :Tb phosphor, Appl. Surf. Sci. 257 (2011) 10147–10155. Y. Lu, X. Tang, L. Yan, K. Li, X. Liu, M. Shang, C. Li, J. Lin, Synthesis and luminescent properties of GdNbO 4 :RE 3+ (RE = Tm, Dy) nanocrystalline phosphors via the sol-gel process, J. Phys. Chem. C 117 (2013) 21972–21980. Y. Zeng, K. Qiu, Z. Yang, Y. Bu, W. Zhang, J. Li, Enhanced red emission of NaSrVO 4 :Eu 3+ phosphor via Bi 3+ co-doping for the application to white LEDs, Ceram. Int. 43 (2017) 830–834. L. G. Van Uitert, Characterization of Energy Transfer Interactions between Rare Earth Ions, J. Electro. Chem. Soc. 114 (1967) 1048-1053. H.Ye, M. Y. He, T. S. Zhou, Q. F. Guo, J. L. Zhang, L. B. Liao, L. F. Mei, H. K. Liu, M. Runowski, A novel reddish-orange fluorapatite phosphor, La 6-x Ba 4 (SiO 4 ) 6 F 2 : x Sm 3+ - Structure, luminescence and energy transfer properties, J. Alloys Compd. 757 (2018) 79-86. S. Liu, Z.J. Wang, Q. Bao, X. Li, Y. Chen, Z.P. Wang, L. Guan, Abnormal thermal quenching and blue-shift of Zn 3 (BO 3 )(PO 4 ):Inducing host defect by doping Mn 2+ and Tb 3+ , Dyes and Pigments, 165 (2019) 44–52. J.J. Chen, Y. Zhao, Z.Y. Mao, D.J. Wang, L.J. Bie. Investigation of thermal quenching and abnormal thermal quenching in mixed valence Eu co-doped LaAlO 3 phosphor, J Lumin, 186 (2017) 72–76. Tables Table 1 The CIE chromaticity coordinates of Ca 2 Sr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + at different temperature values. Temperature (K) x y 298 323 0.3917 0.4101 0.3795 0.3879 348 0.3918 0.4099 373 0.3905 0.4086 398 0.3890 0.4067 423 0.3876 0.4051 448 0.3803 0.3925 473 0.3840 0.4002 498 0.3811 0.3967 523 0.3786 0.3935 Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-577287","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":30288462,"identity":"2c7633bc-65b8-41ea-94d4-fed9c2f16c02","order_by":0,"name":"Qingfeng Guo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA0ElEQVRIiWNgGAWjYDAC5gNAogKIJYCYhygtbAlA4gzJWhjbSNHCd4z9msTHeXaJ86MbGB+8bWOQNyekRfIYT5nkzG3JiRvvHGA2nNvGYLizgYAWg/s9adK82w4kbpyRwCbN28aQYHCAkJZjPEAtc8Ba2H8TqYX9mDRvw4HE+RIJbMxEaQH6hdlyxrFk4w0yB5sl55yTMNxASAswxB7e+FBjJzt/dvPBD2/KbOQJ2sJwgMcA4sIDjA0MkNghqIX9AZiWbyBC8SgYBaNgFIxMAADHx0I8yEa4IgAAAABJRU5ErkJggg==","orcid":"","institution":"China University of Geosciences Beijing","correspondingAuthor":true,"prefix":"","firstName":"Qingfeng","middleName":"","lastName":"Guo","suffix":""},{"id":30288463,"identity":"e21dcd06-0b02-463e-9511-837d1edc3c90","order_by":1,"name":"Yuying Chen","email":"","orcid":"","institution":"China University of Geosciences Beijing","correspondingAuthor":false,"prefix":"","firstName":"Yuying","middleName":"","lastName":"Chen","suffix":""},{"id":30288464,"identity":"7ffb9180-65bf-4636-ac04-2e45a8d09a58","order_by":2,"name":"Qiang Hao","email":"","orcid":"","institution":"The University of Sydney","correspondingAuthor":false,"prefix":"","firstName":"Qiang","middleName":"","lastName":"Hao","suffix":""},{"id":30288465,"identity":"e692c8c4-e285-4f9d-863c-fedceb858ead","order_by":3,"name":"Stefan Lis","email":"","orcid":"","institution":"Adam Mickiewicz University: Uniwersytet im Adama Mickiewicza w Poznaniu","correspondingAuthor":false,"prefix":"","firstName":"Stefan","middleName":"","lastName":"Lis","suffix":""},{"id":30288466,"identity":"caac06dc-7aee-49e8-91a9-49cefc5847b7","order_by":4,"name":"Haikun Liu","email":"","orcid":"","institution":"Dongguan University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Haikun","middleName":"","lastName":"Liu","suffix":""},{"id":30288467,"identity":"d952bb78-4134-4a97-80ea-915dfcd099a6","order_by":5,"name":"Marcin Runowski","email":"","orcid":"","institution":"Adam Mickiewicz University: Uniwersytet im Adama Mickiewicza w Poznaniu","correspondingAuthor":false,"prefix":"","firstName":"Marcin","middleName":"","lastName":"Runowski","suffix":""},{"id":30288468,"identity":"c96e46ad-9a10-44ac-88cc-f988b4d40e4c","order_by":6,"name":"Lefu Mei","email":"","orcid":"","institution":"China University of Geosciences Beijing","correspondingAuthor":false,"prefix":"","firstName":"Lefu","middleName":"","lastName":"Mei","suffix":""},{"id":30288469,"identity":"e1195a6b-a714-4148-80bf-219424399b7a","order_by":7,"name":"Libing Liao","email":"","orcid":"","institution":"China University of Geosciences Beijing","correspondingAuthor":false,"prefix":"","firstName":"Libing","middleName":"","lastName":"Liao","suffix":""}],"badges":[],"createdAt":"2021-05-31 14:30:00","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-577287/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-577287/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":9966129,"identity":"49aee782-6dcf-48e1-acd9-8daadc999f90","added_by":"auto","created_at":"2021-06-03 23:12:44","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":27530,"visible":true,"origin":"","legend":"XRD patterns of the CaSr2-2x(PO4)2:xDy3+,xLi+ phosphors (x = 0.01, 0.03, 0.06, 0.12, 0.18, 0.24, 0.27 and 0.3) and the standard pattern of Ca3(PO4)2 (JCPDS no. 09-0169) is shown as a reference.","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-577287/v1/80a3de34a0fc324ed5f5b7e7.png"},{"id":9966130,"identity":"80c051c8-1c7d-4056-9c47-34432dc90c35","added_by":"auto","created_at":"2021-06-03 23:12:44","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":24398,"visible":true,"origin":"","legend":"XRD patterns of the CaSr2-1.5x(PO4)2:xDy3+ phosphors (x = 0.01, 0.03, 0.05, 0.07, 0.09, 0.12, 0.15 and 0.18) and the standard pattern of Ca3(PO4)2 (JCPDS no. 09-0169) is shown as a reference. ","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-577287/v1/25a1001a9a632f12db9ade70.png"},{"id":9966245,"identity":"96f8ed94-0e32-4e00-9ed1-54c3331410d3","added_by":"auto","created_at":"2021-06-03 23:15:44","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":194943,"visible":true,"origin":"","legend":"(a) The elemental analysis result of the sample measured by the EDS method and the inset showing the SEM image of the CaSr1.88(PO4)2:0.06Dy3+,0.06Li+ sample; (b) The elemental mapping of CaSr1.88(PO4)2:0.06Dy3+,0.06Li+ phosphors.","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-577287/v1/a57292b833d5d7844eba2937.png"},{"id":9966243,"identity":"b05b9f2e-ab63-4245-85d9-fda8e777ed2f","added_by":"auto","created_at":"2021-06-03 23:15:44","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":28963,"visible":true,"origin":"","legend":"PLE spectra of the CaSr2-2x(PO4)2:xDy3+,xLi+ (x = 0.01–0.3) phosphors, measured at room temperature; the inset shows the dependence of the excitation intensity at 351 nm on the Dy3+ doping concentration.","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-577287/v1/61ed11a8226b4b9d97bf2a66.png"},{"id":9966576,"identity":"c9f8ae6a-f565-4966-9180-aaa2b1150f2d","added_by":"auto","created_at":"2021-06-03 23:18:44","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":36866,"visible":true,"origin":"","legend":"PL spectra of the CaSr2-2x(PO4)2:xDy3+,xLi+ (x = 0.01–0.3) phosphors, measured at room temperature with the excitation of 351 nm (6H15/2 to 6P3/2); the inset shows the dependence of the emission intensity at 486 and 578 nm on the Dy3+ doping concentration. ","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-577287/v1/cbba48b8bcb85fb864d7760f.png"},{"id":9966248,"identity":"690c2275-4efe-4600-8580-90474a90e9a8","added_by":"auto","created_at":"2021-06-03 23:15:44","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":108414,"visible":true,"origin":"","legend":"(a) Emission spectra of the CaSr2-2x(PO4)2:xDy3+,xLi+ (x =0.01, 0.03 and 0.12) and CaSr2-1.5x(PO4)2:xDy3+ (x =0.01, 0.05 and 0.9) phosphors; (b) Emission spectra of the CaSr2-2x(PO4)2:xDy3+,xLi+ (x = 0.01 and 0.03) and CaSr2-2x(PO4)2:xDy3+, xNa+ (x = 0.01 and 0.03) phosphors.","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-577287/v1/065873783c7ab5892ff1e87a.png"},{"id":9966246,"identity":"bf2b8977-4ad3-4468-a4a9-6bc79ea3df8e","added_by":"auto","created_at":"2021-06-03 23:15:44","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":20985,"visible":true,"origin":"","legend":"The relation of log(I/x) vs. log(x) for the CaSr2-2x(PO4)2:xDy3+,xLi+ (x = 0.06, 0.12, 0.18, 0.24, 0.27 and 0.3) peaks at 486 and 578 nm.","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-577287/v1/9f5cea1542a0fc31ad3d2965.png"},{"id":9966133,"identity":"ec7e2ada-dbee-4079-8725-eff2e436d450","added_by":"auto","created_at":"2021-06-03 23:12:44","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":35576,"visible":true,"origin":"","legend":"Temperature dependent emission spectra of the CaSr1.88(PO4)2:0.06Dy3+,0.06Li+ phosphor from 298 to 523 K with 351 nm excitation; the inset shows the relative emission intensities at 486 and 578 nm as a function of temperature.","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-577287/v1/060832fa3b496eaceb711269.png"},{"id":9966618,"identity":"7c038768-0d4b-4585-b7c5-5dc1be6a170b","added_by":"auto","created_at":"2021-06-03 23:21:44","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":31816,"visible":true,"origin":"","legend":"The PL intensity of CaSr1.88(PO4)2:0.06Dy3+,0.06Li+ phosphor with respect to time, monitored under 351 nm excitation continuously for 60 min at (a) 100˚C and (b) 150˚C for 30 min at each temperature with a time interval of 5 min.","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-577287/v1/344fb3e0239a020e7b298ef2.png"},{"id":9966577,"identity":"c4d63896-3ee2-4e5f-9757-511744a7a50a","added_by":"auto","created_at":"2021-06-03 23:18:44","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":49041,"visible":true,"origin":"","legend":"The decay curves of Dy3+ emission for CaSr2-2x(PO4)2: xDy3+,xLi+ (x = 0.03, 0.09, 0.15 and 0.18) samples excited at 351 nm and monitored at 578 nm.","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-577287/v1/807edb8727e614db3fdbc3d8.png"},{"id":9966139,"identity":"553807ee-fa64-4c57-9b08-9f0ebe0d8d70","added_by":"auto","created_at":"2021-06-03 23:12:44","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":297445,"visible":true,"origin":"","legend":"(a) Chromaticity coordinates for the CaSr1.88(PO4)2:0.06Dy3+,0.06Li+ material, shown in the CIE diagram; the inset shows a digital photograph of the white-emitting phosphor excited at 351 nm; (b) The CIE chromaticity coordinates of the Ca2Sr1.88(PO4)2:0.06Dy3+,0.06Li+ luminophore at different temperature values.","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-577287/v1/ac8d1d9dbd89708acbdb2cc7.png"},{"id":15673385,"identity":"dbdf3f59-9a41-4519-8c95-ac7fc86ed532","added_by":"auto","created_at":"2021-11-18 14:17:43","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1048945,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-577287/v1/48357a0d-43ee-4369-9447-c6a67459ef97.pdf"}],"financialInterests":"","formattedTitle":"\u003cp\u003eCrystal Structure and Luminescence Properties of a Thermally Stable Single-Phase White Emitting Phosphor CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e: Dy\u003csup\u003e3+\u003c/sup\u003e, Li\u003csup\u003e+\u003c/sup\u003e \u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn recent years, white light-emitting diodes (LEDs) have been widely applied to various purposes due to their superior performance, like device indicators, automobile headlights and general illumination [1\u0026ndash;3]. Commercial manufacture of white LEDs is typically achieved through coating a yellow-emitting phosphor on a blue LED chip, because of its lower cost and higher technical development [4,5]. However, these white LEDs exhibit a low color rendering index (CRI \u0026lt; 80) and high correlated color temperature (CCT) due to the deficiency of the red component [6]. Nowadays, many researchers have attempted to obtain the white LEDs by combining a near-UV LED chip with red, green and blue-emitting (RGB) phosphors [7,8]. These RGB systems, however, are expensive to manufacture, and may have a low efficiency of blue emission due to its reabsorption by the red and green-emitting phosphors [9\u0026ndash;11]. Therefore, efficient, durable, and single-phase white light-emitting phosphors attract increasing research interest because of these disadvantages.\u003c/p\u003e\n\u003cp\u003eUntil now, various single-phase white phosphors have been explored, such as Eu\u003csup\u003e2+\u003c/sup\u003e-Mn\u003csup\u003e2+\u003c/sup\u003e, Mn\u003csup\u003e2+\u003c/sup\u003e-Tb\u003csup\u003e3+\u003c/sup\u003e, Ce\u003csup\u003e3+\u003c/sup\u003e-Tb\u003csup\u003e3+\u003c/sup\u003e, Eu\u003csup\u003e2+\u003c/sup\u003e-Eu\u003csup\u003e3+\u003c/sup\u003e co-doped silicates, phosphates and so on [12\u0026ndash;16]. However, energy transfer between activators is needed for white light in these systems, which results in the serious loss of energy. Luminescence spectrum of trivalent dysprosium (Dy\u003csup\u003e3+\u003c/sup\u003e) ions consists of two intense emission bands in the blue (470\u0026ndash;500 nm) and yellow regions (560\u0026ndash;600 nm), which correspond to their \u003csup\u003e4\u003c/sup\u003eF\u003csub\u003e9/2\u003c/sub\u003e\u0026ndash;\u003csup\u003e6\u003c/sup\u003eH\u003csub\u003e13/2\u003c/sub\u003e and \u003csup\u003e4\u003c/sup\u003e F\u003csub\u003e9/2\u003c/sub\u003e\u0026ndash;\u003csup\u003e6\u003c/sup\u003eH\u003csub\u003e15/2\u003c/sub\u003e electron transitions, respectively [17,18]. By adjusting the intensity ratio of yellow emission to blue emission, it is possible to obtain near-white light emission [19]. Particularly, the luminescence properties of Dy\u003csup\u003e3+\u003c/sup\u003e-doped phosphate phosphors have attracted a lot of attention due to their significant use in solid state light, especially in the case of white light emission. Therefore, doping Dy\u003csup\u003e3+\u003c/sup\u003e into the appropriate host is an important strategy to obtain single-phase white light-emitting phosphors. For instance, CdSiO\u003csub\u003e3\u003c/sub\u003e: Dy\u003csup\u003e3+\u003c/sup\u003e [20], M\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003eN\u003csub\u003e8\u003c/sub\u003e: Dy\u003csup\u003e3+\u003c/sup\u003e [21], Ca\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e: Dy\u003csup\u003e3+\u003c/sup\u003e [22], MZr\u003csub\u003e4\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e6\u003c/sub\u003e: Dy\u003csup\u003e3+\u003c/sup\u003e [23], GdVO\u003csub\u003e4\u003c/sub\u003e:Dy\u003csup\u003e3+\u003c/sup\u003e [24], SrAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e:Dy\u003csup\u003e3+\u003c/sup\u003e [25], GdScO\u003csub\u003e3\u003c/sub\u003e:Dy\u003csup\u003e3+\u003c/sup\u003e [26], and Y\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e:Dy\u003csup\u003e3+\u003c/sup\u003e [27] have been extensively studied as white-light emitting phosphors for UV chip-based white LEDs. On the basis of literature studies and the summary of knowledge, we found that similar to the M\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e (M = Ca, Sr, Ba) structure [28-29], among which CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e is also a suitable host for phosphors [30]. The effect of Li\u003csup\u003e+\u003c/sup\u003e, Na\u003csup\u003e+\u003c/sup\u003e and K\u003csup\u003e+\u003c/sup\u003e co-doping on the luminescence enhancement of CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:Dy\u003csup\u003e3+\u003c/sup\u003e phosphors for white light-emitting diodes has been reported [31]. However, the focus was on the effect of the phosphor reinforcing agent. Moreover, the concentration quenching and thermal stability of Dy\u003csup\u003e3+ \u003c/sup\u003ewere not studied in CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e. Hence, to study in detail the luminescence properties and the concentration quenching, the thermal stability of Dy\u003csup\u003e3+ \u003c/sup\u003ein CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e is essential.\u003c/p\u003e\n\u003cp\u003eHere, a series of CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e, \u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e phosphors was synthesized by a high-temperature solid-state reaction method in an air atmosphere. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence excitation (PLE) and emission (PL) spectra, including time-resolved spectroscopy (luminescence decay curves). The XRD pattern of the CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e phase is similar to the whitlockite mineral that crystallizes in the space group R3c. In addition, the as-prepared phosphors have good thermal stability, confirmed by temperature-dependent emission spectra. The obtained results indicate that the as-prepared CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:Dy\u003csup\u003e3+\u003c/sup\u003e,Li\u003csup\u003e+\u003c/sup\u003e phosphor can act as a UV convertible, white phosphor for \u003cem\u003ew\u003c/em\u003e-LEDs.\u003c/p\u003e"},{"header":"2. Experimental Section","content":"\u003cp\u003e\u003cstrong\u003e2.1. Materials and synthesis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePowder samples CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e (\u003cem\u003ex\u003c/em\u003e= 0\u0026ndash;0.4) were synthesized by a high temperature solid-state method. The starting materials, CaCO\u003csub\u003e3\u003c/sub\u003e (99.9%), SrCO\u003csub\u003e3\u003c/sub\u003e (99.9%), Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e (99.9%), (NH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eHPO\u003csub\u003e4\u003c/sub\u003e (99.9%), and Dy\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (99.999%) were purchased from Aldrich. First, according to stoichiometric ratios, these starting materials were mixed and thoroughly ground in an agate mortar. The pre-sintered samples were then transferred evenly to a tube furnace and calcined at 800 ℃ for 1 h of decomposition of the calcium carbonate and strontium carbonate in air. Finally, the as-prepared powders were calcined again at 1250 ℃ for 5 h in an air atmosphere, and the furnace was cooled down to room temperature before their removal.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2. Characterization \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe obtained CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e, \u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e phosphors were studied by XRD analysis (XD-3, PGENERAL, China) in the 2\u0026theta; range of 10\u0026deg; to 70\u0026deg;, with Cu K\u0026alpha; radiation (\u0026lambda;= 0.15406 nm) operating at 40 kV and 40 mA. The PL and PLE spectra of the phosphors were measured on a F-4600 spectrofluorometer (HITACHI, Japan) with a photomultiplier tube operating at 500 V, and a 150 W Xe lamp used as an excitation source. A 400 nm cut-off filter was used to eliminate the second-order emission. Temperature-dependent PL spectra were also measured using the same spectrofluorometer with the home-made heating controller. The CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e were observed by SEM using the HitachiS-520 instrument. The room-temperature luminescence decay curves were recorded with a spectrofluorometer (Horiba, JobinYvon TBXPS), using a tunable pulse laser radiation as the excitation source.\u003c/p\u003e"},{"header":"3. Results And Discussion","content":"\u003cp\u003e\u003cstrong\u003e3.1. Crystal structure\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe valence charge is unbalanced when Mn\u003csup\u003e+\u003c/sup\u003e ions are substituted by M\u003csup\u003e(n+1)\u003c/sup\u003e. This is undesirable for the phosphor materials and decreases the luminescence intensity. To avoid the charge unbalance and the formation of vacancy in the sample, Li\u003csup\u003e+\u003c/sup\u003e ions were employed as charge compensators and added along with Dy\u003csup\u003e3+\u003c/sup\u003e ions. The mechanism of charge compensation is based on the fact that two Sr\u003csup\u003e2+\u003c/sup\u003e ions are replaced by one Dy\u003csup\u003e3+\u003c/sup\u003e ion and one Li\u003csup\u003e+\u003c/sup\u003e ion. Therefore, the crystallinity of CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:Dy\u003csup\u003e3+\u003c/sup\u003e,Li\u003csup\u003e+\u003c/sup\u003e improves because the doping with Li\u003csup\u003e+\u003c/sup\u003e lowers the crystallization temperature. The XRD patterns of the synthesized CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e (\u003cem\u003ex\u003c/em\u003e = 0.01, 0.03, 0.06, 0.12, 0.18, 0.24, 0.27 and 0.3) are shown in Fig. 1. The XRD patterns of the synthesized CaSr\u003csub\u003e2-1.5x\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:xDy\u003csup\u003e3+\u003c/sup\u003e, (\u003cem\u003ex\u003c/em\u003e = 0.01, 0.03, 0.05, 0.07, 0.09, 0.12, 0.15 and 0.18) and CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e are shown in Fig. 2. It is clearly seen that all the XRD patterns can be well fitted with the standard patterns of the Ca\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e (JCPDS no. 09-0169) belonging to the trigonal crystal system, with the space group of R3\u003cem\u003ec\u003c/em\u003e (161). The introduction of Li\u003csup\u003e+\u003c/sup\u003e did not have any significant influence on the structure of the as-prepared samples, which suggests that all samples were crystallized in a single phase. All diffraction peaks shifted to the smaller 2\u0026theta; angle side (larger d-spacing, i.e. interplanar distances), which can be ascribed to the substitution of Ca\u003csup\u003e2+\u003c/sup\u003e by larger Sr\u003csup\u003e2+\u003c/sup\u003e. Dy\u003csup\u003e3+\u003c/sup\u003e and Li\u003csup\u003e+\u003c/sup\u003e ions have been successfully embedded into the crystal lattice, and also the Dy\u003csup\u003e3+\u003c/sup\u003e and Li\u003csup\u003e+\u003c/sup\u003e dopant ions had negligible influence on the structure of the host CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e in varying amounts of doping.\u003c/p\u003e\n\u003cp\u003eThe crystal structure of CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e and different coordination environments for Ca/Sr atoms have already been discussed in the literature. [32] The crystalline structure of CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e is a result of the lattice deformation of Ca\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e by partial substitution of Ca\u003csup\u003e2+\u003c/sup\u003e with Sr\u003csup\u003e2+\u003c/sup\u003e. In the crystal structure of CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e, the Ca\u003csup\u003e2+ \u003c/sup\u003e(Sr\u003csup\u003e2+\u003c/sup\u003e) ions are distributed between five crystallographic sites, four of which can be occupied by both Ca\u003csup\u003e2+\u003c/sup\u003e and Sr\u003csup\u003e2+\u003c/sup\u003e ions in various ways defined by chemical composition, and one site only can be occupied by Ca\u003csup\u003e2+\u003c/sup\u003e ions, which are too small to be occupied by Sr\u003csup\u003e2+\u003c/sup\u003e ions. If the number of Sr\u003csup\u003e2+\u003c/sup\u003e ions continues to increase in the Ca\u003csub\u003e2\u003c/sub\u003e\u003csub\u003e\u0026minus;\u003c/sub\u003e\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003eSr\u003csub\u003e1+\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e structure, the replacement of Ca\u003csup\u003e2+\u003c/sup\u003e by Sr\u003csup\u003e2+\u003c/sup\u003e in all these compounds does not change the space group, and the crystal structure remains unchanged. If all crystallographic sites are occupied by Sr\u003csup\u003e2+\u003c/sup\u003e ions, the structure of CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e would change to Sr\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e, which is different from the former one. In the crystal structure of Sr\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e there are two nonequivalent crystallographic sites for the Sr\u003csup\u003e2+\u003c/sup\u003e ions (Sr1 and Sr2). The Sr1 atoms are located on the threefold axis, and show 10 coordination sites with 6 oxygen atoms as nearest neighbors belonging to the PO\u003csub\u003e4\u003c/sub\u003e group and site symmetry C3v slightly closer than other coordination sites with 8 oxygen atoms. The other Sr2 atoms in the unit cell show an octahedral configuration, coordinated with eight oxygen atoms. The Sr2 site has 12 coordination numbers and is the largest site in this structure from the PO\u003csub\u003e4\u003c/sub\u003e tetrahedral on the six-fold axis.\u003c/p\u003e\n\u003cp\u003eIt was found that the formation of CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e structure is more favorable than the formation of Sr\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2 \u003c/sub\u003e[33]. This phenomenon can be explained by the fact that the formation energy of CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e from Ca\u003csub\u003e2\u003c/sub\u003eSr(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e is lower by 17.34 eV than the formation energy of CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e from Sr\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e [34]. In the crystal structure of CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e, the Ca\u003csup\u003e2+\u003c/sup\u003e/Sr\u003csup\u003e2+\u003c/sup\u003e ions are distributed between five crystallographic sites - all of them are occupied by both Ca\u003csup\u003e2+\u003c/sup\u003e and Sr\u003csup\u003e2+\u003c/sup\u003e ions in various ways. The Sr1/ Ca1, Sr2/Ca2, Sr3/Ca3, Sr4/Ca4 and Sr5/Ca5 positions are coordinated with six, six, seven, three and six oxygen atoms, respectively. It is worth noting that the Ca4 is three0fold coordinated, suggesting weak bonding and the formation of crystal defects.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;To investigate the composition and morphology of the material, the CaSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e: 0.06Dy\u003csup\u003e3+\u003c/sup\u003e, 0.06Li\u003csup\u003e+\u003c/sup\u003e phosphor was selected as a representative example for measurements. Fig. 3(a) displays the results elemental analysis of the sample measured by the EDS method, and the inset shows SEM images of the CaSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e: 0.06Dy\u003csup\u003e3+\u003c/sup\u003e, 0.06Li\u003csup\u003e+\u003c/sup\u003e sample with elemental mapping of CaSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e: 0.06Dy\u003csup\u003e3+\u003c/sup\u003e, 0.06Li\u003csup\u003e+\u003c/sup\u003e phosphor provided in Fig. 3(b). The EDS results indicate that the synthesized phosphor is composed of Ca, Sr, P, O, and Dy, which is consistent with the composition of CaSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e: 0.06Dy\u003csup\u003e3+\u003c/sup\u003e, 0.06Li\u003csup\u003e+\u003c/sup\u003e material. Besides, the results revealed the contents of each element in Table 1. The SEM image reveals that the as-prepared samples are well-crystallized. The substances synthesized by solid-state method are usually agglomerated, but the sample obtained consist of irregular crystal sizes, which meet well the requirements of phosphor used in \u003cem\u003ew\u003c/em\u003e-LEDs. Moreover, the mapping results showed that the distribution of the elements in this material is very uniform.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.2. Photoluminescence properties\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFig. 4 depicts the excitation spectrum (\u0026lambda;\u003csub\u003eem\u003c/sub\u003e = 578 nm) of various Dy\u003csup\u003e3+\u003c/sup\u003e-doped CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e phosphors. The excitation spectrum of the Dy\u003csup\u003e3+\u003c/sup\u003e ion, monitored at 580 nm emission (corresponding to the \u003csup\u003e4\u003c/sup\u003eF\u003csub\u003e9/2\u003c/sub\u003e \u0026rarr; \u003csup\u003e6\u003c/sup\u003eH\u003csub\u003e13/2 \u003c/sub\u003etransition), consists of several sharp peaks centered at 299, 328, 351, 390, and 455 nm, which are assigned to the f-f transitions of Dy\u003csup\u003e3+\u003c/sup\u003e from its ground state \u003csup\u003e6\u003c/sup\u003eH\u003csub\u003e15/2\u003c/sub\u003e to the excited states \u003csup\u003e4\u003c/sup\u003eM\u003csub\u003e17/2\u003c/sub\u003e, \u003csup\u003e6\u003c/sup\u003eP\u003csub\u003e7/2\u003c/sub\u003e, \u003csup\u003e6\u003c/sup\u003eP\u003csub\u003e3/2\u003c/sub\u003e, \u003csup\u003e4\u003c/sup\u003eF\u003csub\u003e7/2\u003c/sub\u003e and \u003csup\u003e4\u003c/sup\u003eG\u003csub\u003e11/2\u003c/sub\u003e, respectively. Excitation peaks between 320 and 400 nm indicate that CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:Dy\u003csup\u003e3+\u003c/sup\u003e phosphors can be effectively excited by near-UV LED-chips. Fig.5 shows the emission spectra of the CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e (\u003cem\u003ex\u003c/em\u003e = 0.01\u0026ndash;0.3) phosphors, measured at room temperature, wavelength with the excitation of 403 nm (\u003csup\u003e6\u003c/sup\u003eH\u003csub\u003e15/2\u003c/sub\u003e \u0026rarr; \u003csup\u003e6\u003c/sup\u003eP\u003csub\u003e3/2\u003c/sub\u003e); the inset shows the dependence of the emission intensity at 486 and 578 nm on the Dy\u003csup\u003e3+\u003c/sup\u003e doping concentration. Different from other Dy\u003csup\u003e3+\u003c/sup\u003e- doped phosphors [35], as the Dy\u003csup\u003e3+\u003c/sup\u003e amount increases, the emission intensity increases and reaches a maximum at 6 mol.% Dy\u003csup\u003e3+\u003c/sup\u003e doping content, which is considered as the optimum concentration. A higher Dy\u003csup\u003e3+\u003c/sup\u003e ion concentration results in a reduction of the luminescence intensity associated with concentration quenching phenomenon. This is because when the concentration of Dy\u003csup\u003e3+\u003c/sup\u003e increases, the distance between the ions reduces and enhances the energy transfer cross-relaxation processes between the dopant ions.\u003c/p\u003e\n\u003cp\u003eFig. 6(a) shows the emission spectra of the CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e (\u003cem\u003ex\u003c/em\u003e = 0.01 and 0.03) and CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eNa\u003csup\u003e+\u003c/sup\u003e (\u003cem\u003ex\u003c/em\u003e = 0.01 and 0.03) phosphors, under 351 nm excitation. The emission spectra of CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:0.03 Dy \u003csup\u003e3+\u003c/sup\u003e,Li\u003csup\u003e+ \u003c/sup\u003e/ Na\u003csup\u003e+\u003c/sup\u003e samples include some peaks centered at 486 and 578 nm, which are similar to those mentioned in Fig. 5. This indicates that co-doping with Li\u003csup\u003e+\u003c/sup\u003e enhances the luminescence intensity of phosphors and it is more effective than co-doping with Na\u003csup\u003e+\u003c/sup\u003e. The maximum luminescence intensity is achieved with the content of 3% Li\u003csup\u003e+\u003c/sup\u003e doping. Fig. 6(b) illustrates the emission spectra of CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e, \u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e and CaSr\u003csub\u003e2-1.5\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e phosphors, showing the beneficial effect of Li\u003csup\u003e+\u003c/sup\u003e co-doping on the luminescence intensity of the samples. The phosphors need to produce ion defects to maintain the charge balance because the replacement of Sr\u003csup\u003e2+\u003c/sup\u003e with Dy\u003csup\u003e3+\u003c/sup\u003e is not equal. However, too many defects produce crystal lattice distortions that reduce the luminescence intensity. Instead, the introduction of Li\u003csup\u003e+\u003c/sup\u003e ions means that the charge compensation takes place according to the following formula: 2Sr\u003csup\u003e2+\u003c/sup\u003e = Dy\u003csup\u003e3+\u003c/sup\u003e + Li\u003csup\u003e+\u003c/sup\u003e. This limits the number of crystal defects and leads to the enhanced luminescence intensity. Moreover, Li\u003csup\u003e+\u003c/sup\u003e co-doping changes symmetry of the local coordination environment and the related crystal field strength. The reduced site symmetry may also lead to the enhanced luminescence intensity.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.3. Energy transfer mechanism \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe quenching of the emission intensity depends on the critical distance \u003cem\u003eRc\u003c/em\u003e, which is the shortest average distance between the nearest dopant Dy\u003csup\u003e3+\u003c/sup\u003e ions at a critical concentration \u003cem\u003ex\u003csub\u003ec\u003c/sub\u003e\u003c/em\u003e. The critical distance \u003cem\u003eRc\u003c/em\u003e is described by the following equation [36]:\u003c/p\u003e\n\u003cp style=\"text-align: left;\"\u003e\u003cimg src=\"data:image/png;base64,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\" alt=\"\" /\u003e\u003c/p\u003e\n\u003cp\u003ein which V stands for the volume of the unit cell, \u003cem\u003ex\u003csub\u003ec\u003c/sub\u003e\u003c/em\u003e is the critical concentration of activator ion (Dy\u003csup\u003e3+\u003c/sup\u003e) beyond the concentration quenching, and N represents the number of host cations in one unit cell. In our case, N = 6 V was estimated to be 3744.14 \u0026Aring;, and \u003cem\u003ex\u003csub\u003ec\u003c/sub\u003e\u003c/em\u003e is 0.06 according to the above discussions. According to the eq. (1), Rc was changed to 27.08 \u0026Aring; (\u003cem\u003ex\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e = 0.06). It is well-known that exchange interactions play a crucial role in the energy transfer mechanism when the critical distance between the sensitizer and the activator ions is less than 4 \u0026Aring;. With a much higher \u003cem\u003eRc\u003c/em\u003e value, the energy transfer mechanism is considered to be an electric multipolar interaction. Based on the Dexter's theory, if the energy transfer occurs by electric multipolar interactions, then the relationship between the luminescent intensity (\u003cem\u003eI\u003c/em\u003e) and the activator concentration (\u003cem\u003ex\u003c/em\u003e) can be expressed by the following equation [37]:\u003c/p\u003e\n\u003cp style=\"text-align: left;\"\u003e\u003cimg src=\"data:image/png;base64,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\" alt=\"\" /\u003e\u003c/p\u003e\n\u003cp\u003ewhere \u003cem\u003ex\u003c/em\u003e is the activator concentration, \u003cem\u003eK\u003c/em\u003e and \u003cem\u003e\u0026beta;\u003c/em\u003e are constants for each interaction at the same excitation. \u003cem\u003e\u0026theta;\u003c/em\u003e is a multipolar interaction constant equal to 3, 6, 8 or 10, corresponding to the nearest-neighbor ions, i.e., dipole\u0026ndash;dipole (\u003cem\u003ed\u003c/em\u003e\u0026ndash;\u003cem\u003ed\u003c/em\u003e), dipole\u0026ndash;quadrupole (\u003cem\u003ed\u003c/em\u003e\u0026ndash;\u003cem\u003eq\u003c/em\u003e) and quadrupole\u0026ndash;quadrupole (\u003cem\u003eq\u003c/em\u003e\u0026ndash;\u003cem\u003eq\u003c/em\u003e) interactions, respectively. We chose the CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e (\u003cem\u003ex\u003c/em\u003e = 0.06, 0.12, 0.18, 0.24, 0.27 and 0.3) samples for the constant emission intensity measurements at 578 nm exceeding the quenching concentration. The relation of log(I/\u003cem\u003ex\u003c/em\u003e) vs. log(\u003cem\u003ex\u003c/em\u003e) for the CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e (\u003cem\u003ex\u003c/em\u003e = 0.06, 0.12, 0.18, 0.24, 0.27 and 0.3) peaks at 486 and 578 nm was plotted and depicted in Fig. 7. All \u003cem\u003eq\u003c/em\u003e values are close to 6, hence we can conclude that the quenching mechanism between Dy\u003csup\u003e3+\u003c/sup\u003e ions in the CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e samples conforms to the dipole\u0026ndash;dipole (\u003cem\u003ed\u003c/em\u003e\u0026ndash;\u003cem\u003ed\u003c/em\u003e) interactions.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.4. Temperature-dependent luminescent properties\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIt is well-known that the luminescence intensity for most phosphors decreases if the operating temperature exceeds a certain value (thermal quenching). [38] Thus, the thermal stability of the phosphor is a key issue for high-power \u003cem\u003ew\u003c/em\u003e-LEDs. One of the key requirements for a good phosphor is to maintain the performance at the operating temperature of the device. Usually, the luminescence intensity of the phosphors at 423 K with respect to that at room temperature is used to assess the thermal stability. The reason for this has been reported as an increase in the non-radiative transition probability in the configurational coordinated diagram. [39] Fig. 8 shows the temperature dependent emission spectra of the CaSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:0.06Dy\u003csup\u003e3+\u003c/sup\u003e,0.06Li\u003csup\u003e+\u003c/sup\u003e phosphor from 298 to 523 K, at 351 nm excitation; the inset shows the relative emission intensities at 486 and 578 nm as a function of temperature. The shape of the emission bands remains unchanged with increasing temperature (Fig. 8), suggesting that the phosphor has excellent color stability, which is crucial in LEDs or high temperature LEDs. Upon heating the phosphor samples in the temperature range from 298K to 523K, the emission intensity decreased slightly, since the probability of nonradiation is increased and luminescent c enter is released through the crossing point between the excitation state and the ground state, causing the luminescence quenching. [40] Besides, Fig. 9 illustrated the PL intensity of CaSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:0.06Dy\u003csup\u003e3+\u003c/sup\u003e,0.06Li\u003csup\u003e+\u003c/sup\u003e phosphor with respect to time, monitored under 351 nm excitation continuously for 60 min at (a) 100˚C and (b) 150˚C for 30 min at each temperature with a time interval of 5 min, and the emission profile of phosphor maintained at 100˚C and 150˚C continuously for 30 min at each temperature showed that the emission intensity are very stable and remain unchanged, as well as without variation of the emission wavelength. In addition, the emission intensities of CaSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:0.06Dy\u003csup\u003e3+\u003c/sup\u003e,0.06Li\u003csup\u003e+\u003c/sup\u003e maintains 77% of the initial emission intensity corresponding to a temperature of 423K, revealing that the CaSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:0.06Dy\u003csup\u003e3+\u003c/sup\u003e,0.06Li\u003csup\u003e+\u003c/sup\u003e phosphors have good thermal stability, which confirms the stable chromaticity coordinates of CaSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:0.06Dy\u003csup\u003e3+\u003c/sup\u003e,0.06Li\u003csup\u003e+\u003c/sup\u003e phosphors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.5. Luminescence decay curves and chromaticity coordinates\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFig. 10 shows the decay curves of Dy\u003csup\u003e3+\u003c/sup\u003e emission for CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e (\u003cem\u003ex\u003c/em\u003e = 0.03, 0.09, 0.15 and 0.18) samples excited at 351 nm and monitored at 578 nm. All decay curves can be well fitted to a bi-exponential decay equation as follows:\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eI\u003c/em\u003e = A\u003csub\u003e1 \u003c/sub\u003eexp(-\u003cem\u003ex\u003c/em\u003e/\u0026tau;\u003csub\u003e1\u003c/sub\u003e) + A\u003csub\u003e2 \u003c/sub\u003eexp(-x/\u0026tau;\u003csub\u003e2\u003c/sub\u003e)\u003c/p\u003e\n\u003cp\u003ewhere I is the luminescence intensity at time t, A1 and A2 are amplitudes, and t1 and t2 are the luminescence lifetimes. The average emission lifetimes \u0026tau; were calculated by following formula:\u003c/p\u003e\n\u003cp style=\"text-align: left;\"\u003e\u003cimg src=\"data:image/png;base64,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\" alt=\"\" /\u003e\u003c/p\u003e\n\u003cp\u003eThe calculated average emission lifetimes decrease with increasing Dy\u003csup\u003e3+\u003c/sup\u003e concentration, i.e. 0.74, 0.47, 0.22 and 0.11 ms for the CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e samples with \u003cem\u003ex\u003c/em\u003e = 0.03, 0.09, 0.15 and 0.18, respectively. This is due to the decreasing distance between Dy\u003csup\u003e3+\u003c/sup\u003e\u0026ndash;Dy\u003csup\u003e3+\u003c/sup\u003e ions, resulting in the observed concentration quenching phenomenon (enhanced cross-relaxations processes), as well as enhanced probability of energy transfer to the luminescence killer sites. Thereby, the luminescence lifetimes of Dy\u003csup\u003e3+\u003c/sup\u003e ions are shortened due to the favorable nonradiative energy transfer processes when the Dy\u003csup\u003e3+\u003c/sup\u003e concentration increases. The observed bi-exponential character of the decay curves could be because of the non-equal occupation of the cation sites by the emitting activator Dy\u003csup\u003e3+\u003c/sup\u003e that has a concentration-dependent preferential occupation in one of the sites as well.\u003c/p\u003e\n\u003cp\u003eThe Commission International de I\u0026prime;Eclairage (CIE) chromaticity coordinates for the representative sample (Ca\u003csub\u003e2\u003c/sub\u003eSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:0.06Dy\u003csup\u003e3+\u003c/sup\u003e,0.06Li\u003csup\u003e+\u003c/sup\u003e) were calculated based on the corresponding emission spectra, and the results are shown in Fig. 11(a). It can be seen that the coordinates (x = 0.3450, y = 0.3787) are located in the near-white region. In addition, we also calculated the CIE chromaticity coordinates of Ca\u003csub\u003e2\u003c/sub\u003eSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:0.06Dy\u003csup\u003e3+\u003c/sup\u003e,0.06Li\u003csup\u003e+\u003c/sup\u003e at different temperature values (Table 1), and we found that with the temperature rising the phosphor exhibits almost no change in the color of emission, which is showed in Fig. 11(b). Thus, the CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e material can be potentially used as a white phosphor for \u003cem\u003ew\u003c/em\u003e-LEDs in solid-state lighting applications.\u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn summary, a series of CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e, CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eNa\u003csup\u003e+ \u003c/sup\u003eand CaSr\u003csub\u003e2-1.5x\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:xDy\u003csup\u003e3+ \u003c/sup\u003ephosphors were prepared by a conventional solid-state reaction. The phase structure, luminescence properties, thermal quenching and emission decay curves were investigated. Under the excitation of 351 nm, the CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e phosphors showed two emission bands centered at 486 and 578 nm, characteristic of\u0026nbsp; Dy\u003csup\u003e3+\u003c/sup\u003e. The excitation spectra showed narrow excitation bands from 250 to 550 nm with a maximum at 403 nm. This means that the phosphor can be effectively excited by UV chips for potential applications in \u003cem\u003ew\u003c/em\u003e-LEDs. The optimum dopant concentration of Dy\u003csup\u003e3+\u003c/sup\u003e ions is 0.06 mol.%. Furthermore, the quenching mechanism between the Dy\u003csup\u003e3+\u003c/sup\u003e ions was recognized as dipole\u0026ndash;dipole (\u003cem\u003ed\u003c/em\u003e\u0026ndash;\u003cem\u003ed\u003c/em\u003e) interactions. The dependence of the emission spectra on temperature indicated that the phosphor has a good thermal stability in both emission color and intensity. The CIE chromaticity coordinates of the selected Ca\u003csub\u003e2\u003c/sub\u003eSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:0.06Dy\u003csup\u003e3+\u003c/sup\u003e,0.06Li\u003csup\u003e+\u003c/sup\u003e sample were calculated (\u003cem\u003ex\u003c/em\u003e =0.3450, \u003cem\u003ey\u003c/em\u003e = 0.3787), and they are located in the white-light region. These results indicate that CaSr\u003csub\u003e2-2\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003ex\u003c/em\u003eDy\u003csup\u003e3+\u003c/sup\u003e,\u003cem\u003ex\u003c/em\u003eLi\u003csup\u003e+\u003c/sup\u003e phosphors have a good potential for their use as white-emitting luminophores for the phosphor-converted \u003cem\u003ew\u003c/em\u003e-LEDs. Thus, the results of this work indicate the potential applications of these effective phosphors in white light-emitting diodes, excited with a near-UV chip.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eConflicts of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere are no conflicts to declare.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe present work is supported by the National Natural Science Foundation of China (Grant No. 41802040) and the Fundamental Research Funds for China University of Geosciences, Beijing (Grant No. 2652019080).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eM.M. Shang, G.G. Li, D.L. Geng, D.M. Yang, X.J. Kang, Y. Zhang, H.Z. Lian, J. 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Liu, Effect of A\u003csup\u003e+\u003c/sup\u003e (A = Li, Na and K) co-doping on the luminescence enhancement of CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:Dy\u003csup\u003e3+\u003c/sup\u003e phosphors for white light-emitting diodes, Ceram. Int. 43 (2017) 15493-15499.\u003c/li\u003e\n\u003cli\u003eH. Ji, Z. Huang, Z. Xia, M.S. Molokeev, V.V. Atuchin, M. Fang, S. Huang, New yellow-emitting whitlockite-type structure Sr\u003csub\u003e1.75\u003c/sub\u003eCa\u003csub\u003e1.25\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:Eu\u003csup\u003e2+\u003c/sup\u003e phosphor for near-UV pumped white light-emitting devices, Inorg. Chem. 53 (2014) 5129\u0026ndash;5135.\u003c/li\u003e\n\u003cli\u003eW. Tang, Z. 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Runowski, A novel reddish-orange fluorapatite phosphor, La\u003csub\u003e6-x\u003c/sub\u003eBa\u003csub\u003e4\u003c/sub\u003e(SiO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e6\u003c/sub\u003eF\u003csub\u003e2\u003c/sub\u003e: \u003cem\u003ex\u003c/em\u003eSm\u003csup\u003e3+\u003c/sup\u003e - Structure, luminescence and energy transfer properties, \u003cem\u003eJ. Alloys Compd.\u003c/em\u003e 757 (2018) 79-86.\u003c/li\u003e\n\u003cli\u003eS. Liu, Z.J. Wang, Q. Bao, X. Li, Y. Chen, Z.P. Wang, L. Guan, Abnormal thermal quenching and blue-shift of Zn\u003csub\u003e3\u003c/sub\u003e(BO\u003csub\u003e3\u003c/sub\u003e)(PO\u003csub\u003e4\u003c/sub\u003e):Inducing host defect by doping Mn\u003csup\u003e2+\u003c/sup\u003e and Tb\u003csup\u003e3+\u003c/sup\u003e, Dyes and Pigments, 165 (2019) 44\u0026ndash;52.\u003c/li\u003e\n\u003cli\u003eJ.J. Chen, Y. Zhao, Z.Y. Mao, D.J. Wang, L.J. Bie. Investigation of thermal quenching and abnormal thermal quenching in mixed valence Eu co-doped LaAlO\u003csub\u003e3\u003c/sub\u003e phosphor, J Lumin, 186 (2017) 72\u0026ndash;76.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003e\u003cstrong\u003eTable 1\u003c/strong\u003e The CIE chromaticity coordinates of Ca\u003csub\u003e2\u003c/sub\u003eSr\u003csub\u003e1.88\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:0.06Dy\u003csup\u003e3+\u003c/sup\u003e,0.06Li\u003csup\u003e+ \u003c/sup\u003eat different temperature values.\u003c/p\u003e\n\u003ctable border=\"1\"\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd\u003e\n\u003cp\u003eTemperature (K)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003ex\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003ey\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd rowspan=\"2\"\u003e\n\u003cp\u003e298\u003c/p\u003e\n\u003cp\u003e323\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3917\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.4101\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3795\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3879\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003e\n\u003cp\u003e348\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3918\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.4099\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003e\n\u003cp\u003e373\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3905\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.4086\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003e\n\u003cp\u003e398\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3890\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.4067\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003e\n\u003cp\u003e423\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3876\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.4051\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003e\n\u003cp\u003e448\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3803\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3925\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003e\n\u003cp\u003e473\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3840\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.4002\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003e\n\u003cp\u003e498\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3811\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3967\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003e\n\u003cp\u003e523\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3786\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd\u003e\n\u003cp\u003e0.3935\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"CaSr2(PO4)2, Phosphor, Dy3+, Thermal stability","lastPublishedDoi":"10.21203/rs.3.rs-577287/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-577287/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSingle-phase CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:Dy\u003csup\u003e3+\u003c/sup\u003e,Li\u003csup\u003e+ \u003c/sup\u003ephosphors were prepared using the high-temperature solid-state method in the air. To characterize the luminescence properties of the synthesized phosphors, Powder X-ray diffraction patterns (XRD), scanning electron microscopy images (SEM), photoluminescence spectra, and concentration-dependent emission spectra were measured to characterize the luminescence properties of the synthesized phosphors. The results showed that the CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:Dy\u003csup\u003e3+\u003c/sup\u003e,Li\u003csup\u003e+ \u003c/sup\u003ephosphors exhibited white luminescence, and the emission spectra of the phosphors consisted of two sharp peaks at ≈486 and ≈578 nm (the most intense one). The optimum concentration of Dy\u003csup\u003e3+\u003c/sup\u003e doping was determined to 0.06 mol.%. On the basis of the Dexter's theory, the mechanism of energy transfer between the Dy\u003csup\u003e3+\u003c/sup\u003e ions was determined to dipole–dipole interactions. The results of the temperature-dependent luminescence confirmed that the as-prepared phosphors are proved to be promising UV-convertible material capable of white light emitting in UV-LEDs due to its excellent thermal stability and luminescence properties. Luminescence intensity and decay time of the CaSr\u003csub\u003e2\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e:Dy\u003csup\u003e3+\u003c/sup\u003e,Li\u003csup\u003e+\u003c/sup\u003e phosphors were improved remarkably with the addition of charge compensators (Li\u003csup\u003e+\u003c/sup\u003e ions), which would promote their applications in white light-emitting diodes based on the near-UV chip.\u003c/p\u003e","manuscriptTitle":"Crystal Structure and Luminescence Properties of a Thermally Stable Single-Phase White Emitting Phosphor CaSr2(PO4)2: Dy3+, Li+","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2021-06-03 23:12:42","doi":"10.21203/rs.3.rs-577287/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"46d4791e-8781-4c85-a169-101b43a064fc","owner":[],"postedDate":"June 3rd, 2021","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":4775368,"name":"Ceramics"},{"id":4775369,"name":"Optical Materials and Devices"},{"id":4775370,"name":"Electronic Materials and Devices"},{"id":4775371,"name":"Magnetics Materials and Devices"}],"tags":[],"updatedAt":"2021-06-03T23:12:43+00:00","versionOfRecord":[],"versionCreatedAt":"2021-06-03 23:12:42","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-577287","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-577287","identity":"rs-577287","version":["v1"]},"buildId":"_2-kVJe1T_tPrBINL-cwx","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}