Mixed Polaron and Bipolaron Transport in (xV2O5 – (65-x) Sb2O3 – 35P2O5) Glasses

This study presents the electrical and optical properties of phosphate glasses in the 35P2O5 -xV2O5 -(65–x) Sb2O3 (PVS) system with 0 ≤ x ≤ 65 mol%. DC resistivity was measured using the Van der Pauw method and optical absorption spectra were recorded in the UV–VIS–NIR range. Electrical transport is attributed to simultaneous hopping of small polarons (SPs) between V4+ and V5+ (vanadium ion) sites and small bipolarons (SBPs) between Sb3+ and Sb5+ (antimony ion) sites. The resistivity exhibits a non- linear dependence on the ionic fraction of vanadium, defined as, [V]([V]+[Sb]) where [V] and [Sb] are the concentrations of the respective cations in the glasses. Whereas the resistivity exhibits a minimum in the composition range 0 ≤ nV ≤ 0.3, a resistivity maximum was observed in the range 0.3 ≤ nV ≤ 0.5. On further increasing nv, the resistivity exhibits a monotonic decline. In the composition range 0 ≤ nV ≤ 0.3, where the hopping distance between V ions decreases, while that between the Sb ions increases, the resistivity minimum has been shown to be the consequence of decreasing tunneling distance of SPs between the V4+ and V5+ ion sites. Though the probability of successful hops of SBPs decreases on account of increasing separation between Sb3+ and Sb5+ ions, the far superior mobility of SPs than that of SBPs more than offsets the decreasing contribution of SBPs to the total conductivity. In the composition range 0.3 ≤ nV ≤ 0.5, the resistivity, activation energy for DC conduction, glass transition temperature and also density exhibit their respective maxima even though the separation between V4+ and V5+ sites continues to decrease. This feature is explained by enhanced localization of electrons on account of increased disorder (entropy) among the SPs and SBPs, similar to that of Anderson localization. This argument is further supported by a shift of the polaronic optical absorption bands associated with the SPs and SBPs toward higher energies. The transport behavior of all the glasses except the x = 0 composition has been explained by adiabatic transport, principally, by the SPs on V ions while the Sb ions contribute little to the total transport process. On the contrary, Sb ions is the principal source that produces the highly disordered potential field leading to a resistivity maximum at a [V/Sb] ratio of ~ 0.5 in the glass composition.