Growth of diamond in liquid metal at 1 atmosphere pressure

Abstract Natural diamonds were (and are) formed (some, billions of years ago) in the Earth’s upper mantle in metallic melts in a temperature range of 900–1400°C and at pressures of 5–6 GPa1,2; indeed, diamond is thermodynamically stable under high pressure and high temperature (HPHT) conditions as per the phase diagram of carbon3. Scientists at General Electric invented and used a HPHT apparatus in 1955 to synthesize diamonds from melted iron sulfide at about 7 GPa and 1600°C4–6. There is an existing paradigm that diamond can be grown using liquid metals only at both high pressure (typically 5–6 GPa) and high temperature (typically 1300–1600°C) where it is the stable form of carbon7. Here, we describe the growth of diamond crystals and polycrystalline diamond films with no seed particles using liquid metal but at 1 atmosphere pressure, and at 1025°C, breaking this paradigm. Diamond grew at the interface of liquid metal composed of gallium, iron, nickel, and silicon and a graphite crucible, by catalytic activation of methane and diffusion of carbon atoms in the subsurface region of the liquid metal. Raman spectroscopy with 13C-labeling proves that methane introduced into the growth chamber is the carbon source for many of the regions of newly grown diamond. The new growth diamonds were studied by Raman spectroscopy, scanning and transmission electron microscopy, X-ray diffraction, and photoluminescence. Growth of (metastable) diamond in liquid metal at moderate temperature and 1 atm pressure opens many possibilities for further basic science studies and for the scaling of this type of growth.