Local structure of molten AuGa2 under pressure: Evidence for coordination change and planetary implications

In situ x-ray diffraction measurements and inverse Monte Carlo simulations of pair distribution functions were used to characterize the local structure of molten AuGa₂ up to 16 GPa and 940 K. Our results document systematic changes in liquid structure due to a combination of bond compression and coordination increase. Empirical potential structure refinement shows the first-neighbor coordination of Ga around Au and of Au around Ga to increase from about 8 to 10 and 4 to 5, respectively between 0 and 16 GPa, and the inferred changes in liquid structure can explain the observed melting-point depression of AuGa₂ up to 5 GPa. As intermetallic AuGa₂ is an analogue for metallic SiO₂ at much higher pressures, our results imply that structural changes documented for non-metallic silicate melts below 100 GPa are followed by additional coordination changes in the metallic state at pressures in the 0.2–1 TPa range achieved inside large planets.

Melting of cubic boron nitride at extreme pressures

Due to its large pressure range of stability and inert nature, cubic boron nitride has been proposed as a potential pressure standard for high pressure experiments. It is extremely refractive upon compression, although its melting temperature is not known beyond 10 GPa. We apply first-principles molecular dynamics to evaluate the thermodynamics of zincblende structured (cubic) and liquid boron nitride at extreme temperatures and pressures, and compute the melting curve up to 1 TPa by integration of the Clapeyron equation. The resulting equations of state reveal that liquid boron nitride becomes denser than the solid phase at pressures of around 0.5 TPa. This is expressed as a turnover in the melting curve, which reaches a maximum at 510 GPa and 6550 ± 700 K. The origin of this density crossover is explained in terms of the underlying liquid structure, which diverges from that of the zincblende structured solid as the phases are compressed.