First-principles simulations of CaO and CaSiO3 liquids: structure, thermodynamics and diffusion

Abstract

We have performed first-principles molecular dynamics simulations of CaO and CaSiO3 liquids over broad ranges of pressure (0–150 GPa) and temperature (2,500–8,000 K) within density-functional theory. The simulated liquid structure changes considerably on compression with the mean cation–anion coordination numbers increasing nearly linearly with volume. The Ca–O coordination number increases from 5 (7) near the ambient pressure to 8 (10) at high pressure for CaO (CaSiO3) liquid. The Si–O coordination number increases from 4 to 6 over the same pressure regime. Our results show that both liquids are much more compressible than their solid counterparts implying the possibility of liquid–solid density crossovers at high pressure. The Gruneisen parameter of both the liquids increases with pressure, which is opposite in case of crystalline phases. The calculated self-diffusion coefficients strongly depend on temperature and pressure, thereby requiring non-Arrhenian representation with variable activation volume. The diffusivity differences between the two liquids tend to be large at low-temperature and low-pressure regime. Also, comparisons with MgSiO3 liquid suggest that network modifier cations Ca and Mg behave similarly though Ca is more coordinated and more mobile as compared to Mg.