Molecular dynamics simulations of Na₂Si₄O₉ liquid at high pressure

Molecular dynamics (MD) simulations are used to investigate the structural and transport properties of Na₂Si₄O₉ liquid as a function of pressure. Simulations were performed at 6000 K at a variety of pressures, ranging from 1 atm to 100 GPa. The calculated oxygen self-diffusivity increases with increasing pressure, up to approximately 10-15 GPa, as found in previous simulations and experimental studies. Above this pressure, the O^2- diffusivity decreases slightly with increasing pressure. From the MD results, we distinguish two distinct mechanisms for the pressure-induced coordination change of silicon. The first, occurring in the lower pressure regime, below 15 GPa, involves formation of ^[5]Si species via a reaction with the nonbridging oxygen atoms. The second mechanism occurs at high pressures via a reaction of the bridging oxygen atoms and results in the formation of ^[3]O species.