The equation of state of B2-type NaCl

Abstract

The equation of state (EOS) of B2-type NaCl has been investigated to 270 GPa and 3000 K using the first-principles molecular dynamics method and high-pressure experiments in a diamond anvil cell. We used the high-pressure experimental data to determine the compressibility at room temperature, and used the generalized gradient approximation (GGA) and the projector augmented-wave method (PAW) in simulations to calculate the thermal pressure. A Vinet EOS fitted to the room temperature data yielded an isothermal bulk modulus of BT0 = 39.25 GPa and a pressure derivative of BT0' = 4.72. The high-temperature data from the first-principles calculations were fitted to the thermal pressure EOS. The resulting calculated parameters of the thermal pressure, αBT(V0,T) and (δBT /δT)V, were 3.28 × 10−3 (GPa/K) and 4.3 ×10−4 (GPa/K), respectively. A small volume dependence of the thermal pressure of B2-type NaCl was revealed from the analysis of our data. A significant temperature dependence of the calculated Grüneisen parameters was confirmed. This indicates that the conventional approach using the Mie-Grüneisen approximation is likely to have a significant uncertainty in determining the EOS for B2-type NaCl, and that an intrinsic anharmonicity should be considered to analyze the EOS.