First principles versus spherical ion models of the B1 and B2 phases of NaCl

Abstract

The electron density‐functional formalism is used to compute first‐principles equations of state and charge distributions for the B1 and B2 phases of NaCl. The B1 equation of state is in excellent agreement with available static, acoustic, and shock wave data. The theoretical B2 equation of state differs significantly from available diamond anvil data but agrees well with shock wave data. Analysis of the theoretical and experimental isotherms suggest that the former may be a little low in pressure, while the latter is almost certainly too stiff. The computed charge distribution and valence band energies indicate that Cl‐Cl interaction may dominate the energetics of the B2 phase and that, consequently, the Cl ions are significantly distorted. The theoretical zero pressure Cl radius is smaller than the classical crystallographical radius, is sensitive to coordination, and expands significantly across the B1–B2 transformation. In contrast, the Na ion is insensitive to coordination and seems to retain classical ionic properties (+1 charge, spherical charge distribution) up to the highest pressures investigated (∼70 GPa). These properties are largely responsible for the failure of Born‐Mayer‐type structure independent pair potentials to account accurately for the changes in elastic properties of NaCl across the B1–B2 transformation.