Pressure-volume equation of state of the high-pressure B2 phase of NaCl

We have measured the unit-cell volume of the high-pressure B2 phase of NaCl up to 68 GPa in a diamond anvil cell with the laser annealing technique and synchrotron radiation. Laser annealing promotes the release of nonhydrostatic stress in the sample chamber, resulting in a quasihydrostatic condition for the sample at high pressures and an associated precision increase in the unit-cell volume determination. Since the B2 phase is only stable over 30 GPa, it is difficult to use the conventional Birch-Murnaghan equation of state (EOS) due to the lack of volume information at zero pressure. We adopted a modified data treatment which uses a reference volume V-r at any pressure instead of the zero-pressure volume V-0. The modified third-order Birch-Murnaghan EOS is expressed by the pressure (P-r), bulk modulus (K-r), and pressure dependence of the bulk modulus (K-r') at the reference point. We also fitted our data to the modified universal EOS, with which infinite zero-pressure volume can be treated. All these treatments yield reasonably consistent results. We calculated the pressure dependence of the bulk modulus and compared the result with other materials generally used as pressure media. The bulk modulus of the B2 phase of NaCl is similar to that of the B1 phase around the transition pressure, but it increases faster with pressure. The bulk modulus data show that, at P=100 GPa, the B2 phase of NaCl (Ksimilar to420 GPa) is harder than argon (Ksimilar to360 GPa), but still softer than MgO (Ksimilar to500 GPa).