Pressure-Induced Phase Transformation in NaCl

A pressure-induced phase transformation in NaCl which occurs rapidly and reversibly at approx 300 kbar and room temperature has been observed in a diamond-anvil high-pressure cell. X-ray diffraction data indicate that the high-pressure polymorph has the cesium chloride (B2) structure. The lattice parameters of the low- (B1) and high- (B2) pressure phases at the transformation pressure are, respectively, 4.872±0.004 Å and 2.997±0.004 Å, and the volume change for the transformation is − 1.00±0.05 cm3 mole−1. The entropy change for the phase transformation has been calculated from the volume change and from the high-temperature-pressure data obtained by the shock experiments of Fritz et al. and found to be 1.5±0.3 cal deg−1 mole−1. Comparison with other alkali chlorides indicates that a linear relationship exists between the entropy change and the volume change for the B1-B2 phase transformation. A thermodynamic equation accounting for this relationship has been derived under the assumption that the Grüneisen parameter is proportional to the Ath power of the volume. An equation which relates this factor A to the adiabatic bulk modulus and its pressure and temperature derivatives has also been derived.