Determination of the phase boundary between the B1 and B2 phases in NaCl by in situ x-ray diffraction

The equilibrium phase boundary between the B1 and B2 phases in NaCl was determined at temperatures between 1150 and 2000 K by in situ x-ray diffraction, using a Kawai-type apparatus in a beam line of SPring-8, Hyogo, Japan. The press was oscillated during data acquisition in order to reduce the effect of grain growth on the diffraction pattern of NaCl at high temperature, obtained by the energy-dispersive method with a fixed angle. Forward (B1 to B2) and backward (B2 to B1) transitions were observed independently at temperatures between 1150 and 1600 K; the P-T conditions where these transitions occur fall on a straight line, indicating that there is no kinetic effect between the forward and the backward transitions over this temperature range. The equilibrium phase boundary is represented by the linear equation P (in GPa)=30.6(2)–0.0053(2) T (in K). Liquid NaCl was observed at 20.1 GPa and 2100 K, consistent with the melting curve obtained in a previous study using a laser-heated diamond anvil cell. According to our experimental results, the triple point between B1, B2, and liquid NaCl would be located at 19.7±0.5 GPa and 2050±50 K.