Measurement of theB1-B2 transition pressure in NaCl at high temperatures

The phase boundary and kinetics of the B1-B2 transition in NaCl are documented between 298 and 670 K by visual observation into an externally heated diamond cell. We find an equilibrium (reversed) transition pressure Ptr=26.8 GPa at room temperature, with a Clapeyron slope dP_tr/dT=-8.9 MPa/K at elevated temperatures; the corresponding entropy change is 7.4 J/mol K. An apparent activation energy of 5.3 kJ/mol is obtained from the observed decrease in transition hysteresis with temperature. Previous determinations of the NaCl B1-B2 transition pressure have been biased upward by 3 GPa or more due to kinetic hindrances. Extrapolating our results to 0 K, however, we find good agreement ({~}12% difference) with the transition pressure derived from ab initio pseudopotential calculations.