Pressure Calibration to 100 kbar Based on the Compression of NaCl

Abstract

An independent determination has been made of the transformation pressures assigned to the high‐pressure transformations in Ba and Bi which are generally used for calibration points. The metals were imbedded in NaCl, and the transitions were detected by the resistance change of the metal while the NaCl lattice parameter was simultaneously measured by x‐ray diffraction. The NaCl compression values determined at the fixed points were related to pressure by reference to the semiempirical compression curve of Decker. The Ba I–II and Bi III–V transformations at room temperature were assigned thermodynamic equilibrium values of 53.3±1.2 kbar and 73.8±1.3 kbar, respectively, corresponding to NaCl linear compression values of Δa/a₀=0.0510 and 0.0637, respectively. Hysteresis in the transformation pressures was investigated and sample hysteresis was separated from apparatus hysteresis. The hysteresis measurements imply a stress energy which inhibits nucleation of the new phase and causes a sizable sample hysteresis in solid‐media systems.