Compression of Ag and phase transformation of NaCl

Abstract

The isothermal compression of silver up to 309 kbar has been measured at 23±3°C by high‐pressure x‐ray diffraction employing a diamond‐anvil cell. The B1 phase of NaCl mixed with the silver was used both as a pressure transmitting medium and as an internal pressure calibrant. The data fitted to two forms of the Birch equation yield K₀=1180±50 kbar, K₀′=3.8±0.5, and K₀=1105±95 kbar, K₀′=4.6±0.8. When the pressure exceeded 300 kbar, the NaCl underwent a phase transformation to the B2 phase. Increasing the load did not result in the complete disappearance of the B1 phase up to a pressure of 311 and 334 kbar based on the extrapolation of the Ag and Re compression data, respectively. The average volume change measured for the B1‐B2 transformation is −0.83±0.11 cm³/mole, which is 17% smaller than the value reported by Bassett et al. This smaller volume change improves the agreement of NaCl with the linear relationship between volume change and entropy change proposed by Bassett et al. for the B1‐B2 transformation in the alkali halides.