Isothermal equation of state for sodium chloride by the length-change-measurement technique

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Abstract
The change in length of a 1‐m‐long NaCl single crystal has been determined as a function of hydrostatic pressure up to 7.5 kbar and at temperatures of 29.5 and 40.4 °C, to an accuracy of 500 Å using a Fabry‐Perot–type He‐Ne laser interferometer. The best values of the isothermal bulk modulus and its pressure derivative at atmospheric pressure and at 29.5 °C are B0=237.7±0.3 kbar, B′0 =5.71±0.25, and B″0=−0.10±0.05 kbar−1, respectively. These are the averages of the values obtained by a least‐squares fit of several different equations of state to the present isothermal data. From these low‐pressure measurements along, it is not possible to conclude which one of these equations provides a better fit to the data than the others. However, when other high‐pressure data are taken into account, it appears that Keane’s equation best represents the measurements. When Keane’s equation is fitted to the data, and the published lattice parameter of NaCl at the Bi‐III‐V transition and at the NaCl B1‐B2 transition are used, the respective transition pressures are found to be 75.8 kbar, within 1.2 kbar of the presently accepted value and 262 kbar, respectively. Considering the precision of the experiment the values of B0, B′0, and B″0 represent the best measurements so far. The determination of B″0 isothermally represents the first measurement of its kind.