7 ' 1~ pressure tlepentlrnce of the thermal expansivity ( a ) of NaCl crystal has been studied using various relations hetween a and the Anderson parameter 6. The variation of 6 with pressure has been estimated anti t,akeri into account for determining the pressure dependence of a. A comparison of the results derived from various models has been presented with available experimental data. It is found that the Anderson power law for the pressure dependence of 6 is more consistent with the experimental data.

The second-order elastic constants (711, (712, and C 4 4 , isothcrmal bulk modulus BT, and volume thermal expansion coefficient a are calculated for NaCl crystals in the temperature range (294 to 766 K) using Tallon's model based on the concept of modified Anderson-Griineisen parameters. The results obtained in the present study arc compared with the experimental values reported in the literature. There is good agreement between calculated and experimental values for V/Vo, C11, Cd4, BT, and a. It is found that C11, (244, arid BT decrease whereas C 1 2 and a increase with increase in temperature.

Various equations of state reported in the literature are used to investigate the variation of the bulk modulus for NaCl crystals under the effect of pressure up to 300 x lo8 Pa. The results are compared with the corresponding values obtained in the present study using interionic potentials as well as with the available experimental data. Values of bulk modulus as a function of pressure are also calculated by taking into account the variation of the Anderson parameter with pressure. The results thus obtained are found to present best agreement with those estimated from the universal equation of state.

The Birch equation of state (BES) derived from the theory of finite strain is used to calculate the pressure versus volume compression data at room temperature for eleven alkali halide crystals, viz. LiF, LiCl, LiBr, LiI, NaF, NaC1, NaBr, NaI, CsC1, CsBr, and Csl. These crystals can be compressed to rather high pressures without appearance of any structural phase transformation. The parameters of BES are determined from the ultrasonically measured experimental values of the bulk modulus and its first pressure derivative and the calculated values of second pressure derivative of bulk modulus derived from interionic potential models. The resulting P-Vdata for all the crystals compare well with the corresponding experimental values reported by Vaidya and Kennedy and by Yagi.

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