Recently developed isothermal Kholiya’s EOS is modified to study the temperature dependent volume expansion and applied for NaCl crystal. The results obtained with the present model are in quite close agreement to the experimental values. The model is therefore extended to study the variation of bulk modulus and the coefficient of volume thermal expansion with temperature. Comparison of the obtained results with the experimental data demonstrates that an isothermal EOS may also be modified to study the temperature dependent elastic properties. The present study also reveals that the quasi harmonic approximation, that is, the product of bulk modulus and the coefficient of volume thermal expansion as constant, is valid in case of NaCl crystal. 1. Introduction The behavior of solids under the effect of high pressure and high temperature has truly developed into an interdisciplinary area which has important implications for an application in the area of physics, biology, engineering, and technology apart from the discovery of various novel and unexpected phenomena, high pressure high temperature research has provided new insight into the behavior of matter [1]. Strength and elastic properties of a solid depend on the strength of its interatomic forces. Therefore, the application of temperature which changes the interatomic distance of the substances changes its physical properties. The EOS gives us valuable information about the relationship between the changes in thermodynamic variable, namely, pressure, volume, and temperature. Every thermodynamic system has its own EOS, independent of others. An EOS expresses the peculiar behavior of one individual system which distinguishes it from the others. In order to determine the EOS of a system, the thermodynamic variables of the system are accurately measured and a relation is expressed between them. Attempts have been made to derive a compressibility equation from molecular theory, but none of them has resulted in convenient equation expressing the results of experiments with adequate accuracy. To meet this need some empirical equations have been proposed, the sole justification of which is that it works. In spite of impressive advances on the theoretical front over the past decades, the need for the search of an EOS continues to exist. Although, modern electronic band structure calculation allows the predictions of EOS for solids yet the calculation is time consuming as well as expansive. In the literature, there are number of equations of states, and these arise from an unchecked and improvable assumption
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