Shanker formulation needs modification at high pressures

It is found that the Shanker formulation widely used in the literature to study the thermal expansion of solids (at constant pressure) works under the effect of pressure (at constant temperature) up to a limited range (≈30 kbar). Large deviations occur, when the pressure range is increased, demonstrating the failure of the relation under high pressure (at constant temperature). We, therefore, propose the modification in the formulation on an empirical basis. The modified relation is used to study the compression behavior of ionic solids viz. NaF, NaCl, NaBr and NaI crystals. The results obtained with the modified relation are compared with the experimental data in the light of the results obtained from Shanker formulation and Birch–Murnaghan equation of state. A good agreement between theory and experiment demonstrates the validity of the modification presented in the present note.