The Thermal Expansion of Rubidium Bromide by the Shell Model

Abstract

The equation of state and thermal expansion of rubidium bromide have been studied on the basis of the simple shell model taking the electronic polarisabilities of both the ions into account. The static pressure has been evaluated from the Coulomb interaction supplemented by a repulsive potential of the Born‐Mayer type. The thermal pressure has been calculated from the vibration spectrum of the lattice. Out of the seven parameters of the model three, namely, the ionic and shell charges have been assumed to be volume independent although the individual values of these vary with volume. The vibration frequencies have been computed for a one thousand‐point mesh in the Brillouin zone at eleven closely spaced volumes. The total pressure has been obtained by adding the static and thermal pressures. The isotherms have been drawn in the temperature range 0 to 950 K, whose intercepts on the zero‐pressure axis give the thermal expansion coefficients. In agreement with Herzfeld and Mayer's theory the isotherm at 950 K, which is the melting point of this salt, has a minimum on the zero pressure axis. The calculated thermal expansion coefficients are in satisfactory agreement with the measured values of James and Yates and of Deshpande and Sirdeshmukh.