Pm3m-Fm3m transformations of alkali halides and the Born-Mayer model. CsCl and its solid solutions with RbCl, KCl and CsBr

Abstract

Some energy aspects of the Pm3m—Fm3m transformation of CsCl have been studied quantitatively. Effects of addition of RbCl, KCl and CsBr to CsCl on the transformation have been examined. Above a critical concentration of RbCl or KCl, the transformation becomes irreversible and at much higher concentrations the Fm3m form gets stabilized. Lattice energy differences between the Pm3m and Fm3m forms of CsCl as well as its solid solutions have been calculated employing various forms of the modified Born-Mayer model with three, and four, structure-dependent parameters. Both the three and the four parameter equations are satisfactory when a higher van der Waals term is used. The four parameter equation satisfactorily explains the variation in the transformation energy of the solid solutions of CsCl with the mole fraction of RbCl.