Phase transition and elastic properties of La - compounds

The phase transition and elastic properties of La - monochalcogenides have been investigated under pressure by means of a modified charge - transfer potential model which incorporates the Coulomb interaction modified by Coulomb screening due to the delocalization of electron of rare - earth atom leading to many - body interactions, covalency effect and overlap repulsion extended up to second nearest neighbours. Under high pressure the coordination increases and they transform from rock - salt to CsCI structure. The calculated values of cohesive energy, lattice constant, phase transition pressure, relative volume collapse and harmonic elastic moduli their agree well with the available measured data and better than those computed by earlier workers. Present model is capable of explaining the Cauchy's discrepancy correctly. (author)