B1-B2 structural phase transition and elastic properties of UX (X = S, Se, and Te) compounds at high pressure

Pressure induced structural phase transformation and mechanical properties of NaCl- type ( B1) to CsCl- type ( B2) structure in uranium monochalcogenides ( UX; X = S, Se, and Te) are presented. An effective interionic interaction potential is constructed, consisting of the long- range Coulomb and the Hafemeister and Flygare type short- range overlap repulsion extended up to the second- neighbour ions and the van der Waals ( vdW) interaction. Particular attention is devoted to evaluate the vdW coefficients following the Slater - Kirkwood variational method, as both the ions are polarizable. Our calculated results have revealed reasonably good agreement with the available experimental data on the phase transition pressures ( P-t = 81, 21, 13 GPa) and the elastic properties of UX ( X = S, Se, and Te). The equation of state curves ( plotted between V ( P)/ V ( 0) and pressure) for both the NaCl- type ( B1) and CsCl- type ( B2) structures obtained by us are in fairly good agreement with the experimental results. The calculated values of the volume collapses (Delta V ( P)/ V ( 0)) are also closer to their observed data. The variations of the second- and third- order elastic constants with pressure have followed systematic trends, which are almost identical to those exhibited by the measured and observed data in other compounds of the NaCl- type structure family.