Pressure-induced B1-B2 structural phase transition and elastic properties of UxLa1-xS solid solution

A comprehensive phenomenological calculation is reported for the pressure-dependent structural phase transition, equation of state and elastic properties of UxLa1-xS solid solution. We develop an effective interionic interaction potential incorporating the long-range Coulomb and the Hafemeister and Flygare form of short-range overlap repulsion extended up to the second-neighbour ions and van der Waals (vdW) multipole interactions. The overlapping of the f orbitals with the p orbitals of the nearest neighbour in UxLa1-xS that influences the effective Coulomb interactions is noticed to be a potential parameter in revealing the structural phase transition (from NaCl type (B1) to CsCl type (B2)) as well the equations of state of UxLa1-xS (x = 0, 0.08, 0.40, 0.50, 0.60, 0.80 and 1.0) rare-earth compounds. The calculated results have revealed reasonably good agreement with the available data on the phase transition pressures (P-t = 25.5 (LaS), 30 (U0.08La0.92S), 34 (U0.40La0.60S), 35 (U0.50La0.50S), 47 (U0.60La0.40S), 59 (U0.80La0.20S) and 81 (US) GPa). 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. Deduced values of the volume collapses [Delta V(P)/V(0)] are also closer to their observed data. Further, the variations of the second- and third-order elastic constants with pressure have followed a systematic trend, that is almost identical to those exhibited by the measured and observed data in other compounds of NaCl-type structure family.