Structural phase transition in lanthanum monochalcogenides induced by hydrostatic pressure

An effective interionic interaction potential (EIOP) is developed to investigate the pressure induced phase transitions from NaCl-type (B1) to CsCl-type (B2) structure in lanthanum monochalcogenides LaX [X = S, Se, Te] compounds. The long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction up to second-neighbor ions within the Hafemeister and Flygare approach with modified ionic charge are properly incorporated in EIOP. The vdW coefficients are computed following the Slater-Kirkwood variational method, as both the ions are polarizable. The estimated value of the phase transition pressure (P-t) and the magnitude of the discontinuity in volume at the transition pressure are consistent with the reported data. A large volume discontinuity in the pressure-volume phase diagram identifies the structural phase transition from B1 to B2 structure. We also study the second order elastic constants for these La compounds. It appears that the vdW interaction is effective in determining the elastic and structural properties of these test compounds. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.