Study of elastic properties and their pressure dependence of lanthanum monochalcogenides

An effective interionic interaction potential (EIOP) approach is employed for the description of phase transitions and equations of state of lanthanum monochalcogenides LaX {X = S, Se, Te} compound semiconductors. 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 an EIOP. We have obtained a reasonably good agreement between the present theoretical and available experimental values on the phase transition pressures (Pt = 25.5, 12.4 and 16.8 GPa) and elastic properties in all materials under consideration. The associated volume collapses at the phase transitions have been found to be 8.1%, 10.4% and 7.2% for lanthanum monochalcogenides. The vast volume discontinuity in pressure volume phase diagram identifies the structural phase transition from NaCl type (B1) to CsCl type (B2) structure. The variations of elastic constants and their combinations with pressure follow a systematic trend identical to that observed in other compounds of NaCl type structure family. The present approach has also succeeded in predicting the Born and relative stability criteria.