Thermodynamics and P-T Phase Diagram of Lanthanum Monosulfide

Abstract

Self-consistent plane-wave basis set and ultrasoft pseudopotential for electron-ion interaction have been used to obtain the ground state properties for lanthanum monosulfide. Phonon dispersions and phonon density of states at ambient as well as at high temperature conditions are also derived using the quasiharmonic density functional perturbation theory. For high-T thermophysical properties, quasiharmonic GIBBS code was used with necessary inputs provided by the ab initio calculations. The present study shows good agreement for cohesive and certain finite temperature thermodynamic properties including entropy, specific heats (C-V and C-P) and equation of state. However, it also raises subtle question regarding the microscopic understanding of the phase stability. Pressure corresponding to B1 (NaCl-structure) to B2 (CsCl-structure) phase transition estimated by free energy calculation and based on the phonon-instability criterion are different. In particular, the B2 phase of LaS is dynamically stable even at ambient condition, and can be understood only in terms of total energy. This leaves an open question for awaiting study. A small discrepancy observed in the elastic properties is attributed to the treatment of core-valence electrons in constructing the pseudopotentials. Further, a general scheme of combining the Lindemann's melting law with quasiharmonic Debye model is used to construct the P-T phase diagram. In lieu of other data to us, we believe that the present results may serve as the guideline to future study.