Systematic first-principles study of pressure-induced phase transitions and lattice properties in lanthanide monosulfides

Abstract

We present a systematic ab-initio investigation of the structural, elastic, and vibrational properties of lanthanide monosulfides (LnS, Ln = La-Lu) under pressure using density-functional theory within the plane-wave pseudopotential framework implemented in Quantum ESPRESSO. Totalenergy calculations combined with Birch-Murnaghan equations of state reveal that all compounds are thermodynamically stable in the NaCl-type (B1) structure at ambient conditions and undergo a pressure-induced phase transition to the CsCl-type (B2) structure upon compression. The calculated transition pressures increase monotonically along the lanthanide series, from 22 GPa in LaS to 66 GPa in LuS, reflecting the contraction of the lanthanides. A notable exception is YbS, which exhibits an anomalously low bulk modulus and transition pressure because of the stability of the divalent Yb electronic configuration at ambient pressure. Elastic constant calculations confirm the mechanical stability of both B1 and B2 phases and reveal systematic trends in ductility and stiffness throughout the series. Phonon dispersion relations show that both phases are dynamically stable, with the B2 structure exhibiting softer lattice vibrations and enhanced compressibility relative to the B1 phase. The combined analysis of structural, elastic, and vibrational properties provides a comprehensive description of the high-pressure behavior of lanthanide monosulfides and offers predictive guidance for future experimental studies.