Pressure-induced valence transitions in rare earth chalcogenides and pnictides

The electronic structure of rare earth chalcogenides and pnictides is calcu lated with the ab-initio self-interaction corrected local-spin-density appr oximation (SIC-LSD). This approach allows both an atomic-like description o f the rare earth f-electrons and an itinerant description of other electron ic degrees of freedom. Specifically, different formal valencies of the rare earth atom, corresponding to different f-shell occupancies, can be studied and their energies compared, leading to a first-principles theory for. pre ssure-induced valence transitions. SIC-LSD calculations for cerium monopnic tides and monochalcogenides. Yb monochalcogenides, and EuS are presented. T he observed equilibrium lattice constants are well reproduced assuming a tr ivalent Cc configuration and divalent Eu and Yb configurations. The trends in the high pressure behavior of these systems are discussed. With applied pressure, isostructural phase transitions are found to occur in CeP and CeS , caused by the delocalization of the Ce f-electron, while in the heavier C c compounds, the structural B1 --> B2 transition happens before f-electron delocalization occurs. Similarly, both Eu and Yb chalcogenides transfer to trivalent configurations with pressure, in accordance with observation.