Pressure-Induced Valence Transitions in Rare Earth Chalcogenides and Pnictides

The electronic structure of rare earth chalcogenides and pnictides is calculated with the ab-initio self-interaction corrected local-spin-density approximation (SIC-LSD). This approach allows both an atomic-like description of the rare earth f-electrons and an itinerant description of other electronic 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. pressure-induced valence transitions. SIC-LSD calculations for cerium monopnictides and monochalcogenides. Yb monochalcogenides, and EuS are presented. The observed equilibrium lattice constants are well reproduced assuming a trivalent 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 Cc 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.