Effect of the d electrons on phase transitions in transition-metal sesquioxides

We present a systematic density-functional study of phase relations in three 4d-transition-metal sesquioxides: Y₂O₃, Rh₂O₃, and In₂O₃. Y₂O₃ and In₂O₃ undergo pressure-induced transitions to phases with larger cation coordination number (from 6 to 7) at low pressures. However, this does not occur in Rh₂O₃ at least up tõ 300 GPa. This cannot be explained by usual arguments based on ionic-radii ratios often used successfully to explain phase relations in simple-metal and rare-earth sesquioxides and sesquisulfides. Inspection of their electronic structures shows that, in Rh₂O₃, the electronic occupancy of 4d orbitals, 4d⁶, plays a fundamental role in the extraordinary stability of the Rh₂O₃(II)-type phase with respect to coordination increase. We point out that d-orbital occupancy is a fundamental factor in explaining phase relations in transition-metal sesquioxides and sesquisulfides.