Structural and electronic properties of APO₄ (A= Sc, Lu, Y) orthophosphates under high pressure: a first-principles study

We investigated the structural and electronic properties of APO4 (A= Sc, Lu, Y) orthophosphates under hydrostatic pressure up to 100 GPa using first-principles calculations based on density functional theory. Our results confirm the experimentally observed sequence of pressure-induced phase transitions: zircon-to-scheelite for ScPO₄ (with a predicted scheelite-to-monazite transition at 94 GPa), zircon-to-monazite-to-scheelite for YPO₄, and zircon-to-scheelite for LuPO₄. The calculated critical pressures, pressure induced volume collapses and bulk moduli for each crystal phase align well with experimental data. We found that in the zircon phase two distinct sets of A-O bonds within the AO8 dodecahedron exhibit significantly different compressibilities. We explain this phenomenon, and relate it to an unusual increase of the AO8 volume at the critical pressures that trigger the zircon-to-scheelite phase transition. In the scheelite phase, both sets of A-O bonds show approximately equal compressibilities. Despite undergoing pressure-induced phase transitions, all compounds exhibit remarkable electronic structure stability, with consistent band characteristics dominated by O 2p- (valence band) and A d-states (conduction band). The bandgap increases with pressure within each phase but decreases abruptly at phase boundaries due to expansion of the more compressible A–O bond set. Presented results confirm a strong mechanical resistance to compression and electronic resilience of rare-earth orthophosphates, supporting their potential applications under extreme conditions.