Roles of (NH4)2HPO4 and NH4H2PO4 in the phase transition and luminescence enhancement of YPO4:Eu

Investigating crystal phase transition helps in understanding new crystal structures and obtaining novel performance. The conventional approaches to realizing phase transition usually require extreme conditions such as high temperature, high pressure, or heavy ion doping. In this work, we achieve phase transition from hexagonal to tetragonal YPO4:Eu crystals just by increasing the concentration ratio of (NH4)2HPO4 to NH4H2PO4 in the precursor. When increasing the ratio, the underlying transition mechanism modifies the free energy of the crystalline system and in turn selects the tetragonal phase with higher formation energy. The difference in preferred growth direction endows tetragonal and hexagonal microcrystals with sphere- and prism-like morphology, respectively. The luminescence intensity is stronger in the tetragonal than the hexagonal phase, due to stronger non-radiative energy transfer induced by high-frequency vibrations in the hexagonal lattice. The efficient method provided here allows convenient production of tetragonal samples with high luminescence intensity on a large scale, and could be well applied to other phosphate systems for phase selection.