Photoluminescence behavior of YPO₄:Tb³⁺ crystallized in monoclinic, hexagonal or tetragonal phase obtained by hydrothermal process

The luminescent intensity of rare earth-doped lanthanide orthophosphates (LnPO₄:Re³⁺) vary, depending on morphology and crystal structure. In this context, terbium-doped yttrium orthophosphate (YPO₄:Tb³⁺) nanoparticles with different morphologies and crystalline structures were successfully obtained by hydrothermal process. The morphology and crystal structure were modified by varying the pH, temperature and dopant concentration. The YPO₄:Tb³⁺ nanocrystals were characterized by XRD, FT-IR, SEM, TEM and photoluminescence. X-ray patterns show that monoclinic, hexagonal and tetragonal phases of YPO₄ depend on the pH and temperature of synthesis; besides synthesis conditions, the hexagonal structure is influenced by the terbium concentration. The monoclinic phase is transformed into the tetragonal phase at synthesis temperatures higher than 100° C. Both structures, monoclinic and hexagonal, change to the tetragonal phase when subjected to heat treatment. According to SEM images, the morphology depends on the crystal structure. Finally, the photoluminescent properties of YPO₄:Tb³⁺ with different crystal structures were investigated and compared.