Structures and Polymorph-Sensitive Luminescence Properties of BiPO₄/Eu Grown in Hydrothermal Conditions

In this work, BiPO₄/Eu of different polymorphs was prepared using a simple hydrothermal method. It is found that a hexagonal phase (HP) was formed at 100 °C, which transformed to a low-temperature monoclinic phase (LTMP) when hydrothermal temperature was increased to 160 °C. This LTMP transformed reversely to HP when the temperature increased beyond 180 °C. This phase evolution is quite distinct from those when using high-temperature calcinations (CrystEngComm, 2011, 13, 6251–6257). Accompanying this phase evolution, sample morphology varied from homogeneous rodlike shape to prismlike shape, while the lattice strain features changed from tensile to compressive, and then to tensile, as followed by a decrease in symmetry of tetragonal PO₄ groups from pseudo-<i>T</i>_<i>d</i> to <i>C</i>₁. These variations were closely related to the reaction mechanism in solution chemistry, in which two interfacial processes were indicated, differing from the ionic diffusion process during calcination. As a consequence, luminescence properties become sensitive to polymorphs. All Judd–Ofelt parameters (Ω_λ, λ = 2, 4, 6) were calculated to evaluate the asymmetric nature for the polymorph-sensitive luminescence. It is found that the Ω₂ value for HP was 1.66 × 10^–20 cm², which increased to 2.10 × 10^–20 cm² for LTMP; meanwhile, the quantum efficiency increased from 9.33% for HP to 36.09% for LTMP. These results demonstrated that BiPO₄ of different polymorphs can be obtained through the hydrothermal method, which may enrich the significance of solution chemistry in preparing advanced materials of tailored functionalities