Preparation and polymorph-sensitive luminescence properties of BiPO4:Eu, Part I: room-temperature reaction followed by a heat treatment

In this work, BiPO4:Eu of three different polymorphs was prepared using a room-temperature reaction followed by a heat treatment. The formation mechanism, morphologies, structures, and luminescence properties of these three polymorphs were investigated in detail. It is found that a hexagonal phase (HP) was formed at room temperature, which transformed to a low-temperature monoclinic phase (LTMP) and then a high-temperature monoclinic phase (HTMP) when the treatment temperature was increased to 500 and 700 degrees C, respectively. With the phase transformation, the morphology changed from homogeneous rod-like shape to nearly spherical-like shape, while the lattice strain features varied from the compressive to tensile, and the symmetry of tetragonal PO43- groups decreased from pseudo-T-d to C-1. Such a change in the symmetry of PO43- groups also showed an impact on the local environment of the lattice sites for Eu3+ and moreover the luminescence properties. Judd-Ofelt parameters (Omega(2)) were estimated to understand the asymmetric nature of the dopant Eu3+ in these polymorphs. It is indicated that the Omega(2) value for HP was 8.24 x 10(-20) cm(2), which is slightly increased to 8.42 x 10(-20) cm(2) for LTMP, and the quantum efficiency increased from 12.76% for HP to 70.77% for LTMP. These results demonstrate that rare-earth doped BiPO4 materials can be tailored to show optimum luminescence properties through kinetic control over the polymorphs.