Multi-Color Luminescent m-LaPO4:Ce/Tb Monospheres of High Efficiency via Topotactic Phase Transition and Elucidation of Energy Interaction.

Monoclinic (m-) structured (La0.96- xCe0.04Tb x)PO4 phosphor monospheres ( x = 0-0.12) of excellent dispersion and morphology uniformity were calcined (≥600 °C) from their precipitated precursor spheres (∼2.0 μm) of a hexagonal (h-) structure for efficient and multicolor luminescence. The h → m phase transition, driven by dehydration, was originally proposed to proceed in a topotactic manner, which involves displacement of the RE-O polyhedra (RE: rare-earth) along the a/ b axis and slight expansion of the {010} and {100} interplanar spacings of the hydrated h-phase to form the {120} and {100} planes of the anhydrous m-phase, respectively. Analysis of the energy process involving the optically active Ce3+ and Tb3+ ions found efficient Ce3+ → Tb3+ energy transfer occurring via electric dipole-quadrupole interaction, whose efficiency reached the highest value of ∼44.48% at x = 0.10. The Tb3+ codoped phosphors simultaneously displayed the characteristic emissions of Ce3+ (∼313 nm) and Tb3+ (∼545 nm) upon exciting the Ce3+ ions with 275 nm UV light, with which the emission color was finely tuned from dark blue to green by increasing the Tb3+ content. Fluorescence decay analysis found decreasing and almost constant lifetime values for the Ce3+ and Tb3+ emissions at a higher Tb3+ content, respectively, and the phosphor presented the highest external quantum efficiency of ∼84.67% at x = 0.10. The excellent luminescent performance and morphology uniformity may allow the monospheres to find application in lighting and display technologies.