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

Monoclinic (m-) structured (La 0.96– x Ce 0.04 Tb x )PO 4 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 Ce 3+ and Tb 3+ ions found efficient Ce 3+ → Tb 3+ energy transfer occurring via electric dipole–quadrupole interaction, whose efficiency reached the highest value of ∼44.48% at x = 0.10. The Tb 3+ codoped phosphors simultaneously displayed the characteristic emissions of Ce 3+ (∼313 nm) and Tb 3+ (∼545 nm) upon exciting the Ce 3+ ions with 275 nm UV light, with which the emission color was finely tuned from dark blue to green by increasing the Tb 3+ content. Fluorescence decay analysis found decreasing and almost constant lifetime values for the Ce 3+ and Tb 3+ emissions at a higher Tb 3+ 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.