New type of symmetry inhomogeneity in β-TCP-type phosphors: symmetrical features of mixed oxysalts

This study shows the modification of photoluminescent properties (PL) of red-emitting β-Ca₃(PO₄)₂-type (β-TCP) phosphor through anionic sublattice substitution. A new series of Eu³⁺-activated Ca₉La(PO₄)_7(1–x)(VO₄)_7x were synthesized using the solid-state method. X-ray powder diffraction study shows that all sample have β-TCP structure, however the differences in symmetry between the samples was found by second harmonic generation method. The unusual centrosymmetric structure appeared at 0.2 ≤ x ≤ 0.5 in Ca₉La(PO₄)_7(1–x)(VO₄)_7x:Eu³⁺. Study of the series' photoluminescence properties revealed significant improvement in Eu³⁺-emission intensity for centrosymmetric samples. The PO₄ ↔ VO₄ substitution at 0.2 ≤ x ≤ 0.5 resulted in a 1.6-fold enhancement of Eu³⁺ photoluminescent properties. The emission intensity changes correspond to variations in the anionic part of the structure while maintaining constant activator concentration. The optical band-gap of the synthesized phosphors was estimated at ∼3 eV. Characteristic lifetimes decreased from 1.796 to 0.538 ms with increasing VO₄ content. Quantum yields reached 34 % with just 1 mol.% of Eu³⁺. The crystal structure variation proposed here serves as an effective tool for improving photoluminescence properties of rare-earth ions.