Red-emitting phosphor series: Ca₉Y(PO₄)_7(1-x)(VO₄)_7x:Eu³⁺ (x = 0 − 1) with improved luminescence thermal stability by anionic polyhedron substitution

Superior thermal stability of the phosphors is crucial to retain the light balance of phosphor-converted WLEDs. At present, the fundamental research on the development of the phosphors with high thermal stability remains a challenging task. In this work, a series of red-emitting phosphors Ca₉Y(PO₄)_7(1−x)(VO₄)_7x:Eu³⁺ (x  =  0 − 1), in which phosphate groups were gradually replaced by vanadate groups, were synthesized via a high temperature solid-state reaction method. Their XRD patterns, Raman spectra, photoluminescent excitation and emission spectra, as well as temperature dependent emission properties and decay curves were investigated. The results of XRD pattern and Raman spectra indicated that a solid solution formed in Ca₉Y(PO₄)_7(1−x)(VO₄)_7x:Eu³⁺ from x = 0 to x = 1. The photoluminescent properties study revealed that emission intensity upon the excitation 393 nm decreased while that under the irradiation of 464 nm increased with the improvement of vanadate groups. Moreover, the substitution of vanadium ions can increase the red to orange intensity ratio values of Eu³⁺. Especially, a limited amount of vanadium in Ca₉Y(PO₄)₇:Eu³⁺ will improve the luminescence thermal stability. This was confirmed by temperature dependent lifetime values for the phosphors with and without vanadium. The possible reason affecting the above properties have been considered in details.