Preparation and properties of Eu2+/Eu3+ co-activated Ca9Lu(PO4)7 phosphors for multichannel photoluminescence

Abstract

A series of single-phase white light-emitting phosphors Ca₉Lu(PO₄)₇:xEu^2+/3+ (x = 0.01–0.2) were successfully synthesized by a general high-temperature solid-state reaction in a CO atmosphere. Their crystallization and optical properties were investigated using powder X-ray diffraction (XRD), photoluminescence excitation (PLE), emission spectroscopy (PL) and fluorescence decay. The photoluminescence properties indicate that the excitation band of the obtained phosphors is located in the range of 240 to 460 nm, which matches well with that of the n-UV chip. For the emission spectra, a broad emission band around 484 nm and several sharp peaks due to the 4f–5d electronic dipole-allowed transitions of Eu²⁺ and the 4f–4f transition of Eu³⁺ ions can be observed. The emission colors turn from blue-green to white and eventually to red by simply adjusting the valence mixing of Eu²⁺/Eu³⁺ ions. The crystallographic sites of Eu²⁺ and Eu³⁺ ions in the Ca₉Lu(PO₄)₇ host were investigated by site-selective excitation and emission spectroscopy in the ⁵D₀ → ⁷F₀ region and showed that europium shares positions M1, M2, M3 and M5 with calcium. It is suggested that Eu²⁺/Eu³⁺ ions have four different crystallographic sites in the doped Ca₉Lu(PO₄)₇ host, namely Ca(1), Ca(2), Ca(3) and Ca(5). Finally, the thermal quenching of Ca₉Lu(PO₄)₇:xEu^2+/3+ was investigated. As a result, the single-phase Ca₉Lu(PO₄)₇:xEu^2+/3+ phosphor may serve as a potential phosphor for application in white light-emitting diodes.