Photoluminescence properties of Eu³⁺-doped glaserite-type orthovanadates CsK₂Gd[VO₄]₂

Undoped and Eu³⁺-doped glaserite-type orthovanadates CsK ₂Gd_1-xEu_x[VO₄]₂ with various Eu³⁺ concentrations of x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 were synthesized via the solid-state reaction. The formation of a single phase compound was verified through the X-ray diffraction studies. The photoluminescence (PL) and PL excitation (PLE) spectra, PL decay curves, and absolute quantum efficiency (QE) were investigated. Unlike the conventional Eu³⁺-doped vanadates, these Eu³⁺-doped samples showed not only several sharp emission lines due to Eu³⁺ but also a broad emission band with a maximum at 530 nm due to the [VO ₄]^3- host. The intensities of the host and Eu³⁺ emissions increased when the Eu³⁺ concentration was increased from x = 0 to x = 0.6 and decreased above x = 0.6. Similar concentration dependence was observed for QE. The host emission, even if in the Eu³⁺-condensed host of CsK₂Eu(VO₄), was never quenched indicating inefficient energy transfer from the host [VO₄]^3- to Eu³⁺. This inefficient energy transfer is understood by suppression of the energy transfer by the V-O-Eu bond angle deviated from 180° and the separation of Eu³⁺ ions at the Gd³⁺ site from [VO ₄]^3-. Like the 530 nm charge transfer [VO ₄]^3- emission, two broad and intense PLE bands with maxima at 330 and 312 nm were observed for the Eu³⁺ emission. A maximum QE of 38.5% was obtained from CsK₂Gd_1-xEu_x[VO ₄]₂ (x = 0.6). A white-colored emission was obtained by the combination of the broad 530 nm emission band and the intense sharp lines due to Eu³⁺ at 590-620 nm.