Structural and optical properties of langbeinite-related red-emitting K₂Sc₂(MoO₄)(PO₄)₂:Eu phosphors

The concentration series of langbeinite-related solid solutions K₂Sc₂(MoO₄)(PO₄)₂:xEu (x = 0.1, 0.2, 0.6, 0.8, and 1.0 mol%) has been prepared via a solid state route and the effects of europium content on the phase composition, morphology, crystal structure and luminescence properties have been studied by scanning electron microscopy, X-ray powder diffraction, UV-vis, IR and luminescence spectroscopy. The band gap values have been estimated from UV-vis spectra and are in the range of 3.7-3.8 eV for all concentrations studied. The electronic band structure calculations have shown that Sc d, Mo d and O^phos p states dominate in the band edge region and determine the optical transitions in the K₂Sc₂(MoO₄)(PO₄)₂ host. The photoluminescence (PL) spectra, intensity and decay time dependences on the Eu³⁺ concentration revealed complex behavior of europium-containing emitting centers. The PL characteristics indicated the presence of at least two types of luminescence centers. One of them (Eu_K) is shown to be formed by the Eu³⁺ ion located within K sites, while the other one is formed by the Eu³⁺ ions that reside in Sc sites (Eu_Sc). The luminescence color coordinates calculated for K₂Sc₂(MoO₄)(PO₄)₂:xEu indicated that these ceramics can be potential candidates for UV-based lighting applications as efficient red phosphors.