On the phosphors Na5M(WO4)4 (M = Y, La–Nd, Sm–Lu, Bi) – crystal structures, thermal decomposition, and optical and magnetic properties

Abstract

The pentasodium rare-earth tungstates Na₅M(WO₄)₄ are closely related to the sodium rare-earth double tungstates Na₅M(WO₄)₂ both adopting the scheelite structure type (space group I4₁/a, no. 88). After the preparation of polycrystalline powders via flux syntheses improving the phase purity significantly, the crystal structures of Na₅M(WO₄)₄ (M = Y, La–Nd, Sm–Lu, Bi) were determined by single crystal XRD and Rietveld analysis. Na₅M(WO₄)₄ is a promising phosphor material both as a host and as a 100% phosphor due to the possible charge transfer of the tungstate group and the absence of any concentration quenching. Na₅M(WO₄)₄ incongruently melts to Na₅M(WO₄)₂ and Na₂WO₄. After the clarification of the crystallographic relationship of Na₅M(WO₄)₄ and Na₅M(WO₄)₂ based on a rare isomorphic transition of index 5 (i5) the non-linear trend of the decomposition temperature within the row of rare earth ions is explained systematically taking into account the existence of domains within the crystal structure predetermining the posterior decomposition. A miscibility gap for solid solutions of Na₅Y(WO₄)₄ and Na₅Eu(WO₄)₄ or Na₅Tb(WO₄)₄ is identified and its temperature dependence is investigated. Furthermore, the investigation of the fluorescent properties of Na₅M(WO₄)₄ (M = Pr, Sm, Eu, Tb, Tm, Bi), Na₅Y_1−xEu_x(WO₄)₄ and Na₅Y_1−yTb_y(WO₄)₄ provided insights into the weak ligand field and the energy transfer from WO₄²⁻ to M³⁺ governed by the emission of the sensitiser within Na₅M(WO₄)₄. Additionally, the compounds were characterised by magnetic measurements and vibrational, UV/Vis and ¹⁵¹Eu Mössbauer spectroscopy.