On the Phosphors Na5M(WO4)4 (M = Y, La-Nd, Sm-Lu, Bi) - Crystal Structures, Thermal Decomposition, Optical and Magnetic Properties
The pentasodium rare-earth tungstates Na5M(WO4)4} are closely related to the sodium rare earth double tungstates NaM(WO4)2 adopting both the scheelite type (space group I41/a, no. 88). After 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 or as a 100% phosphor due to the possible charge transfer of the tungstate group and the absence of any concentration quenching. Na5M(WO4)4} is incongruently melting to NaM(WO4)2 and Na 2WO4. After clarification of the crystallographic relationship of Na5M(WO4)4} and NaM(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. Further, the investigation of the fluorescent properties of Na5M(WO4)4} (M = Pr, Sm, Eu, Tb, Tm, Bi), Na5Y1-xEux(WO4)4 and Na5Y1-yTby(WO4)4 yielded insights into the weak ligand field and the energy tranfer from WO42– to M3+ governed by the emission of the sensitiser within Na5M(WO4)4}. Additionally, the compounds were characterised by magnetic measurements, vibrational, UV/Vis and 151Eu Mößbauer spectroscopy.