On the phosphors Na 5 M(WO 4 ) 4 (M = Y, La–Nd, Sm–Lu, Bi) – crystal structures, thermal decomposition, and optical and magnetic properties

The pentasodium rare-earth tungstates Na 5 M(WO 4 ) 4 are closely related to the sodium rare-earth double tungstates Na 5 M(WO 4 ) 2 both adopting the scheelite structure type (space group I 4 1 / a , no. 88). After the preparation of polycrystalline powders via flux syntheses improving the phase purity significantly, the crystal structures of Na 5 M(WO 4 ) 4 (M = Y, La–Nd, Sm–Lu, Bi) were determined by single crystal XRD and Rietveld analysis. Na 5 M(WO 4 ) 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. Na 5 M(WO 4 ) 4 incongruently melts to Na 5 M(WO 4 ) 2 and Na 2 WO 4 . After the clarification of the crystallographic relationship of Na 5 M(WO 4 ) 4 and Na 5 M(WO 4 ) 2 based on a rare isomorphic transition of index 5 ( i 5) 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 Na 5 Y(WO 4 ) 4 and Na 5 Eu(WO 4 ) 4 or Na 5 Tb(WO 4 ) 4 is identified and its temperature dependence is investigated. Furthermore, the investigation of the fluorescent properties of Na 5 M(WO 4 ) 4 (M = Pr, Sm, Eu, Tb, Tm, Bi), Na 5 Y 1−x Eu x (WO 4 ) 4 and Na 5 Y 1−y Tb y (WO 4 ) 4 provided insights into the weak ligand field and the energy transfer from WO 4 2− to M 3+ governed by the emission of the sensitiser within Na 5 M(WO 4 ) 4 . Additionally, the compounds were characterised by magnetic measurements and vibrational, UV/Vis and 151 Eu Mössbauer spectroscopy.