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

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), Na5Y1-xEux(WO4)4 and Na5Y1-yTby(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.

Physical and Magnetocaloric Properties of TbPdAl2 and the Ferromagnetic Solid Solution Tb1-xLuxPdAl2 (x = 0.1-0.9).

TbPdAl2 and the solid solution Tb1-xLuxPdAl2 (x = 0.1-1) have been synthesized via arc-melting techniques using the elements as starting materials and crystallize in the orthorhombic MgCuAl2-type structure (Cmcm). As expected, the unit cell volumes decrease due to the lanthanide contraction from the Tb to Lu compounds, thus showing Vegard behavior because of the differences of the ionic radii of the trivalent rare-earth cations. TbPdAl2 orders ferromagnetically below TC = 85.5(5) K and shows partial magnetic saturation already at low fields. The magnetic phase transition has been additionally investigated by heat capacity measurements, showing a broadened λ anomaly at THC = 83.2(1) K. The electrical resistivity is almost linear above TC, indicating dominant electron-phonon interactions. Below the ordering temperature, electron-spin wave scattering with a ρ ∼ T2 behavior is evident. In the solid solution Tb1-xLuxPdAl2 (x = 0-1), the ferromagnetic Curie temperatures decrease in a linear fashion with increasing Lu content. Investigations of the magnetocaloric properties of TbPdAl2 obtained a maximum magnetic entropy change of -ΔSMmax = 1.2, 2.2, 3.1, and 3.6 J kg-1 K-1 for the field changes of ΔH = 10, 20, 50, and 70 kOe, respectively. The rather low values are caused by entropy losses due to hysteresis behavior. The relative cooling power for TbPdAl2, therefore, also exhibits comparably low values of 32, 81, 142, and 178 J kg-1.