Structure determination of α-La6W2O15

Selective Crystallization of Four Tungstates (La2W3O12, La2W2O9, La14W8O45, and La6W2O15) via Hydrothermal Reaction and Comparative Study of Eu3+ Luminescence

Hydrothermal reaction at 200 °C was systematically undertaken in wide ranges of solution pH (4-13) and W/La molar ratio ( R = 0.5-2), without using any organic additive, to investigate the effect of hydrothermal parameter on product property and the underlying mechanism. Combined analysis by X-ray diffraction (XRD), inductively coupled plasma (ICP) spectroscopy, elemental mapping, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that either a decreasing pH or increasing R value yielded a product richer in W and, conversely, richer in La. The results were interpreted from the solution chemistry of La³⁺ and tungstate ions. As an outcome of our 40 well-designed experiments, four La tungstates-La₂W₃O₁₂, La₂W₂O₉, La₁₄W₈O₄₅, and La₆W₂O₁₅-were successfully obtained in a phase-pure form by calcining their hydrothermal precursors. Phase and morphology evolution, structure features, and properties of Eu³⁺ emission were, for the first time, comparatively investigated for the four compounds. Spectral analysis found that the 5 at. % Eu³⁺-doped La₂W₃O₁₂ phosphor exhibits the highest quantum efficiency (∼47%), more red component, and the shortest fluorescence lifetime of luminescence (∼0.72 ms).

Crystal structure of Re-substituted lanthanum tungstate La5.4W1−y Re y O12–δ (0 ≤ y ≤ 0.2) studied by neutron diffraction

A precise determination of sample composition and water uptake of La6−x WO12−δ (0.4 ≤ x ≤ 0.8) and Re-substituted La5.4W1−y Re y O12−δ (0 ≤ y ≤ 0.2) lanthanum tungstate is carried out. Sample compositions and water uptake were determined by electron probe micro-analysis and thermogravimetry, respectively. A single-phase region of Re-substituted lanthanum tungstates is reported. The crystal structure of two selected specimens produced by the citrate-complexation route based on the Pechini method, namely La5.4WO12−δ and La5.4W0.8Re0.2O12−δ, was investigated by neutron diffraction in the temperature range 1.5 ≤ T ≤ 1200 K. The structural model for lanthanum tungstates is validated, according to which the Wyckoff site shared by La and W (Fm{\overline 3}m space group, 24d site) is split with half-site occupancies (Fm{\overline 3}m space group, 48h site). Replacement of W by up to 20 mol% Re does not change the lattice structure, and Re atoms substitute for W statistically in both 4a and 48h Wyckoff sites of the Fm{\overline 3}m space group, as shown by combining the average neutron scattering length procedure, thermogravimetry and electron probe micro-analysis. Using the Willis and Pryor approach to anisotropic displacement parameters it is shown that the remaining static disorder in the unit cell found in La5.4WO12−δ and La5.4W0.8Re0.2O12−δ structures is comparable, when the Fm{\overline 3}m space group with split 48h site is employed. Through the estimation of the Debye temperature for both compounds, extracted from the analysis of thermal expansion coefficients and from the Willis and Pryor approach, anion anharmonic vibrations like those in yttria-stabilized zirconia are proven to exist in LaWO for the first time.

On halide derivatives of rare-earth metal(III) oxidomolybdates(VI) and -tungstates(VI)

Halide derivatives of rare-earth metal(III) oxidomolybdates(VI) have been investigated comprehensively over the last decade comprising the halogens fluorine, chlorine, and bromine. Iodide-containing compounds are so far unknown. The simple composition REXMoO4 (RE=rare-earth element, X=halogen) is realized for X=F almost throughout the complete lanthanide series as well as for yttrium. While ytterbium and lutetium do not form any fluoride derivative, for lanthanum, only a fluoride-deprived compound with the formula La3FMo4O16 is realized. Moreover, molybdenum-rich compounds with the formula REXMo2O7 are also known for yttrium and the smaller lanthanoids. For X=Cl the composition REClMoO4 is known for yttrium and the whole lanthanide series, although, four different structure types were identified. Almost the same holds for X=Br, however, only two different structure types are realized in this class of compounds. In the case of halide derivatives of rare-earth metal(III) oxidotungstates(VI) the composition REXWO4 is found for chlorides and bromides only, so far. Due to the similar size of Mo6+ and W6+ cations, the structures found for the tungstates are basically the same as for the molybdates. With the larger lanthanides, the representatives for both chloride and bromide derivates exhibit similar structural motifs as seen in the molybdates, however, the crystal structure cannot be determined reliably. In case of the smaller lanthanoids, the chloride derivatives are isostructural with the respective molybdates, although the existence ranges differ slightly. The same is true for rare-earth metal(III) bromide oxidotungstates(VI).