Monoclinic superstructure in orthorhombic Ce10W22O81 from transmission electron microscopy

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).

Origin of the superstructure elucidated by atomic resolution HAADF-STEM and HREM in the Ce10W22O81lanthanide tungstate phase

The present paper provides new information on the attribution of the cationic sites of the orthorhombic Ce10W22O81crystal phase prepared in the CeO2–Ce2O3–WO3ternary system. Atomic resolution HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy) and HREM (high-resolution electron microscopy) investigations have highlighted the presence of two mixed columns of Ce and W cations along theaaxis that were previously assigned to pure W cations in the asymmetric unit. This discovery explains the presence of a commensurate superstructure doubling the orthorhombic unit-cell lengthao.

Sr1/2Ce5/14□1/7WO4: a new modulated ternary scheelite compound

For the first time, a ternary tetragonal scheelite structure tungstate with strontium and cerium cations, (Sr,Ce)WO₄, was synthesized. As much as 35% Ce could be inserted into the structure, leaving 1\over 7 of the (Sr,Ce) cation sites vacant. Partial ordering of Sr and Ce, with atomic displacements, were shown by high-resolution electron microscopy. Two-dimensional incommensurate modulations occur in this material, in small domains 20 nm in size. The band gap of this compound is significantly lower than the band gap of SrWO₄ and this was related to the distortions of WO₄ and (Sr,Ce)O₈ polyhedra. The band gap value of 3.2 eV makes Sr_1/2Ce_5/14□_1/7WO₄ a promising candidate for violet luminescence.