Oxide Ion-Conducting Materials Containing Tetrahedral Moieties: Structures and Conduction Mechanisms

This Review presents an overview from the perspective of tetrahedral chemistry on various oxide ion-conducting materials containing tetrahedral moieties which have received continuous growing attention as candidates for key components of various devices, including solid oxide fuel cells and oxygen sensors, due to the deformation and rotation flexibility of tetrahedral units facilitating oxide ion transport. Emphasis is placed on the structural and mechanistic features of various systems ranging from crystalline to amorphous materials, which include a variety of gallates, silicates, germanates, molybdates, tungstates, vanadates, aluminates, niobate, titanates, indium oxides, and the newly reported borates. They contain tetrahedral units in either isolated or linked manners forming different polyhedral dimensionality (0 to 3) with various defect properties and transport mechanisms. The development of oxide ion conductors containing tetrahedral moieties and the elucidation of the roles of tetrahedral units in oxide ion migration have demonstrated diverse opportunities for discovering superior electrolytes for solid oxide fuel cells and other related devices and provided useful clues for uncovering the key factors directing fast oxide ion conduction.

Bottleneck Effect Explained by Le Bail Refinements: Structure Transformation of Mg-CUK-1 by Confining H2O Molecules

The structure transformation of Mg-CUK-1 due to the confinement of H₂O molecules was investigated. Powder X-ray diffraction (PXRD) patterns were collected at different H₂O loadings and the cell parameters of the H₂O-loaded Mg-CUK-1 material were determined by the Le Bail strategy refinements. A bottleneck effect was observed when one hydrogen-bonded H₂O molecule per unit cell (18% relative humidity (RH)) was confined within Mg-CUK-1, confirming the increase in the CO₂ capture for Mg-CUK-1.

Extension of the A nB nO3 n+2 Slicing/Oxidizing Process from 2D Layered to 1D Columnar Perovskites: The La7W7-xM xO30 Structural Family with Di-, Tri-, and Tetravalent Transition Metal M Substitutes to Tungsten

The A₇B₇O₃₀ structural type is a columnar-perovskite-type arrangement originally evidenced in La₇Mo₇O₃₀ [Goutenoire et al. J. Solid State Chem. 1999, 142, 228], an intermediate reduction product of fast oxide-ion conductor La₂Mo₂O₉. Subsequently, this new structural family has been extended to La₇W⁶⁺₄M⁵⁺₃O₃₀ (M = Nb, Ta) [Goutenoire et al. J. Solid State Chem. 2005, 178, 2811]. Here we show that it is possible to further extend the solid solution through partial substitution of hexavalent tungsten by di-, tri-, or tetravalent transition metals. The general formula is La³⁺₇W⁶⁺_{7- x}M ^m+ _xO₃₀ with x = 3/(6 - m), or m = 6 - 3/ x. Single phase samples with M ^m+ = Zn²⁺, Fe³⁺, and Ti⁴⁺ have been prepared, and their crystal structure refined using X-ray powder diffraction. In the structure, the transition metal with lower valence is preferentially located in the octahedral site shared by consecutive trans-connected perovskite cages, whose diagonal runs along the threefold axis of the structure. Close similarities are noticed with the cationic repartition and distortion of coordination polyhedra in compounds belonging to the A _nB _nO_{3 n+2} series of layered perovskites. A rationale is proposed, which includes the A₇B₇O₃₀ structural type as a subsequent 1D step to the 3D → 2D A _nB _nO_{3 n+2} scheme of perovskite framework slicing by progressive oxygen insertion through decreasing n.

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).

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).

Dineodymium(III) ditungstate(VI), Nd(2)W(2)O(9)

Single crystals of monoclinic Nd₂W₂O₉ were obtained by growth from tungsten borate flux in an atmosphere of air. The crystal structure consists of chains of distorted [WO₆] octa­hedra that run along the c axis of the structure, and of [NdO₉] polyhedra that are connected via common faces and common edges to form a three-dimensional framework.