Rare-gas mobility in some anisotropic ceramic oxides: Al2O3, Cr2O3, Fe2O3, TiO2, U3O8

The mobility of the inert gases xenon or radon in five anisotropic oxides (hexagonal corundum Al₂O₃, Cr₂O₃, Fe₂O₃, tetragonal rutile TiO₂, and orthorhombic U₃O₈) was studied. The gases were introduced by ion bombardment. The oxides were in the form of powders, sinters, or single crystals. Normal volume diffusion was found at low gas concentration, the activation energies in kilocalories per mole being 85 for Al₂O₃, 73±5 for Cr₂O₃, 68±5 for Fe₂O₃, 78±5 for TiO₂, and 85±8 for U₃O₈, and the pre-exponential termsD ₀ falling into the ideal range of about 3×10^–1±1 cm²/sec. Structural radiation damage, the annealing of which coincided with gas release at low temperatures, and, in some cases, retardation of the gas release were found at higher gas concentrations. Some evidence is presented that grain boundaries, pre-existing vacancy clusters, and dislocation loops may act as trapping sites for gas atoms (or bubbles) and may be stabilised after trapping of the gas.     

Interfacial reactions between titanium film and single crystal α-Al2O3

Titanium is commonly used to join metals and ceramics by active metal brazing methods. In this work, titanium was sputter deposited on to single-crystal -Al₂O₃ substrates and the interfacial reactions between the titanium film and the Al₂O₃ substrate were studied. Al₂O₃ was reduced by titanium when samples were annealed at 973 and 1173 K for 300 s in an argon gas flow. Metallic aluminium was produced at the interface, and this diffused from the interface into the titanium film. At 1173 K, the intermetallic compound Ti₃Al and the intermediate titanium oxides, such as Ti₂O and TiO, were formed. The Al⁰ diffusion is important in stimulating interfacial reactions.     

Crystal and molecular structure of uranyl acetylacetonate dimer, [UO<Subscript>2</Subscript>(C<Subscript>5</Subscript>H<Subscript>7</Subscript>O<Subscript>2</Subscript>)<Subscript>2</Subscript>]<Subscript>2</Subscript>

The crystal and molecular structure of uranyl acetylacetonate dimer was determined by single crystal X-ray diffraction. The compound crystallizes in the tetragonal system, a = 7.9420(2), c = 40.1240(13) Å (at 100 K), Z = 4, space group P4₁2₁2. Dimeric uranyl acetylacetonate molecules in the crystal are formed by bridging bonding of one of O atoms of the acetylacetonate ligands with U atoms, so that the coordination polyhedra of U atoms (distorted pentagonal bipyramids) share a common equatorial edge. The dimer has a nonplanar structure, being significantly bent along the conventional line connecting the bridging O atoms.     

Nonstoichiometry in quaternary transition metal oxides with the rutile structure: A case study of NiV2Nb2O10

NiV₂Nb₂O₁₀ forms with the rutile structure. The Ni-V-Nb-O phase shows a large homogeneity range with respect to NiO, V₂O₄, V₂O₅, NbO₂ or Nb₂O₅ with retention of the rutile structure. The lattice parameter variations observed as a function of each of the constituent oxide concentrations respectively, within the rutile homogeneity range are discussed in terms of ionic size effects and cation-cation interactions along the tetragonal c axis.     

A structural investigation of stabilized oxygen evolution catalysts

A ternary mixed oxide containing SnO₂, RuO₂ and IrO₂ was shown to exhibit a more stable electrocatalytic behaviour than simpler catalysts consisting of noble metal oxides only. X-ray diffraction and transmission electron microscopy were used to characterize the structure of these oxide catalysts. It was found that precipitation of the fine catalyst powders resulted in the formation of a mixed (Sn, Ru, Ir)O₂ rutile phase. This phase was shown to be thermodynamically unstable, decomposing to SnO₂ and (Ru, Ir)O₂ on annealing at 800° C. X-ray photoelectron spectroscopy revealed that the surface composition of these mixed oxide catalysts varies considerably from the bulk composition. The formation of such metastable, mixed oxides is discussed.     

Structure and Some Properties of a Double Neptunyl(V) Cesium Molybdate,Cs[NpO<Subscript>2</Subscript>MoO<Subscript>4</Subscript>(H<Subscript>2</Subscript>O)]

A double neptunyl(V) cesium molybdate, Cs[NpO₂MoO₄(H₂O)], was studied by single crystal X-ray diffraction. Crystal data: rhombic system, a = 9.478(2), b = 7.900(1), c = 10.499(2) Å; space group Pnna, Z = 4, d = 5.05 g cm^?3, R = 0.030. The compound has a framework structure; the coordination polyhedron of the Np atom is a distorted pentagonal bipyramid with the equatorial positions occupied by four O atoms of four molybdate groups and an O atom of the coordinated water molecule. The IR and visible absorption spectra of this compound and of Cs₂[(NpO₂)₂Mo₂O₈] whose structure had been determined previously were measured. The NpO ₂ ⁺ stretching vibration frequencies in the IR spectra of these compounds virtually coincide. Incorporation of the O atom of the Mo-O-Mo bridge into the first coordination sphere of the neptunyl(V) ion in Cs₂[(NpO₂)₂Mo₂O₈] exerts the same disturbing effect on the electronic absorption spectrum as does the cation-cation interaction with one of the O atoms acting as a bridge between two Np atoms.     

Fine Structure of α-Al2O3-Based Solid Solutions

The fine structure of solid solutions between isomorphic p-block and 3dtransition-metal oxides was investigated experimentally and theoretically in the technologically important systems -Al₂O₃–M₂O₃(M = Ti, V, Cr, Fe). It was shown that the thermodynamic theory of isomorphic miscibility and conventional approaches to estimating solubility limits fail to explain and, at best, only predict the phase relations in the systems examined. The proposed magnetochemical method offers the possibility of studying in detail the M–M interactions in first-row oxides M₂O₃and (M _x Al_{1 – x} )₂O₃solid solutions, probing their homogeneity, and revealing their salient structural features. The marked difference in the solubilities of isomorphic M₂O₃oxides in -Al₂O₃is interpreted in terms of the magnetic structure of the oxides.     

Mn and Co substitution in δ-FeOOH and its decomposition products

Cobalt and manganese substituted -FeOOH were obtained and used in the preparation of Fe, Co and Mn, Fe mixed oxides. Cobalt is present in the oxyhydroxide with 2.8–3.0, oxidation states. Lattice parameters and oxidation state decrease with cobalt content. Mn-substituted phases contain manganese in oxidation states decreasing from 4.0 to 3.3 as the degree of substitution is higher. These Mn-oxidation states imply cation vacancies which are alternatively ordered in octahedral sites along the [001] direction of the -oxyhydroxide structure. The thermal decomposition of the cobalt and Mn-substituted samples at 200 ° C leads to mixed oxides with distorted -Fe₂O₃ structure and high manganese and cobalt oxidation states. From 200 to 400 ° C, a reduction process takes place, leading to Mn-substituted sesquioxides, while Co-containing samples decompose into mixtures of spinel and -Fe₂O₃-structure phases.     

Synthesis and crystal structure of americium(V) dichromate, Cs3AmO2(Cr2O7)2·H2O

A new Am(V) chromate complex with outer-sphere cesium cation, Cs₃AmO₂(Cr₂O₇)₂·H₂O, was prepared from aqueous solution. Its composition and structure were determined by single crystal X-ray diffraction. The Am(V) atom has coordination surrounding in the form of a distorted pentagonal bipyramid with the O atoms of the AmO ₂ ⁺ group in apical positions. The equator of the bipyramid is formed by the O atoms of four dichromate groups and of the water molecule. The mean bond lengths (Å) are as follows: Am=O 1.802(5), Am-O(Cr₂O₇) 2.443(6), and Am-O_w 2.519(6). The Am(V) pentagonal bipyramids are combined via bidentate bridging Cr₂O ₇ ^2? anions into infinite chains [AmO₂(Cr₂O₇)₂H₂O] _n ^3? arranged in layers parallel to the series of diagonal planes (111). A system of hydrogen bonds links the chains in a layer and the layers with each other. The Cs cations are arranged between the layers, forming cationic interlayers.     

Preparation of new niobates with the rutile structure

CrVNbO₆, FeVNbO₆ and NiV₂Nb₂O₁₀ have been prepared. The compounds are isostructural with the rutile phase of FeNbO₄, are nonstoichiometric with respect to all three constituent metal atoms, and show appreciable electrical conductivity.     

Photo-e.m.f. measurements on oxides

Various metal oxides were investigated using laser pulse photo-electromotive force measurements. This method is possible if the band gap of these oxides is smaller than the laser radiation energy. The sign of the photovoltage and its decay rate depend on the specific surface potential, which is influenced by adsorption layers. The adsorption of acids and bases on ZnO, TiO₂, -Fe₂O₃ and -Fe₂O₃ microcrystals shows the dependence of the photovoltage on the type of space charge layer on the surface of the oxides. Oxides with a depletion layer exhibit a rising photovoltage with rising pH, whilst oxides with an accumulation layer at the surface exhibit the opposite behaviour.     

Photo-induced properties of anodic oxide films on Ti6Al4V

Photocatalytic activity and wettability of the anodic oxide layer on Ti6Al4V prepared by anodization in a sulfuric acid electrolyte are explored. The oxide is composed mainly of TiO₂ with V₂O₅, VO₂ and Al₂O₃. The crystal structure of the TiO₂ varies from anatase to rutile with the sulfuric acid concentration in the electrolyte. Anatase exhibits better photocatalytic activity compared with rutile, which is different from those on Ti and Ti-Nb-Sn alloy. Contact angles of the oxides decrease with ultraviolet light illumination, and hydrophilicity is observed in the rutile oxide. Both photocatalytic activity and hydrophilicity are inferior to the corresponding Ti and Ti-Nb-Sn alloy, which is explained by the presence of Al₂O₃ in the anodic oxide.     

Change in the mechanism of conductivity in ZrO<Subscript>2</Subscript>-based crystals depending on the content of stabilizing Y<Subscript>2</Subscript>O<Subscript>3</Subscript> additive

The interrelationship between the structure, phase composition, and transport characteristics of solid electrolytes based on ZrO₂ has been studied as dependent on the content of stabilizing Y₂O₃ additive. It is established that twin boundaries do not lead to the appearance of additional mechanism of ionic conductivity acceleration in ZrO₂–Y₂O₃ crystals. The maximum conductivity has been observed in ZrO₂–(8–10) mol % Y₂O₃ crystals containing a t” phase, in which oxygen atoms are displaced from high-symmetry positions characteristic of the cubic phase.     

Heterogeneous Phase Equilibria in Rare Earth–Mn–O Systems in Air

The phase relations in rare earth–Mn–O systems in air are considered. Most of the phase diagrams of these systems fall into two distinct groups: R"–Mn–O (R" = Y, Ho–Lu) and R"–Mn–O (R" = Pr, Nd, Sm–Dy). In addition, the Sc–Mn–O, La–Mn–O, and Ce–Mn–O systems have phase-diagram features of their own. The Ce–Mn–O system contains no ternary oxides or solid solutions: there are only mixtures of cerium and manganese oxides. The Sc–Mn–O system has phase-diagram features in common with both the R"–Mn–O and M–Mn–O (M = Mg, Al, 3d transition metal) systems. The La–Mn–O phase diagram can be thought of as a degenerate diagram of the R"–Mn–O group, since LaMn₂O₅ exists at oxygen pressures higher than atmospheric pressure. The R"–Mn–O and R"–Mn–O systems contain two chemical compounds, RMnO₃ and RMn₂O₅, but differ in the crystal structure of RMnO₃: hexagonal in the R" group and orthorhombic perovskite-like in the R" group. A key role in determining the structure of RMnO₃ is played by the size factor. In both groups, the RMn₂O₅ compounds dissociate in air by the reaction \({\text{RMn}}_{\text{2}} {\text{O}}_{\text{5}} {\text{ = RMnO}}_{\text{3}} + \frac{1}{3}{\text{RMn}}_{\text{3}} {\text{O}}_{\text{4}} + \frac{1}{3}{\text{O}}_{\text{2}} \). The dissociation temperature of RMn₂O₅ is shown to correlate with the atomic number of R, the total number of 4f electrons, the number of unpaired 4f electrons, and the ionic radius of R³⁺.     

Structural and Transport Properties of La2/3 – x Li3x □4/3 – 2x Ta2O6 Perovskite-Like Solid Solutions

The homogeneity range of La_2/3–x Li_{3x 4/3 – 2x} Ta₂O₆ perovskite-like solid solutions was determined. The transport properties of the La_2/3–x Li_{3x 4/3 – 2x} Ta₂O₆ oxides were shown to correlate with their lattice parameters and composition. Lithium-doped lanthanum metatantalate with an orthorhombically distorted perovskite-like structure is a good solid electrolyte with high lithium ion conductivity.     

Hexamethylenetetramine: A New Fuel for Solution Combustion Synthesis of Complex Metal Oxides

Hexamethylenetetramine, (CH₂)₆N₄ (HMT) has been employed as a fuel for the first time to synthesize several binary, ternary, and quaternary oxides by the solution combustion method. Technologically important oxides, namely, zirconia, ceria, and their solid solutions, -Al₂O₃, Cr³⁺-doped -Al₂O₃, spinels, MAl₂O₄ (M = Ca, Mg, Zn and Ni), perovskites, LaMO₃ (M = Mn, Cr and Al), La_0.67Sr_0.33MnO₃, and La_0.67Ca_0.33MnO₃, have been synthesized. The oxides synthesized are characterized by X-ray powder diffraction, resistivity, fluorescence spectra, particle size, and surface area measurements. Decomposition of nickel nitrate-HMT complex has been investigated by TG, DTA, TPD, and evolved-gas analysis to elucidate the mechanism.     

Dependency of mechanochemical reactions forming complex oxides on the crystal structures of starting oxides

Mechanochemical reactions applicable to syntheses of perovskite-type compounds, LaMO₃, where M is trivalent metals, are taken as examples to investigate effects of crystal structures of starting materials, M ₂O₃, on the reactions. The investigated oxides M ₂O₃ are categorized into two groups: one is the corundum structure and the other the non-corundum structure. Under the experimental conditions, it has been found that the oxides with non-corundum structure react easily with La₂O₃ by their co-grinding at room temperature to form the corresponding compounds. On the contrary, none of such reactions can be occurred for the oxides with corundum structure. The difference has been discussed based on the grinding-induced phase transformation from loose packing of atoms to close packing of atoms. Based on the results, the extension to the synthesis of other compounds such as CrO₂ is presented.     

Identification of ground-state spin ordering in antiferromagnetic transition metal oxides using the Ising model and a genetic algorithm

Abstract

We identify ground-state collinear spin ordering in various antiferromagnetic transition metal oxides by constructing the Ising model from first-principles results and applying a genetic algorithm to find its minimum energy state. The present method can correctly reproduce the ground state of well-known antiferromagnetic oxides such as NiO, Fe₂O₃, Cr₂O₃ and MnO₂. Furthermore, we identify the ground-state spin ordering in more complicated materials such as Mn₃O₄ and CoCr₂O₄.     

Electron Correlations Engineer Catalytic Activity of Pyrochlore Iridates for Acidic Water Oxidation

The development of highly efficient oxygen‐evolving catalysts compatible with powerful proton‐exchange‐membrane‐based electrolyzers in acid environments is of prime importance for sustainable hydrogen production. In this field, understanding the role of electronic structure of catalysts on catalytic activity is essential but still lacking. Herein, a family of pyrochlore oxides R₂Ir₂O₇ (R = rare earth ions) is reported as acidic oxygen‐evolving catalysts with superior‐specific activities. More importantly, it is found that the intrinsic activity of this material significantly increases with the R ionic radius. Electronic structure studies reveal that the increased R ionic radius weakens electron correlations in these iridate oxides. This weakening induces an insulator–metal transition and an enhancement of Ir? O bond covalency, both of which promote oxygen evolution kinetics. This work demonstrates the importance of engineering the electron correlations to rationalize the catalytic activity toward water oxidation in strongly correlated transition‐metal oxides.