β-alumina-14H and β-alumina-21R : two chromic Na2-δ(Al,Mg,Cr)17O25 polysomes observed in slags from the production of low-carbon ferrochromium

The crystal structures of unknown phases found in slags from the production of low-carbon ferrochromium were studied by powder and single-crystal X-ray diffraction. Two phases of Na2−δ (Al, Mg, Cr)17O25 composition were found to be composed of an alternating stacking of a spinel-type and a Na-hosting block. Similar structures are known for β-alumina and β”- alumina, NaAl11O17. However, the spinel-type block in Na2-δ(Al, Mg, Cr)17O25 is composed of five cation layers in contrast to three cation layers in the β-alumina spinel-block. The two new phases, β-alumina-14H, P63/mmc, a=5.6467(2), c=31.9111(12) Å, and β-alumina-21R, R m, a=5.6515(3), c=48.068(3) Å have a 14-layer and 21-layer stacking with a 2 × (cccccch) and a 3 × (ccccccc) repeat sequence of oxygen layers in cubic and hexagonal close packing, respectively.http://www.elsevier.com/locate/jssc2017-09-30hb2016Electrical, Electronic and Computer Engineerin

First investigations on the quaternary system Na2O-K2O-CaO-SiO2: synthesis and crystal structure of the mixed alkali calcium silicate K1.08Na0.92Ca6Si4O15

In the course of an exploratory study on the quaternary system Na2O-K2O-CaO-SiO2 single crystals of the first anhydrous sodium potassium calcium silicate have been obtained from slow cooling of a melt in the range between 1250 and 1050 C. Electron probe micro analysis suggested the following idealized molar ratios of the oxides for the novel compound: K2O:Na2O:CaO:SiO2=1:1:12:8 (or KNaCa6Si4O15). Single-crystal diffraction measurements on a crystal with chemical composition K1.08Na0.92Ca6Si4O15 resulted in the following basic crystallographic data: monoclinic symmetry, space group P 2 1/c, a=8.9618(9) Å, b=7.3594(6) Å, c=11.2453(11) Å, beta = 107.54(1), V=707.2(1) Å3, Z=2. Structure solution was performed using direct methods. The final least-squares refinement converged at a residual of R(|F|)=0.0346 for 1288 independent reflections and 125 parameters. From a structural point of view, K1.08Na0.92Ca6Si4O15 belongs to the group of mixed-anion silicates containing [Si2O7]- and [SiO4]-units in the ratio 1:2. The mono- and divalent cations occupy a total of four crystallographically independent positions located in voids between the tetrahedra. Three of these sites are exclusively occupied by calcium. The fourth site is occupied by 54(1)% K and 46%(1) Na, respectively. Alternatively, the structure can be described as a heteropolyhedral framework based on corner-sharing silicate tetrahedra and [CaO6]-octahedra. The network can build up from kröhnkite-like [Ca(SiO4)2O2]-chains running along [001]. A detailed comparison with other A2B6Si4O15-compounds including topological and group-theoretical aspects is presented.(VLID)460432

Water-Gas Shift and Methane Reactivity on Reducible Perovskite-Type Oxides

Comparative (electro)catalytic, structural, and spectroscopic studies in hydrogen electro-oxidation, the (inverse) water-gas shift reaction, and methane conversion on two representative mixed ionic–electronic conducting perovskite-type materials La0.6Sr0.4FeO3−δ (LSF) and SrTi0.7Fe0.3O3−δ (STF) were performed with the aim of eventually correlating (electro)catalytic activity and associated structural changes and to highlight intrinsic reactivity characteristics as a function of the reduction state. Starting from a strongly prereduced (vacancy-rich) initial state, only (inverse) water-gas shift activity has been observed on both materials beyond ca. 450 °C but no catalytic methane reforming or methane decomposition reactivity up to 600 °C. In contrast, when starting from the fully oxidized state, total methane oxidation to CO2 was observed on both materials. The catalytic performance of both perovskite-type oxides is thus strongly dependent on the degree/depth of reduction, on the associated reactivity of the remaining lattice oxygen, and on the reduction-induced oxygen vacancies. The latter are clearly more reactive toward water on LSF, and this higher reactivity is linked to the superior electrocatalytic performance of LSF in hydrogen oxidation. Combined electron microscopy, X-ray diffraction, and Raman measurements in turn also revealed altered surface and bulk structures and reactivities

Ni–perovskite interaction and its structural and catalytic consequences in methane steam reforming and methanation reactions

Metal–support interaction effects and their consequences in CO2/CO methanation and methane steam reforming have been exemplarily studied on two complex Ni–perovskite powder catalyst systems, namely Ni–La0.6Sr0.4FeO3−δ (lanthanum strontium ferrite, LSF) and Ni–SrTi0.7Fe0.3O3−δ (strontium titanium ferrite, STF). Pre-reduction in hydrogen and treatment in catalytic gas mixtures cause a variety of structural effects, including exsolution of iron particles and formation of Ni–Fe alloy particles. These manifestations strongly depend on the reducibility of the perovskite and are hence much more pronounced on LSF. Reactivity differences are strongly influenced by the chemical properties of the respective perovskite support. The more reducible the perovskite support, the stronger the deviation from the catalytic behavior of a Ni/Al2O3 reference catalyst, rendering establishments of direct structure–activity/selectivity relationships difficult. The studies show the extreme variety of the metal–perovskite interface, which helps in judging similar systems of recent high catalytic importance, e.g. metals supported on spinel or other perovskite phases

Nanoindentation, High-Temperature Behavior, and Crystallographic/Spectroscopic Characterization of the High-Refractive-Index Materials TiTa₂O₇ and TiNb₂O₇

Colorless single crystals, as well as polycrystalline samples of TiTa₂O₇ and TiNb₂O₇, were grown directly from the melt and prepared by solid-state reactions, respectively, at various temperatures between 1598 K and 1983 K. The chemical composition of the crystals was confirmed by wavelength-dispersive X-ray spectroscopy, and the crystal structures were determined using single-crystal X-ray diffraction. Structural investigations of the isostructural compounds resulted in the following basic crystallographic data: monoclinic symmetry, space group <i>I</i>2<i>/m</i> (No. 12), <i>a</i> = 17.6624(12) Å, <i>b</i> = 3.8012(3) Å, <i>c</i> = 11.8290(9) Å, β = 95.135(7)°, <i>V</i> = 790.99(10) ų for TiTa₂O₇ and <i>a</i> = 17.6719(13) Å, <i>b</i> = 3.8006(2) Å, <i>c</i> = 11.8924(9) Å, β = 95.295(7)°, <i>V</i> = 795.33(10) ų, respectively, for TiNb₂O₇, <i>Z</i> = 6. Rietveld refinement analyses of the powder X-ray diffraction patterns and Raman spectroscopy were carried out to complement the structural investigations. In addition, <i>in situ</i> high-temperature powder X-ray diffraction experiments over the temperature range of 323–1323 K enabled the study of the thermal expansion tensors of TiTa₂O₇ and TiNb₂O₇. To determine the hardness (<i>H</i>), and elastic moduli (<i>E</i>) of the chemical compounds, nanoindentation experiments have been performed with a Berkovich diamond indenter tip. Analyses of the load–displacement curves resulted in a hardness of <i>H</i> = 9.0 ± 0.5 GPa and a reduced elastic modulus of <i>E</i>_r = 170 ± 7 GPa for TiTa₂O₇. TiNb₂O₇ showed a slightly lower hardness of <i>H</i> = 8.7 ± 0.3 GPa and a reduced elastic modulus of <i>E</i>_r = 159 ± 4 GPa. Spectroscopic ellipsometry of the polished specimens was employed for the determination of the optical constants <i>n</i> and <i>k</i>. TiNb₂O₇ as well as TiTa₂O₇ exhibit a very high average refractive index of <i>n</i>_D = 2.37 and <i>n</i>_D = 2.29, respectively, at λ = 589 nm, similar to that of diamond (<i>n</i>_D = 2.42)

Metastable Corundum-Type In₂O₃: Phase Stability, Reduction Properties, and Catalytic Characterization

The phase stability, reduction, and catalytic properties of corundum-type rhombohedral In₂O₃ have been comparatively studied with respect to its thermodynamically more stable cubic In₂O₃ counterpart. Phase stability and transformation were observed to be strongly dependent on the gas environment and the reduction potential of the gas phase. As such, reduction in hydrogen caused both the efficient transformation into the cubic polymorph as well as the formation of metallic In especially at high reduction temperatures between 573 and 673 K. In contrast, reduction in CO suppresses the transformation into cubic In₂O₃ but leads to a larger quantity of In metal at comparable reduction temperatures. This difference is also directly reflected in temperature-dependent conductivity measurements. Catalytic characterization of rh-In₂O₃ reveals activity in both routes of the water–gas shift equilibrium, which gives rise to a diminished CO₂-selectivity of ∼60% in methanol steam reforming. This is in strong contrast to its cubic counterpart where CO₂ selectivities of close to 100% due to the suppressed inverse water–gas shift reaction, have been obtained. Most importantly, rh-In₂O₃ in fact is structurally stable during catalytic characterization and no unwanted phase transformations are triggered. Thus, the results directly reveal the application-relevant physicochemical properties of rh-In₂O₃ that might encourage subsequent studies on other less-common In₂O₃ polymorphs

Enhanced Kinetic Stability of Pure and Y‑Doped Tetragonal ZrO₂

The kinetic stability of pure and yttrium-doped tetragonal zirconia (ZrO₂) polymorphs prepared via a pathway involving decomposition of pure zirconium and zirconium + yttrium isopropoxide is reported. Following this preparation routine, high surface area, pure, and structurally stable polymorphic modifications of pure and Y-doped tetragonal zirconia are obtained in a fast and reproducible way. Combined analytical high-resolution in situ transmission electron microscopy, high-temperature X-ray diffraction, and chemical and thermogravimetric analyses reveals that the thermal stability of the pure tetragonal ZrO₂ structure is very much dominated by kinetic effects. Tetragonal ZrO₂ crystallizes at 400 °C from an amorphous ZrO₂ precursor state and persists in the further substantial transformation into the thermodynamically more stable monoclinic modification at higher temperatures at fast heating rates. Lower heating rates favor the formation of an increasing amount of monoclinic phase in the product mixture, especially in the temperature region near 600 °C and during/after recooling. If the heat treatment is restricted to 400 °C even under moist conditions, the tetragonal phase is permanently stable, regardless of the heating or cooling rate and, as such, can be used as pure catalyst support. In contrast, the corresponding Y-doped tetragonal ZrO₂ phase retains its structure independent of the heating or cooling rate or reaction environment. Pure tetragonal ZrO₂ can now be obtained in a structurally stable form, allowing its structural, chemical, or catalytic characterization without in-parallel triggering of unwanted phase transformations, at least if the annealing or reaction temperature is restricted to <i>T</i> ≤ 400 °C