Structural, Spectroscopic, and Computational Studies on Tl₄Si₅O₁₂: A Microporous Thallium Silicate

Single crystals of the previously unknown thallium silicate Tl₄Si₅O₁₂ have been prepared from hydrothermal crystallization of a glassy starting material at 500 °C and 1kbar. Structure analysis resulted in the following basic crystallographic data: monoclinic symmetry, space group <i>C</i>2/<i>c</i>, <i>a</i> = 9.2059(5) Å, <i>b</i> = 11.5796(6) Å, <i>c</i> = 13.0963(7) Å, β = 94.534(5)°. From a structural point of view the compound can be classified as an interrupted framework silicate with Q³- and Q⁴-units in the ratio 2:1. Within the framework 4-, 6-, and 12-membered rings can be distinguished. The framework density of 14.4 T-atoms/1000 ų is comparable with the values observed in zeolitic materials like Linde type A, for example. The thallium cations show a pronounced one-sided coordination each occupying the apex of a distorted trigonal TlO₃ pyramid. Obviously, this reflects the presence of a stereochemically active 6s² lone pair electron. The porous structure contains channels running along [110] and [−1 1 0], respectively, where the Tl⁺ cations are located for charge compensation. Structural investigations have been completed by Raman spectroscopy. The interpretation of the spectroscopic data and the allocation of the bands to certain vibrational species have been aided by DFT calculations, which were also employed to study the electronic structure of the compound

Surface Reactivity of YSZ, Y₂O₃, and ZrO₂ toward CO, CO₂, and CH₄: A Comparative Discussion

The C1-surface chemistry of catalytically and technologically relevant oxides (YSZ, ZrO₂, and Y₂O₃) toward CH₄, CO, and CO₂ was comparatively studied by electrochemical impedance (EIS) and spectroscopic (FT-IR) methods. Highly correlated <i>in situ</i> measurements yield a consistent picture with respect to qualitative and quantitative surface modifications as a function of temperature and gas phase composition. This includes not only a detailed study of carbon deposition in methane and adsorption of CO and CO₂ but also proof of the strong influence of surface chemistry. On all studied oxides, carbon deposited during methane treatment grows dynamically forming interconnected islands and eventually a continuous conducting carbon layer at <i>T</i> ≥ 1073 K. Before methane dissociation via gas phase radical reactions/H-abstraction and carbon growth, a complex redox interplay of total oxidation as well as formate and carbonate formation leads to associated surface and grain conductivity changes. For CO adsorption, these measurements yield data on the time and temperature dependence of the adsorbate- and carburization-induced conductivity processes. In that respect, an equivalent circuit model in dry CO allows to disentangle the different contributions of grain interiors, grain boundaries, and electrode contributions. For YSZ, temperature regions with different charge carrier activation energies could be identified, perfectly corresponding to significant changes in surface chemistry. Hydroxyl groups, carbonates, or formates strongly influence the impedance properties, suggesting that the conductivity properties of YSZ, e.g., in a realistic reforming gas mixture, cannot be reduced to exclusive bulk ion conduction. Because of the different degree of hydroxylation and the different ability to chemisorb CO and CO₂, the influence of the surface chemistry on the electrochemical properties is varying strongly: in contrast to ZrO₂, the impact of the studied C1-gases on YSZ and Y₂O₃ is substantial. This also includes the reoxidation/reactivation behavior of the surfaces

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)

Mechanical Properties, Quantum Mechanical Calculations, and Crystallographic/Spectroscopic Characterization of GaNbO₄, Ga(Ta,Nb)O₄, and GaTaO₄

Single crystals as well as polycrystalline samples of GaNbO₄, Ga­(Ta,Nb)­O₄, and GaTaO₄ were grown from the melt and by solid-state reactions, respectively, at various temperatures between 1698 and 1983 K. The chemical composition of the crystals was confirmed by wavelength-dispersive electron microprobe analysis, and the crystal structures were determined by single-crystal X-ray diffraction. In addition, a high-P–T synthesis of GaNbO₄ was performed at a pressure of 2 GPa and a temperature of 1273 K. Raman spectroscopy of all compounds as well as Rietveld refinement analysis of the powder X-ray diffraction pattern of GaNbO₄ were carried out to complement the structural investigations. Density functional theory (DFT) calculations enabled the assignment of the Raman bands to specific vibrational modes within the structure of GaNbO₄. To determine the hardness (<i>H</i>) and elastic moduli (<i>E</i>) of the compounds, nanoindentation experiments have been performed with a Berkovich diamond indenter tip. Analyses of the load–displacement curves resulted in a high hardness of <i>H</i> = 11.9 ± 0.6 GPa and a reduced elastic modulus of <i>E</i>_r = 202 ± 9 GPa for GaTaO₄. GaNbO₄ showed a lower hardness of <i>H</i> = 9.6 ± 0.5 GPa and a reduced elastic modulus of <i>E</i>_r = 168 ± 5 GPa. Spectroscopic ellipsometry of the polished GaTa_0.5Nb_0.5O₄ ceramic sample was employed for the determination of the optical constants <i>n</i> and <i>k</i>. GaTa_0.5Nb_0.5O₄ exhibits a high average refractive index of <i>n</i>_D = 2.20, at λ = 589 nm. Furthermore, <i>in situ</i> high-temperature powder X-ray diffraction experiments enabled the study of the thermal expansion tensors of GaTaO₄ and GaNbO₄, as well as the ability to relate them with structural features

Superstructure of Mullite-type KAl₉O₁₄

Large whiskers of a new KAl₉O₁₄ polymorph with mullite-type structure were synthesized. The chemical composition of the crystals was confirmed by energy-dispersive X-ray spectroscopy, and the structure was determined using single-crystal X-ray diffraction. Nanosized twin domains and one-dimensional diffuse scattering were observed utilizing transmission electron microscopy. The compound crystallizes in space group <i>P</i>2₁/<i>n</i> (<i>a</i> = 8.1880(8), <i>b</i> = 7.6760(7), <i>c</i> = 8.7944(9) Å, β = 110.570(8)°, <i>V</i> = 517.50(9) ų, <i>Z</i> = 2). Crystals of KAl₉O₁₄ exhibit a mullite-type structure with linear edge-sharing AlO₆ octahedral chains connected with groups of two AlO₄ tetrahedra and one AlO₅ trigonal bipyramid. Additionally, disproportionation of KAl₉O₁₄ into K β-alumina and corundum was observed using in situ high-temperature optical microscopy and Raman spectroscopy

Methane Decomposition and Carbon Growth on Y₂O₃, Yttria-Stabilized Zirconia, and ZrO₂

Carbon deposition following thermal methane decomposition under dry and steam reforming conditions has been studied on yttria-stabilized zirconia (YSZ), Y₂O₃, and ZrO₂ by a range of different chemical, structural, and spectroscopic characterization techniques, including aberration-corrected electron microscopy, Raman spectroscopy, electric impedance spectroscopy, and volumetric adsorption techniques. Concordantly, all experimental techniques reveal the formation of a conducting layer of disordered nanocrystalline graphite covering the individual grains of the respective pure oxides after treatment in dry methane at temperatures <i>T</i> ≥ 1000 K. In addition, treatment under moist methane conditions causes additional formation of carbon-nanotube-like architectures by partial detachment of the graphite layers. All experiments show that during carbon growth, no substantial reduction of any of the oxides takes place. Our results, therefore, indicate that these pure oxides can act as efficient nonmetallic substrates for methane-induced growth of different carbon species with potentially important implications regarding their use in solid oxide fuel cells. Moreover, by comparing the three oxides, we could elucidate differences in the methane reactivities of the respective SOFC-relevant purely oxidic surfaces under typical SOFC operation conditions without the presence of metallic constituents