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 I2/m (No. 12), a = 17.6624(12) Å, b = 3.8012(3) Å, c = 11.8290(9) Å, β = 95.135(7)°, V = 790.99(10) Å(3) for TiTa2O7 and a = 17.6719(13) Å, b = 3.8006(2) Å, c = 11.8924(9) Å, β = 95.295(7)°, V = 795.33(10) Å(3), respectively, for TiNb2O7, Z = 6. Rietveld refinement analyses of the powder X-ray diffraction patterns and Raman spectroscopy were carried out to complement the structural investigations. In addition, in situ 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 (H), and elastic moduli (E) 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 H = 9.0 ± 0.5 GPa and a reduced elastic modulus of Er = 170 ± 7 GPa for TiTa2O7. TiNb2O7 showed a slightly lower hardness of H = 8.7 ± 0.3 GPa and a reduced elastic modulus of Er = 159 ± 4 GPa. Spectroscopic ellipsometry of the polished specimens was employed for the determination of the optical constants n and k. TiNb2O7 as well as TiTa2O7 exhibit a very high average refractive index of nD = 2.37 and nD = 2.29, respectively, at λ = 589 nm, similar to that of diamond (nD = 2.42).

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2O7. -Colorless single crystals of TiTa2O7 and TiNb2O7 are prepared by melting stoichiometric mixtures of Nb2O5 or Ta2O5, and TiO2 at 1973 and 1873 K, respectively. The compounds crystallize isostructurally in the monoclinic space group I2/m with Z = 6 (single crystal XRD). Thermal expansion, as well as mechanical and optical properties of the compounds are characterized by powder XRD and Raman spectroscopy. Because of their high average refractive index of n D = 2.37 and nD = 2.29 for TiNb 2O7 and TiTa2O7, resp., their relative high hardness, and their temperature behavior, the two phases could be of interest for applications as optical coatings, synthetic gemstones, or low-thermal-expansion materials. -(PERFLER*, L.; KAHLENBERG, V.; WIKETE, C.; SCHMIDMAIR, D.; TRIBUS, M.; KAINDL, R.; Inorg. Chem. 54 (2015) 14, 6836-6848, http://dx.doi.org/10.1021/acs.inorgchem.5b00733 ; Inst. Mineral. Petrogr., Leopold-Franzens-Univ., A-6020 Innsbruck, Austria; Eng.) -W. Pewestorf 39-004

A description of the new mineral innsbruckite, Mn33(Si2O5)14(OH)38, a hydrous manganese phyllosilicate found in Tyrol, Austria is given. The crystal structure was determined by singlecrystal synchrotron radiation diffraction experiments at the X06DA beamline at the Swiss Light Source (Paul Scherrer Institute, Villigen, Switzerland). The space group is Cm and lattice parameters are a = 17.2760(19), b = 35.957(5), c = 7.2560(8) Å , β = 91.359(7)º, V = 4506.1(10) Å3, Z = 2. Innsbruckite belongs to the group of modulated 1:1 layer silicates and is chemically and structurally quite closely related to bementite, Mn7(Si2O5)3(OH)8. The chemical analysis revealed a close to ideal composition with only minor amounts of Al, Fe and Mg. Using Liebau’s nomenclature for silicate classification the silicate anion can be described as an unbranched siebener single layer. Innsbruckite shows a complex topology of the silicate sheet, exhibiting 4-, 5-, 6- and 8-membered rings. The silicate sheet is fully characterized using vertex symbols, and its topology is compared to those in other complex sheet silicates. Furthermore, the structural investigation is complemented with Raman spectroscopic studies.

Spinel-type Li₂ZnTi₃O₈ and Zn₂TiO₄ are useful for various industrial applications due to their interesting chemical and physical properties, for example, as promising anode materials in Li-ion batteries or as components in dielectric devices [1]. Since Li₂ZnTi₃O₈ and Zn₂TiO₄ are expected to have high refractive indices (n calc. = 2,33 and 2,26) we tried to characterize these materials in more detail including single-crystal X-ray diffraction, nanoindentation, spectroscopic ellipsometry and electron microprobe analysis. Single crystals of Li₂ZnTi₃O₈ and Zn₂TiO₄ were grown directly from melt at 1723 K and 1873 K, respectively. Fragments of sintered polycrystalline Li₂ZnTi₃O₈ and Zn₂TiO₄ precursors were placed on an iridium sheet and fired in a muffle furnace from 1273 to 1723/1873 K with a heating ramp of 15 K/min. After a dwell time of 3 min the melt was cooled down to 1473 K with a ramp of 10 K/min and subsequently quenched in water. Structural investigations of the Li₂ZnTi₃O₈ and Zn₂TiO₄ single crystals resulted in the following basic crystallographic data: cubic, P4₃32, a = 8.3697(2) Å, V = 586.31(3) ų, Z = 4 and Fd-3m, a = 8.46230(17) Å, V = 605.99(2) ų, Z = 8, respectively. Nanoindentation experiments were performed with a Berkovich diamond indenter tip to determine the hardness and elastic modulus of Zn₂TiO₄ and Li₂ZnTi₃O₈. For sample preparation the single crystals were embedded in resin and polished to a mirror-like surface finish. More than 150 indents with a distance of 10 µm were made with a maximum load of 20 mN. Analysis of the load-displacement curves for Zn₂TiO₄ revealed a hardness of 10.51 ± 0.39 GPa and a reduced elastic modulus of 180.90 ± 3.92 GPa. Atomic force micrographs displayed indents with a max. depth of 288 ± 5 nm. Li₂ZnTi₃O₈ exhibited a hardness of 6.86 ± 0.45 GPa and a reduced elastic modulus of 148.88 ± 6 GPa. Zn₂TiO₄ showed a dispersion of 0.09 due to the variation of the refractive index from 2.24 (430,8 nm, Fraunhofer G line) and 2.15 (686,7 nm, B line).