High-pressure molybdenum dioxide (HP-MoO2) was synthesized using a multianvil press at 18 GPa and 1073 K, as motivated by previous first-principles calculations. The crystal structure was determined by single-crystal X-ray diffraction. The new polymorph crystallizes isotypically to HP-WO2 in the orthorhombic crystal system in space group Pnma and was found to be diamagnetic. Theoretical investigations using structure optimization at density-functional theory (DFT) level indicate a transition pressure of 5 GPa at 0 K and identify the new compound as slightly metastable at ambient pressure with respect to the thermodynamically stable monoclinic MoO2 (α-MoO2; ΔEm = 2.2 kJ·mol–1).

Blue-colored molybdenum oxide nitrides of the Mo2(O,N,□)5 type were synthesized by direct nitridation of commercially available molybdenum trioxide with a mixture of gaseous ammonia and oxygen. Chemical composition, crystal structure, and stability of the obtained and hitherto unknown compounds are studied extensively. The average oxidation state of +5 for molybdenum is proven by Mo K near-edge X-ray absorption spectroscopy; the magnetic behavior is in agreement with compounds exhibiting MoVO6 units. The new materials are stable up to ∼773 K in an inert gas atmosphere. At higher temperatures, decomposition is observed. X-ray and neutron powder diffraction, electron diffraction, and high-resolution transmission electron microscopy reveal the structure to be related to VNb9O24.9-type phases, however, with severe disorder hampering full structure determination. Still, the results demonstrate the possibility of a future synthesis of the potential binary oxide Mo2O5. On the basis of these findings, a tentative suggestion on the crystal structure of the potential compound Mo2O5, backed by electronic-structure and phonon calculations from first principles, is given.

•Synthesis of a new oxide nitride LaBa2Ta5O13N2.•Refinement of the crystal structure.•Quantum chemical calculations provided band gap close to the measured value.•New phase shows a higher photocatalytic H2 evolution rate compared to prior tested Ba3Ta5O14N.Yellow LaBa2Ta5O13N2 was successfully synthesized as phase-pure material crystallizing isostructurally to previously reported Ba3Ta5O14N and mixed-valence Ba3TaV4TaIVO15. The electronic structure of LaBa2Ta5O13N2 was studied theoretically with the range-separated hybrid method HSE06. The most stable structure was obtained when lanthanum was placed on 2a and nitrogen on 4h sites confirming Pauling's second rule. By incorporating nitrogen, the measured band gap decreases from ∼3.8 eV for the oxide via 2.74 eV for Ba3Ta5O14N to 2.63 eV for the new oxide nitride, giving rise to an absorption band well in the visible-light region. Calculated fundamental band gaps confirm the experimental trend. The atom-projected density of states has large contributions from N2p orbitals close to the valence band edge. These are responsible for the observed band gap reduction. Photocatalytic hydrogen formation was investigated and compared with that of Ba3Ta5O14N revealing significantly higher activity for LaBa2Ta5O13N2 under UV-light.X-ray powder diffraction pattern of LaBa2Ta5O13N2 with the results of the Rietveld refinements. Inset: Unit cell of LaBa2Ta5O13N2 and polyhedral representation of the crystal structure.

•Partial O/F-substitution in BiVO4 strongly improves the photoelectrochemical performance using EPD-prepared electrodes.•Development of a simple synthesis route to homogeneous F-containing materials.•O/F-substitution leads to the formation of cation vacancies.Fluorine-containing bismuth vanadate (F:BiVO4) powder was synthesized using a new, clean, and simple solid-vapor reaction. Incorporation of fluorine mainly leads to the formation of cation vacancies. Electrodes were fabricated from the pre-synthesized powder samples by electrophoretic deposition onto fluorine-doped tin oxide coated glass slides and subsequent calcination. The photoelectrochemical performance concerning the water oxidation reaction was investigated and compared to pristine BiVO4 revealing strongly enhanced photoelectrochemical behavior for the F-containing BiVO4.Unit cell of F:BiVO4 (left) with Tauc plot for BiVO4 and F:BiVO4 based on diffuse reflectance spectrometry (middle) and the corresponding phototransient measurements of the samples (right).

•Detailed analysis of the mechanochemical synthesis to phase pure kesterite powder.•SnCu/Zn and Cu/ZnSn anti-site defects observed by high temperature X-ray diffraction at 500 °C.•These anti-site defects heal out rapidly during cooling to ambient temperature with a cooling rate of 60 K/h.Kesterite-type Cu2ZnSnS4 was synthesized from the corresponding binary sulfides by a mechanochemical route in a planetary ball mill. The reaction progress during this milling step was followed within a time range of 10–180 min by powder X-ray diffraction. In addition, the crystallization of the milled material was studied in situ by high-temperature X-ray diffraction methods in the temperature range of 300–500 °C. Significant disorder (cation distribution) was observed at 500 °C, strongly decreasing during cooling down to ambient temperature with a rate of 60 K/h.

•Off-stoichiometric CZTS powder samples of B- and C-type were successfully synthesized.•Neutron scattering measurements allowed full cation distribution analyses.•In B-type CZTS ZnCu and ZnSn, in C-type materials CuZn and SnZn point defects were observed.•30–35% CuZn-ZnCu anti-site defect pairs on Wyckoff positons 2c and 2d are present in both off-stoichiometric types.In this work a series of B- and C-type off-stoichiometric CZTS samples was prepared by a mechanochemical synthesis route. For structural characterization X-ray diffraction measurements were performed. Furthermore, compositional analyses of the powder samples were carried out by means of wavelength-dispersive X-ray spectroscopy (WDS). For detailed cation distribution analyses of two off-stoichiometric single-phase samples neutron diffraction methods were used. The method of the average neutron scattering length was applied revealing the occurrence of ZnCu, CuZn, and ZnSn point defects in the Cu-poor/Zn-rich sample (B-type, Cu1.95Zn1.09Sn0.96S4.00), whereas for the Cu-rich/Zn-poor sample (C-type, Cu2.04Zn0.95Sn1.01S4.00) ZnCu, CuZn, and SnZn defects are suggested.

δ-TaON was prepared by reaction of gaseous ammonia with an amorphous tantalum oxide precursor. As a representative of the anatase structure (aristotype) it crystallizes in the tetragonal crystal system with lattice parameters a = 391.954(16) pm and c = 1011.32(5) pm. At temperatures between 800 and 850 °C an irreversible phase transformation to baddeleyite-type β-TaON is observed. While quantum-chemical calculations confirm the metastable character of δ-TaON, its transformation to β-TaON is kinetically controlled. The anion distribution of the anatase-type phase was studied theoretically. In agreement with previous studies, it was found that a configuration with maximal N–N distances is most stable. The calculated band edge energies indicate that δ-TaON is a promising photocatalytic material for redox reactions, e.g., water splitting.

•We successfully prepared CN-mayenite (framework structure with cyanide anions in the cages).•As shown by neutron diffraction studies, CN− is disordered.•Ca12Al14O32(CN)2 shows a surprisingly high ionic conductivity of σ = 1.4∙10−3 S/cm at 1173 K (Ea = 4.3 eV).•Diffusion pathways and activation energies were determined additionally by quantum-chemical calculations.•CN-mayenite can be considered as first example of a new kind of solid anion conductor with a mobile molecular anion.CN-mayenite (Ca12Al14O32(CN)2) is a promising candidate for high-temperature CN− conductivity in a solid due to its special structural features. It was prepared by a solid–gas reaction. The transport processes were investigated by means of in situ high-temperature neutron diffraction, impedance spectroscopy and additional Hebb–Wagner measurements. Diffusion pathways and activation energies were determined additionally by quantum-chemical calculations.Ca12Al14O32(CN)2 shows a surprisingly high ionic conductivity of σ = 1.4∙10−3 S/cm at 1173 K (Ea = 4.3 eV) and can be considered as first example of a new kind of solid anion conductor with a mobile molecular anion.

A sol–gel route for ternary lithium fluorides of transition metals (M) is presented allowing the synthesis of Li3MF6-type and Li2MF5-type compounds. It is based on a fluorolytic process using transition metal acetylacetonates as precursors. The domain size of the obtained powders can be controlled by modifying the conditions of synthesis. 6Li and 7Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is used to study local environments of the Li ions in orthorhombic and monoclinic Li3VF6 as well as Li2MnF5. The number of magnetically inequivalent Li sites found by MAS NMR is in agreement with the respective crystal structure of the compounds studied. Quantum chemical calculations show that all materials have high de-lithiation energies making them suitable candidates to be used as high-voltage battery cathode materials.

Pyrochlore-type H3OSbTeO6 is a remarkable proton conductor exhibiting an outstanding electrical conductivity even at ambient temperature. It consists of a three-dimensional interconnected (Sb,Te)O6 framework, built from randomly distributed corner-shared SbO6 and TeO6 octahedra, forming large cages in which the H3O+ ions are located. The dynamics of the caged species has been investigated by temperature-dependent neutron diffraction, quasielastic neutron scattering, and NMR experiments. Three types of motion may be discerned, namely, stochastic rotations of the H3O group around its 3-fold axis, jumps between four equivalent positions within the cage, and long-range inter-cage translational diffusion. The onset of ionic conductivity is clearly reflected by structural changes. Details of the complex diffusion mechanism are given.Highlights► H3OSbTeO6 is an outstanding proton conductor. ► We investigated the dynamics of the caged hydronium ions as a function of temperature. ► Onset of proton conductivity is reflected by structural changes. ► A complex diffusion mechanism is observed.

A sol–gel route for ternary lithium fluorides of transition metals (M) is presented allowing the synthesis of Li3MF6-type and Li2MF5-type compounds. It is based on a fluorolytic process using transition metal acetylacetonates as precursors. The domain size of the obtained powders can be controlled by modifying the conditions of synthesis. 6Li and 7Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is used to study local environments of the Li ions in orthorhombic and monoclinic Li3VF6 as well as Li2MnF5. The number of magnetically inequivalent Li sites found by MAS NMR is in agreement with the respective crystal structure of the compounds studied. Quantum chemical calculations show that all materials have high de-lithiation energies making them suitable candidates to be used as high-voltage battery cathode materials.