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

Synthetic Access to Cubic Rare Earth Molybdenum Oxides RE₆MoO_12−δ (RE = Tm–Lu) Representing a New Class of Ion Conductors

Materials crystallizing in highly symmetric structures are of particular interest as they display superior physical properties in many relevant technological areas such as solid oxide fuels cells (SOFCs), catalysis, or photoluminescent materials. While the rare earth molybdenum oxides RE₆MoO₁₂ with the large rare earth cations RE = La to Dy crystallize in a cubic defect fluorite structure type (<i>Fm</i>3̅<i>m</i>, no. 225), the compounds with the smaller cations RE = Tm–Lu could hitherto only be synthesized in the rhombohedral defect fluorite structure type (<i>R</i>3̅, no. 148). In the following, new low temperature access to the rare earth molybdenum oxides RE₆MoO_12−δ (RE = Tm–Lu) crystallizing in the highly symmetric cubic bixbyite structure type (<i>Ia</i>3̅, no. 206) will be discussed. The three-step method comprises preparation of the rhombohedral phases by solution combustion (SC) reactions, their reduction including simultaneous structural transitions from the rhombohedral to the cubic phases, and subsequent reoxidations while preserving their cubic structures. Detailed studies on this process were performed on the compound Yb₆MoO_12−δ using TG-DTA, XPS, EDX, and X-ray powder diffraction (XRPD) measurements. In contrast to the rhombohedral phase Yb₆MoO₁₂, which does not show any ionic conductivity, the cubic bixbyite structured compound can be classified as a promising ionic conductor. Electrochemical impedance spectroscopy (EIS) revealed that bulk and grain boundary activation energy determined to be 144.6 kJ mol^–1 and 150.4 kJ mol^–1, respectively, range in the same regime as the conventional ionic conductor 8-YSZ. Furthermore, the new cubic phase Yb₆MoO_12−δ displays improved coloristic properties (UV–Vis spectroscopy) with a yellow hue value (CIE-Lab) being enhanced from <i>b</i>* = 26.0 of the rhombohedral to <i>b</i>* = 46.1 for the cubic phase, which is relevant for the field of inorganic pigments