Structural Redetermination and Photoluminescence Properties of the Niobium Oxyphosphate (NbO)₂P₄O₁₃

The structure of (NbO)₂P₄O₁₃ was solved and refined based on new single-crystal diffraction data revealing considerably more complexity than previously described. (NbO)₂P₄O₁₃ crystallizes in the triclinic space group <i>P</i>1̅ with <i>Z</i> = 6. The lattice parameters determined at room temperature are <i>a</i> = 1066.42(4) pm, <i>b</i> = 1083.09(4) pm, <i>c</i> = 1560.46(5) pm, α = 98.55(1)°, β = 95.57(1)°, γ = 102.92(1)°, and <i>V</i> = 1.7213(2) nm³. The superstructure contains 64 unique atoms including two disordered semioccupied oxygen positions. An unusual 180° bond angle between two [P₄O₁₃]^6– groups was refined to form half-occupied, split positions in agreement with previous reports. The IR and Raman spectra reflect the appearance of overlapping bands assignable to specific group vibrations as well as P–O–P linkages present in the [P₄O₁₃]^6– entities. Investigation of the powdered product concerning its photoluminescence properties revealed an excitability in the UV at 270 nm assigned to O2p–Nb4d charge transfer transitions. A resulting broad-band emission with the maximum in the visible region at 455 nm was determined

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