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 RE 6 MoO 12 with the large rare earth cations RE = La to Dy crystallize in a cubic defect fluorite structure type (Fm3̅ m, no. 225), the compounds with the smaller cations RE = Tm− Lu could hitherto only be synthesized in the rhombohedral defect fluorite structure type (R3̅ , no. 148). In the following, new low temperature access to the rare earth molybdenum oxides RE 6 MoO 12−δ (RE = Tm−Lu) crystallizing in the highly symmetric cubic bixbyite structure type (Ia3̅ , 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 Yb 6 MoO 12−δ using TG-DTA, XPS, EDX, and X-ray powder diffraction (XRPD) measurements. In contrast to the rhombohedral phase Yb 6 MoO 12 , 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 Yb 6 MoO 12−δ displays improved coloristic properties (UV−Vis spectroscopy) with a yellow hue value (CIE-Lab) being enhanced from b* = 26.0 of the rhombohedral to b* = 46.1 for the cubic phase, which is relevant for the field of inorganic pigments.

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The strontium borogermanate Sr(3-x/2)B(2-x)Ge(4+x)O14 (x = 0.32) was synthesized by high-temperature solid-state reaction of SrO, GeO2, and H3BO3 in a NaF/KF flux system using platinum crucibles. The structure determination revealed that Sr(3-x/2)B(2-x)Ge(4+x)O14 (x = 0.32) crystallizes in the trigonal space group P321 (No. 150) with the parameters a = 800.7(2) and c = 488.8(2) pm, with R1 = 0.0281, wR2 = 0.0671 (all data), and Z = 1. The crystal structure of Sr(3-x/2)B(2-x)Ge(4+x)O14 (x = 0.32) consists of distorted SrO8 cubes, GeO6 octahedra, GeO4 tetrahedra, and BO4 tetrahedra. In addition to the structural investigations, Raman and IR spectroscopic investigations were carried out.

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The new lead strontium germanate PbSrGeO4 was synthesized by a high-temperature solid-state reaction of lead(II) oxide, strontium carbonate, and germanium(IV) oxide at a temperature of 900 °C in a platinum crucible. The compound crystallizes in the orthorhombic space group P212121 (no. 19) being isotypic to the structure of BaNdGaO4 [1]. The primitive cell contains four formula units with lattice parameters of a = 995:40(2), b = 732:44(2), c = 599:42(2) pm, V = 437:02(2)Å3, and residuals of R1 = 0:0385 and wR2 = 0:0997 for all data. The main structural element of PbSrGeO4 is an isolated [GeO4]4- tetrahedron. A circular arrangement of six of these [GeO4]4- tetrahedra leads to the formation of large channels along b in which the strontium cations are eightfold coordinated by oxygen atoms. The lead cations are coordinated by three oxygen atoms, and with a lone pair of electrons they show a pseudo-tetrahedral coordination. Next to the structural investigations, PbSrGeO4 was characterized by Raman-spectroscopic investigations and DFT calculations.