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

Abstract

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.

Polymorphism in the family of Ln6−xMoO12−δ(Ln = La, Gd–Lu; x = 0, 0.5) oxygen ion- and proton-conducting materials

“…6 Recently, a low-temperature uorite-like bixbyite polymorph (Ia 3) has been obtained using the solution combustion reaction between 700 and 1000 C in Ln 6 MoO 12 (Ln ¼ Tm, Yb, Lu). 15 In a study of the formation of Ln 6 WO 12 (Ln ¼ Nd, Eu, Er) tungstates between 700 and 1480 C using a sol-gel complexation synthesis method, Escolastico et al 16 obtained low-temperature uorites at temperatures from 700 to 1000 C.…”

Cubic Bixbyite‐Structured Phases of Yb6MoO12 and Y6MoO12 Prepared by the Solution Combustion Method at Low Temperatures

“…Contrary to the previously reported compound Yb 6 MoO 12– δ with b.c.c. structure, only a single oxidation state for molybdenum was detected here (Figure ). The measured binding energy of 232.5 eV can be assigned to the electron configuration 3d 5/2 of Mo 6+ .…”

“…Similar lattice parameters were also found for the recently published reduced b.c.c. phase Yb 6 MoO 12– δ with a = 1045.758(4) pm . However, the title compound Yb 6 MoO 12 contains, contrary to Yb 6 MoO 12– δ , no significant amounts of oxygen vacancies.…”

“…More recently, the synthesis of Yb 6 MoO 12– δ revealed a highly symmetric body‐centered‐cubic (b.c.c.) bixbyite phase Ia 3 formed by a reduction and re‐oxidation approach . Structural studies showed that, due to the presence of both Mo 6+ and Mo 5+ cations in the re‐oxidized cubic compound Yb 6 MoO 12– δ only 6/7 of the Wyckoff positions 24 d and 8 a were occupied, along with anionic vacancies at the Wyckoff position 48 e .…”

“…phase reported for RE 6 WO 12 ( Fm 3 m ), our findings reveal an eightfold larger b.c.c. phase ( Ia 3) for Yb 6 MoO 12 in analogy to the partly reduced compound Yb 6 MoO 12– δ . Detailed structural studies were carried out to provide insight into the phase transformations.…”