Synthetic Access to Cubic Rare Earth Molybdenum Oxides RE6MoO12−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 RE6MoO12 with the large rare earth cations RE = La to Dy crystallize in a cubic defect fluorite structure type (Fm3m, no. 225), the compounds with the smaller cations RE = Tm–Lu could hitherto only be synthesized in the rhombohedral defect fluorite structure type (R3, no. 148). In the following, new low temperature access to the rare earth molybdenum oxides RE6MoO12−δ (RE = Tm–Lu) crystallizing in the highly symmetric cubic bixbyite structure type (Ia3, 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 wh...