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

For the first time, Yb6MoO12 was synthesized in the cubic bixbyite structure type (Ia (3) over bar) under low-temperature conditions. The solution combustion method was used to prepare an amorphous precursor at a temperature of 400 degrees C. Subsequent calcination at around 600 degrees C initiated the crystallization process, accompanied by the decomposition of remnant organic material, leading to a carbonate precursor. Further increase of the temperature to 800 degrees C resulted in a decarboxylation of the precursor, converting it into a highly crystalline cubic phase. A second phase transition into a rhombohedral structure was observed at a temperature around 1000 degrees C. Detailed phase transition studies were performed by powder X-ray diffraction (PXRD), thermal analysis (TG-DTA), attenuated total reflection infrared spectroscopy (ATR-IR), and X-ray photoelectron spectroscopy (XPS) analysis. Furthermore, the analogous yttrium compound Y6MoO12 was prepared by the same method. Contrary to previous reports, the structure analysis revealed that Y6MoO12 crystallizes in the same highly symmetric cubic structure (Ia (3) over bar). Due to the thermal stability differences of the corresponding rare earth oxides and molybdenum oxide, both cubic low-temperature phases Y6MoO12 and Y6MoO12 are presumably not attainable by classical solid-state reactions.