Extrinsic Oxygen Vacancies Formation during Crystallization of Al2W3O12 by Calcination in Air

A2M3O12-type materials are widely studied due to their outstanding chemical flexibility, tunable low positive, negative, to near-zero coefficients of thermal expansion, and vast potential applicability from cooker hobs to housing for optical devices. Among the mechanisms for tuning their coefficient of thermal expansion (CTE) are the substitution of the A3+ cation, the insertion of bulky molecules inside the nanochannels of their crystal structure, and, possibly, the formation of extrinsic oxygen vacancies. Although the formation of oxygen vacancies in already crystalline structures is widely studied for various oxides, their evolution during crystallization from the amorphous precursor is scarcely reported. In this study, the amorphous powder was synthesized by co-precipitation, and different concentrations of extrinsic oxygen vacancies were formed through crystallization of the amorphous material into orthorhombic Al2W3O12 in air atmosphere, within the temperature interval between 500 and 620 °C. Amorphous powder and crystalline Al2W3O12 were characterized by differential scanning calorimetry and thermogravimetry, X-ray powder diffraction, transmission electron microscopy, diffuse reflectance spectroscopy, electron paramagnetic resonance, Raman, and X-ray photoelectron spectroscopies to investigate the influence of the calcination temperature on the formation of extrinsic oxygen vacancies, and to understand the oxygen vacancies formation during crystallization. It was found that the unit-cell volume increased up to 0.36% with the introduction of oxygen vacancies, in comparison to the almost extrinsic vacancy-free phase, owing to the weakening of W–O bonds in WO4 tetrahedra. The formation of orthorhombic Al2W3O12 occurs via a two-stage process. The oxygen-deficient amorphous structure is first formed upon calcination at 500 °C of the as-synthesized amorphous powder by water and hydroxyl group release. The increase in calcination temperature in air causes structural organization, while at temperatures as high as 620 °C, an almost extrinsic-point-defect-free counterpart phase is formed. Therefore, it is possible to crystallize orthorhombic Al2W3O12 with largely different concentrations of oxygen vacancies by varying calcination temperatures in air.