Assessment of hydrothermal parameters in the wet synthesis of α- and β-BiTaO4 by in situ synchrotron X-ray diffraction

Abstract

Synchrotron X-ray diffraction was used for in situ monitoring of hydrothermal precursors calcination to obtain BiTaO4 in the orthorhombic (α) and triclinic (β) crystal phases. Different precursors were prepared varying time (24 and 48 h) and temperature (150 and 200 °C) of hydrothermal treatment. According to the results of in situ X-ray analysis during calcination of the precursors, the α- and β-BiTaO4 phases were achieved from the hydrothermal precursors treated at 200 °C. When the hydrothermal treatment time was carried out for 24 h, a precursor with Bi1−xTaxO1.5−x (fluorite structure, like δ-Bi2O3) and BiOCl phases was obtained, which evolved to Bi24Cl10O31 phase with heating. The presence of the chlorine-containing phases and the low crystallinity of the hydrothermal precursor contributed to the formation of β-BiTaO4. When the precursor is obtained with 48 h of hydrothermal treatment, only the Bi1−xTaxO1.5−x crystalline phase was identified and when heated it evolved to α-BiTaO4 phase at 550 °C. This is the first time that the pure orthorhombic phase is obtained from a wet synthesis method. All identified phases were confirmed by Rietveld refinement.