Crystal structure and optical properties of the Bi–Fe–W–O pyrochlore phase synthesized via a hydrothermal method

Abstract The present work investigated the influence of the hydrothermal synthesis conditions (hydrothermal fluid pH) on the composition, structural parameters, morphology, and optical properties of nanocrystalline particles based on a phase of variable composition that forms in the Bi2O3–Fe2O3–WO3 system and has the cubic pyrochlore structure A2B2O6O′δ = (Bi, Fe, VA)2(Fe, W)2O6O′δ , where VA are cation vacancies at the A-site (hereinafter BFWO). The BFWO phase structural parameters were refined by the Rietveld method, and the distribution of Fe over the structurally nonequivalent sites was established. It was shown that the fraction of Fe at the A-site out of the total amount of Fe depends on the BFWO chemical composition and can vary from 0 to ~15%. The results of the Mossbauer study showed that Fe atoms are characterized by two states that differ significantly in terms of the nearest environment, but it was found impossible to explain the differences in the state of iron atoms by their simultaneous presence at the sites A and B. According to the X-ray diffraction (XRD) data, the average crystallite size was shown to change depending on the conditions of hydrothermal synthesis. When the values of hydrothermal fluid pH increased, the average crystallite size first increases from ~78 nm at pH 2 up to 181 nm at pH 5, and then decreases to ~133 nm at pH 7. The scanning electron microscopy (SEM) data have shown that crystallites in all samples grow together into aggregates, the shape of which is close to spherical. The average aggregate size depends on the hydrothermal fluid pH in a similar way: when pH increases, the aggregate size first increases from ~330 nm at pH 2 up to 880 nm at pH 5, and then decreases to ~340 nm at pH 7. The band gap (Eg) of the obtained materials for direct allowed electronic transitions was calculated from the diffuse reflectance spectrum data and found to be in the 2.16–2.41 eV range. The Eg value was shown to decrease with the enrichment of the BFWO composition in Bi and Fe and with a decrease in the volume fraction of crystallites that make up the surface layer of aggregates and have a size of about 30–40 nm.