Hydrothermal synthesis, phase formation and crystal chemistry of the pyrochlore/Bi2WO6 and pyrochlore/α-Fe2O3 composites in the Bi2O3-Fe2O3-WO3 system

The present study was devoted to the investigation of the influence of the Bi/Fe ratio in precursors on the formation of heterogeneous systems with the pyrochlore composition Biy+0.67 delta FeyW2-yO6O'(delta) (hereinafter BFWO)/Bi2WO6, and BFWO/alpha-Fe2O3 under hydrothermal conditions at 200 degrees C in a nitric acid solution within 6 h. It was shown that at the Bi/Fe ratios of similar to 1-9, the two coexisting phases are bismuth tungstate (Bi2WO6) with the crystallite size of about 7 nm, and BFWO with the crystallite size of about 30 nm, while at the Bi/Fe ratios of similar to 0.05-0.33, the coexisting phases are BFWO with the crystallite size of about 40 nm, alpha-Fe2O3 and the X-ray amorphous Fe2O3. According to the Mossbauer spectroscopy data, the position of iron in BFWO is described by two doublets with similar chemical shifts, which indicates the inhomogeneous distribution of iron in the tungsten sublattice. The results of the local energy dispersive X-ray microanalysis (EDXMA), in combination with the data on the BFWO structure refined by the Rietveld method, as well as the TEM microstructure data, have shown the obtaining of a new pyrochlore structure, that is, BFWO, which forms within a wide range of compositions. In accordance with the refined data on the BFWO structure, it was demonstrated that this phase has the displacively disordered pyrochlore structure, in which bismuth atoms occupy the 96g Wyckoff position (WP).