Pyrochlore phase in the Bi2O3-Fe2O3-WO3-(H2O) system: its formation by hydrother-mal synthesis in the low-temperature region of the phase diagram

The present paper investigates features of a (Bi, Fe, ❑)2(Fe, W)2O6O0 delta cubic pyrochlore-structured phase (hereinafter BFWO) formation in the Bi2O3-Fe2O3-WO3-(H2O) system under the hydrothermal syn-thesis conditions at T < 200 degrees C and in the range of pH < 1. It was found that the BFWO phase is formed even when the amorphous precursor suspension is less than 100 degrees C. The BFWO phase particles have a con-ditionally spherical morphology and are polycrystalline. The dependency of the average particle size on the synthesis temperature correlates well with the dependency of the average crystallite size on this parameter: both values increase abruptly with an increase in the amorphous precursor suspension treatment temperature from 90 to 110 degrees C (from-140 and 70 nm to-180 and 90 nm, respectively), and with a further increase in the hydrothermal treatment temperature to 190 degrees C, they increase more smoothly (up to-210 and 110 nm, respectively). It was found that the average number of crystallites in a particle is-9 units regardless of the synthesis temperature, i.e. an increase in the BFWO phase particle size with the increasing temperature (in the considered temperature range) occurs mainly due to an increase in the size of their constituent crystallites.