Pyrochlore phase in the Bi$_2$O$_3$–Fe$_2$O$_3$–WO$_3$–(H$_2$O) system: its formation by hydrothermal synthesis in the low-temperature region of the phase diagram

The present paper investigates features of a (Bi, Fe, $\square$)$_2$(Fe, W)$_2$O$_6$O'$_\delta$ cubic pyrochlore-structured phase (hereinafter BFWO) formation in the Bi$_2$O$_3$–Fe$_2$O$_3$–WO$_3$–(H$_2$O) system under the hydrothermal synthesis conditions at $T<$ 200$^\circ$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$^\circ$C. The BFWO phase particles have a conditionally 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$^\circ$C (from $\sim$140 and 70 nm to $\sim$180 and 90 nm, respectively), and with a further increase in the hydrothermal treatment temperature to 190$^\circ$C, they increase more smoothly (up to $\sim$210 and 110 nm, respectively). It was found that the average number of crystallites in a particle is $\sim$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.