Influence of hydrothermal synthesis conditions on the composition of the pyrochlore phase in the Bi$_{2}$O$_{3}$-Fe$_{2}$O$_{3}$-WO$_{3}$ system

The paper deals with a study of the effect which the hydrothermal fluid pH has on the formation of a pyrochlore-structured phase in the Bi$_{2}$O$_{3}$-Fe$_{2}$O$_{3}$-WO$_{3}$ system. It was shown that at pH of 1 and 8, the formation of pyrochlore-structured phase particles with crystallite sizes of 38 and 118 nm, respectively, is accompanied by the formation of the Bi$_{2}$WO$_{6}$ compound with the Aurivillius phase structure. At pH values from 2 to 7, only pyrochlore-structured nanocrystalline particles with a variable composition are formed. Under these conditions, the dependence of the average size of crystallites of the pyrochlore-structured phase particles on pH is extreme, as the size increases from $\sim$67 nm at pH 2 up to $\sim$126 nm at pH 5 and then decreases to $\sim$102 nm at pH 7. The samples obtained at pH 3-4 have a composition that is the closest to that specified for the synthesis. When pH increases up to 10, there forms a non-single-phase product that contains the Bi$_{2}$WO$_{6}$ phase and $\delta$-Bi$_{2}$O$_{3}$-based phase.