Pyrochlore phase in the Bi$_2$O$_3$–Fe$_2$O$_3$–WO$_3$–(H$_2$O) system: its stability field in the low-temperature region of the phase diagram and thermal stability

The concentration stability field localization of the pyrochlore-structured compounds of variable composition formed in the Bi$_2$O$_3$–Fe$_2$O$_3$–WO$_3$ system under hydrothermal conditions at a temperature of $T$ = 200$^\circ$C and a pressure of $P$ = 7 MPa was determined. It was found that the pyrochlore-structured compounds stability field is longitudinally limited within the atomic ratios 0.47 $<$ Bi/W $<$ 1.25, and in the transverse direction within 1.14 $<$ Bi/Fe $<$ 1.87. It was shown that the pyrochlore phase cubic unit cell parameter a depends on the compound chemical composition as follows: it increases linearly from $\sim$ 10.3319 $\mathring{\mathrm{A}}$ to $\sim$ 10.4199 $\mathring{\mathrm{A}}$ with an increase in the Bi/W atomic ratio from $\sim$ 0.47 to $\sim$ 1.25. It was established that from the Bi$_2$O$_3$–WO$_3$ system side, there is a region of two-phase equilibrium, in which a pyrochlore phase of variable composition coexists with the Bi$_2$WO$_6$ compound, which is formed in the form of plate-like (thickness $h\sim$ 50–100 nm) nanoparticles. It was shown that from the Bi$_2$O$_3$–Fe$_2$O$_3$ system side, there is a region of compositions, in which the pyrochlore phase of the most enriched in bismuth oxide composition coexists with the Bi$_2$WO$_6$ compound, which is formed in the form of rod-shaped ($h\sim$ 10–30 nm) nanoparticles, and with the X-ray amorphous phase composition, formed in the form of nanocrystalline particles about 10 nm in size. It was found that the higher temperature point of the pyrochlore-structured compounds stability field does not exceed 725$^\circ$C, which allows them to be synthesized only by “soft chemistry” methods.