Pyrochlore phase in the Bi2O3-Fe2O3-WO3-(H2O) system: its stability field in the lowtemperature region of the phase diagram and thermal stability

he concentration stability field localization of the pyrochlore-structured compounds of variable composition formed in the Bi2O3-Fe2O3-WO3 system under hydrothermal conditions at a temperature of T = 200 degrees 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 similar to 10.3319 & Aring; to similar to 10.4199 & Aring; with an increase in the Bi/W atomic ratio from similar to 0.47 to similar to 1.25. It was established that from the Bi2O3-WO3 system side, there is a region of two-phase equilibrium, in which a pyrochlore phase of variable composition coexists with the Bi2WO6 compound, which is formed in the form of plate-like (thickness h similar to 50-100 nm) nanoparticles. It was shown that from the Bi2O3-Fe2O3 system side, there is a region of compositions, in which the pyrochlore phase of the most enriched in bismuth oxide composition coexists with the Bi2WO6 compound, which is formed in the form of rod-shaped (h similar to 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 degrees C, which allows them to be synthesized only by "soft chemistry" methods.