Pyrochlore phase in the Bi2O3-Fe2O3-WO3-(H2O) 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 • 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 ∼ 10.3319 Å to ∼ 10.4199 Å with an increase in the Bi/W atomic ratio from ∼ 0.47 to ∼ 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 ∼ 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 ∼ 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 • C, which allows them to be synthesized only by "soft chemistry" methods.KEYWORDS pyrochlore-structured phase