Features of the Phase Formation of Cr/Mn/Fe/Co/Ni/Cu Codoped Bismuth Niobate Pyrochlore

Abstract

The phase formation process of Bi2Cr1/6Mn1/6Fe1/6Co1/6Ni1/6Cu1/6Nb2O9+Δ containing 3d-ions of transition elements in equimolar quantities was studied in a wide temperature range (400–1050 °C). The complex oxide crystallizes in the structural type of pyrochlore (sp. gr. Fd-3m:2, a = 10.4937(2) Å). The investigation of the multi-element pyrochlore phase formation process showed that the synthesis goes through a series of successive stages, during which the transition from Bi-rich to Bi-depleted compounds takes place. The predecessor of the pyrochlore phase is bismuth orthorhombic modification orthoniobate (α-BiNbO4) with an equimolar ratio of Bi(III)/Nb(V) ions. The pyrochlore phase is formed as a result of bismuth orthoniobate doping with transition element ions. The complex oxides Bi14CrO24, Bi25FeO40, BiNbO4, and Bi5Nb3O15 appeared as intermediate phases during the synthesis. The interaction between the initial oxide precursors is fixed at temperatures above 500 °C. The phase transition of α-Bi2O3 into β-Bi2O3 near 500 °C is observed. Varying the heat treatment duration at each synthesis step did not qualitatively change the phase composition of the sample but had an effect on the quantitative phase ratio. Phase-pure pyrochlore of the given composition by solid-phase synthesis method can be obtained at a temperature no lower than 1050 °C. Ceramics are characterized by low-porous dense microstructure with blurred outlines of grain boundaries.