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

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

Конверсия Cr(III) и Co(III) в процессе синтеза кобальт-хромсодержащего пирохлора на основе ниобата висмута по данным NEXAFS

Cubic pyrochlore of the composition Bi2Co0.5Cr0.5Nb2O9+∆ (sp. gr. Fd-3m, a = 10.4838(8) ˚A) was synthesized in several stages using a solid-phase reaction from oxide precursors at a final temper- ature of 1050 ◦С. Using NEXAFS spectroscopy data, the electronic state of cobalt and chromium ions during the synthesis process was studied. It has been established that before the formation of phase- pure pyrochlore, Cr(III) ions are converted to Cr(VI), and then again to Cr(III); Cobalt ions Co(III) are reduced to Co(II). NEXAFS Cr2p spectra of ceramics synthesized at 650 ◦С, according to the main characteristics of the spectrum, coincide with the spectrum of K2Cr2O7 and indicate the chromium content in the oxide ceramics in the form of tetrahedral CrO2 4 ions, and according to the nature of the Co2p spectrum, cobalt ions are in the Co(II) state and Co(III). In the composition of pyrochlore Bi2Co0.5Cr0.5Nb2O9+∆, synthesized at 1050 ◦С, cobalt and chromium appear predominantly in the form of Co(II) and Cr(III) ions. Analysis of phase transformations showed that changes in the oxidation state of transition element ions and the color of ceramics are associated with the formation of intermediate synthesis productsКубический пирохлор состава Bi2Co0.5Cr0.5Nb2O9+∆ (пр.гр. Fd-3m, а = 10.4838(8) ˚A) синтезировали в несколько этапов методом твердофазной реакции из оксидных прекурсоров при финальной температуре 1050 ◦С. По данным NEXAFS-спектроскопии исследовано электронное со- стояние ионов кобальта и хрома в процессе синтеза. Установлено, что до формирования фазовочи- стого пирохлора ионы Cr(III) превращаются в Сr(VI), а затем снова в Cr(III); ионы кобальта Co(III) восстанавливаются до Сo(II). NEXAFS Cr2p-спектры керамики, синтезированной при 650 ◦С, по основным характеристикам спектра совпадают cо спектром K2Cr2O7 и свидетельствуют о содержа- нии хрома в оксидной керамике в виде тетраэдрических ионов CrO2 4 , а по характеру Co2p-спектра ионы кобальта находятся в состоянии Co(II) и Сo(III). В составе пирохлора Bi2Co0.5Cr0.5Nb2O9+∆, синтезированного при 1050 ◦С, кобальт и хром проявляются преимущественно в виде ионов Co(II) и Сr(III). Анализ фазовых превращений показал, что изменение степени окисления ионов переходных элементов и цвета керамики связано с образованием промежуточных продуктов синтез