Фаза пирохлора в системе Bi₂O₃‒Fe₂O₃‒WO₃‒(H₂O): поле устойчивости в низкотемпературной области фазовой диаграммы и термическая устойчивость

Определено положение концентрационной области устойчивости соединений переменного состава со структурой пирохлора, формирующихся в системе Bi₂O₃-Fe₂O₃-WO₃ в гидротермальных условиях при температуре T = 200 °C и давлении P = 7 МПа. Установлено, что область устойчивости фазы пирохлора в продольном направлении ограничена в пределах атомных соотношений 0.47 < Bi/W < 1.25, а в поперечном направлении в пределах 1.14 < Bi/Fe < 1.87. Показано, что параметр a кубической элементарной ячейки фазы пирохлора зависит от химического состава соединения следующим образом: линейно возрастает от ~ 10.3319 Å до ~ 10.4199 Å с увеличением атомного соотношения Bi/W от ~ 0.47 до ~ 1.25. Установлено, что со стороны системы Bi₂O₃-WO₃ имеется область двухфазного равновесия, в которой фаза пирохлора переменного состава сосуществует с соединением Bi2WO6, формирующемся в виде пластинчатых (толщиной h ~ 50‒100 нм) наночастиц. Показано, что со стороны системы Bi₂O₃-Fe₂O₃ имеется область составов, в которой фаза пирохлора наиболее обогащённого по оксиду висмута состава сосуществует с соединением Bi₂WO₆, формирующемся в виде стержневидных (h ~ 10‒30 нм) наночастиц, и с рентгеноаморфной фазой, формирующейся в виде нанокристаллических частиц размером около 10 нм. Показано, что верхний температурный предел области устойчивости полученных соединений со структурой пирохлора не превышает 725 °C, что позволяет синтезировать их только методами «мягкой химии».

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