Thermal behavior of the amorphous precursors of the HfO2-Fe2O3 system

The amorphous precursors of the HfO2-Fe2O3 system at the HfO2-rich side of the concentration range were prepared by the co-precipitation of the corres ponding salts from aqueous solutions. The thermal behavior of the samples w as followed using differential thermal analysis and X-ray diffraction at hi gh-temperature. The differences in the phase developments during the heatin g of the samples in the presence of air at atmospheric pressure (similar to 10(5) Pa) and low pressure (similar to4 x 10(-3) Pa) were attributed to th e influence of the oxygen vacancies introduced during calcination at low pr essure. The results of phase analysis showed that Fe3+ ions have both a sta bilizing and destabilizing influence on the high-temperature polymorphs of HfO2. The incorporation of 10-30 mol% of Fe2O3 stabilized cubic hafnia duri ng calcination in the presence of air at atmospheric pressure, but further increase of the iron content caused the destabilization of this phase. The destabilizing influence of the Fe3+ ions increased during calcination at lo w pressure. The oxygen vacancies introduced during the calcination at low p ressure partially stabilized the cubic HfO2, but this stabilizing influence significantly decreased in the samples with high iron content. The obtaine d results indicated that both the stabilizing and destabilizing influences of Fe3+ ions are related to the oxygen vacancies introduced in the HfO2 lat tice. The stabilization of the high-temperature polymorphs of HfO2 depended on the amount of oxygen vacancies and their distribution inside the lattic e. The solid solubility limits: of Fe2O3 in HfO2 decreased during heating a t atmospheric pressure from >30 moL %at 700 degreesC to similar to2 mol% at 1000 degreesC. The solubility of Fe2O3 in HfO2 during the heating at low p ressure was higher. (C) 2001 Elsevier Science B.V. All rights reserved.