The influence of thermal treatment on phase development in ZrO2-Fe2O3 and HfO2-Fe2O3 systems

Article abstract of DOI:10.1016/S0925-8388(01)01401-3

The effects of thermal treatment (500, 600, 800 and 1100°C) of the amorphous precursors of ZrO₂-Fe₂O₃ and HfO₂-Fe₂O₃ systems, coprecipitated from aqueous solutions of the corresponding salts, were studied at room temperature (RT) by X-ray powder diffraction (XRD) and, in some cases, by laser Raman spectroscopy. The incorporation of Fe³⁺ ions partially stabilized the high temperature polymorphs of zirconia (ZrO₂) and hafnia (HfO₂). The stabilization of the cubic polymorphs of ZrO₂ and HfO₂ occurred in the solid solutions with an Fe₂O₃ content of ≥10 mol%. The presence of Fe³⁺ ions above their solid solubility limit caused the transition of metastable polymorphs of ZrO₂ and HfO₂ to monoclinic polymorphs. The terminal solid solubility limit of Fe₂O₃ in ZrO₂, as estimated at RT, was ～33 mol% after calcination at 600°C, ～9 mol% after calcination at 800°C and ～2 mol% after calcination at 1100°C. After the same thermal treatment, the terminal solid solubility limits of Fe₂O₃ in HfO₂ proved to be very similar only a little smaller. The unit-cell volume of the cubic polymorphs of ZrO₂ and HfO₂, determined by powder-pattern-fitting methods, decreased linearly with the increase in iron content, but the rate of decrease was greater in the ZrO₂ polymorph. The thermal behavior of the amorphous precursors was also followed by differential thermal analysis and thermogravimetric analysis. The crystallization temperature of both systems increased with an increase in the Fe₂O₃ content, from 405 to 730°C in the ZrO₂-Fe₂O₃ system, and from 520 to 720°C in the HfO₂-Fe₂O₃ system. The results indicated that the amorphous precursors with more than 30 mol% of Fe₂O₃ were single co-gels. The status of the Fe³⁺ ions in the ZrO₂-type lattice and the process of their segregation into the α-Fe₂O₃-phase were examined by⁵⁷Fe Mo?ssbauer spectroscopy. Significantly lower values of the hyperfine magnetic fields compared to that of α-Fe₂O₃ indicated the incorporation of Zr⁴⁺ ions into the α-Fe₂O₃ lattice, the amount of which decreased with the increasing temperature of treatment.