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

The effects of thermal treatment (500, 600, 800 and I 100 degreesC) 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 greater than or equal to 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, estimated at RT, was similar to 33 mol% after calcination at 600 degreesC, similar to9 mol% after calcination at 800 degreesC and similar to2 mol% after calcination at 1100 degreesC. 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 degreesC in the ZrO2-Fe2O3 system, and from 520 to 720 degreesC 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 alpha -Fe2O3-phase were examined by Fe-57 Mossbauer spectroscopy, Significantly lower values of the hyperfine magnetic fields compared to that of alpha -Fe2O3 indicated the incorporation of Zr4+ ions into the alpha -Fe2O3 lattice, the amount of which decreased with the increasing temperature of treatment. (C) 2001 Elsevier Science B.V. All rights reserved.