Thermal behavior of the amorphous precursors in the ZrO2–CrO1.5 system

The amorphous precursors of the ZrO2–CrO1.5 system at the ZrO2-rich side of the concentration range were prepared by co-precipitation from aqueous solutions of the corresponding salts. Thermal behavior of the amorphous precursors was monitored using differential thermal analysis, X-ray powder diffraction (XRD), Raman spectroscopy and Fourier transform infrared spectroscopy. The crystallization temperature of the amorphous precursors increased with an increase in the CrO1.5 content, from 435 °C (0 mol% of CrO1.5) to 822 °C (50 mol% of CrO1.5). The results of phase analysis indicated an extended capability for the incorporation of Cr3+ ions into the metastable ZrO2-type solid solution obtained after crystallization of amorphous co-gels. Maximum solubility of Cr3+ ions in the ZrO2 lattice, obtained after crystallization of the amorphous precursors, was estimated at ∼26 mol%, but significantly decreased with further temperature treatment. The results of XRD and Raman spectroscopy showed that the incorporation of Cr3+ ions partially stabilized only the tetragonal ZrO2 polymorph. However, precise determination of lattice parameters, performed using the whole-powder-pattern decomposition method, showed that the axial ratio cf/af in the t-ZrO2-type solid solutions with Cr3+ content ≥20 mol% approach 1. The incorporation of Cr3+ ions caused a decrease in the unit-cell volume of the t-ZrO2-type solid solutions, but the rate of the decrease turned out to be smaller than the rate obtained after the incorporation of Fe3+ ions. The reason for such behavior was discussed.