Thermal behavior of the amorphous precursors of the ZrO2–SnO2 system

Thermal behavior of the amorphous precursors of the ZrO2–SnO2 system on the ZrO2-rich side of the concentration range, prepared by co-precipitation from aqueous solutions of the corresponding salts, was monitored using differential thermal analysis, X-ray powder diffraction, Raman spectroscopy, field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectrometry (EDS). The crystallization temperature of the amorphous precursors increased with an increase in the SnO2 content, from 405 °C (0 mol% SnO2) to 500 °C (40 mol% SnO2). Maximum solubility of Sn4+ ions in the ZrO2 lattice (∼25 mol%) occurred in the metastable products obtained upon crystallization of the amorphous precursors. A precise determination of unit-cell parameters, using both Rietveld and Le Bail refinements of the powder diffraction patterns, shows that the incorporation of Sn4+ ions causes an asymmetric distortion of the monoclinic ZrO2 lattice. The results of phase analysis indicate that the incorporation of Sn4+ ions has no influence on the stabilization of cubic ZrO2 and negligible influence on the stabilization of tetragonal ZrO2. Partial stabilization of tetragonal ZrO2 in products having a tin content above its solid-solubility limit was attributed to the influence of ZrO2–SnO2 surface interactions. In addition to phases closely structurally related to cassiterite, monoclinic ZrO2 and tetragonal ZrO2, a small amount of metastable ZrSnO4 phase appeared in the crystallization products of samples with 40 and 50 mol% of SnO2 calcined at 1000 °C. Further temperature treatments caused a decrease in and disappearance of metastable phases. The results of the micro-structural analysis show that the sinterability of the crystallization products significantly decreases with an increase in the SnO2 content.