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.