Crystallization and Phase Transformation Process in Zirconia: An in situ High‐Temperature X‐ray Diffraction Study

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Abstract
Amorphous zirconia precursors were made by the precipitation of a zirconium tetrachloride solution with either slow (8 h) or rapid additions of ammonium hydroxide at a pH of 10.5. Following calcination at 500°C for 4 h, the rapidly precipitated precursor exhibited predominantly monoclinic ZrO2 phase, while the slowly precipitated precursor produced the tetragonal ZrO2 phase. The crystallization and phase transformations were followed by in situ high‐temperature X‐ray diffraction (HTXRD) for both specimens in helium and in air. Each amorphous precursor first crystallizes as the tetragonal phase at about 450°C. A tetragonal‐to‐monoclinic phase transformation of the rapidly precipitated material was observed on cooling at about 275°C. Surface impregnation of sulfate ions following precipitation inhibited the tetragonal‐to‐monoclinic transformation for the rapidly precipitated ZrO2 sample. The crystallite size for the t‐ZrO2 of all samples, irrespective of whether they transform to monoclinic, was approximately 11 nm, indicating that the t→m transformation in these materials is not controlled by differences in crystallite size. It is therefore suggested that anionic vacancies control the tetragonal‐to‐monoclinic phase transformation on cooling, and that oxygen adsorption triggers this phase transformation.

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