| Abstract
| - In fine-grained zirconia, the order of stability of the three polymorphs is exactly the reverse of that observedin a coarse-grained sample. Starting from a disorder amorphous material, the crystallization into the cubicform occurs, in general, below 500 °C. It is followed, or accompanied, by the formation of tetragonal ZrO2and, finally, by the crystallization into monoclinic zirconia above 700 °C. This sequence is observed inmesoporous and microporous zirconia. It has been suggested that this anomalous behavior is due to a highsurface energy of the monoclinic phase or to strains present in the small particles. It is shown here that thesurface effect explains satisfactorily and semiquantitatively the observations. This does not rule out otherfactors.
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