Factors influencing the stability of the tetragonal form of zirconia

The pH of the solution that zirconia is precipitated from defines the crystal phase formed after calcination of the material at 400 degrees to 600 degrees C. A metastable tetragonal form is obtained for either low (less than about 5) or high (greater than about 13) pH. The tetragonal phase formed at high pH is much more stable at the calcination temperature than the material obtained at a low pH is. For a material obtained by precipitation at a low pH, monoclinic crystallites, determined by x-ray line broadening, were obtained that were smaller than the tetragonal crystals that produced them. A crystallite size effect, based on x-ray line broadening, is not responsible for the formation or stabilization of the tetragonal phase.