The crystallization process of HfO2 and ZrO2 under hydrothermal conditions

Hydrothermal crystallization of hydrous hafnia at 90 and 120 °C, was monitored at pH 3, 7, 9.5 and 13, using X-ray powder diffraction analyses. The obtained results showed that hydrothermal crystallization proceeded much more slowly in a neutral pH medium than in an acidic or alkaline medium. Phase analysis of the products of hydrothermal treatment showed that, regardless of pH, hydrous hafnia crystallized into m-HfO2. Stabilization of a metastable cubic or tetragonal HfO2 occurred upon adding a sufficient amount of stabilizing Na+ ions. Similarities and differences between the hydrothermal crystallization of HfO2 and the hydrothermal crystallization reported for ZrO2 were discussed. It was concluded that in both cases hydrothermal crystallization proceeds via a dissolution/precipitation mechanism in the whole pH range.