Water-Assisted Tetragonal-to-Monoclinic Phase Transformation of ZrO₂at Low Temperatures

In-situ Raman spectroscopy has been used to characterize the transformation of amorphous zirconium oxyhydroxide to tetragonal ZrO2 and then to monoclinic ZrO2 in both the absence and presence of water vapor. In the absence of H2O vapor, t-ZrO2 forms when zirconium oxyhydroxide is treated in air above 673 K and the tetragonal-to-monoclinic transformation occurs at temperatures above 950 K. Treatment at progressively higher temperatures is accompanied by loss of surface area. The tetragonal-to-monoclinic transformation occurs when the size of the zirconia particles is equal to or greater than a critical size determined from an analysis of the thermodynamic stability of small particles of t- and m-ZrO2. Exposure of t-ZrO2 to 3 kPa of H2O or immersion in liquid water at 298 K results in its extensive (∼80%) transformation to m-ZrO2 without a loss in surface area. This observation is attributed to a decrease in the difference between the surface free energy of m-ZrO2 and that of t-ZrO2 caused by the adsorption of H2O. Treatment in water vapor at 623 K induces the tetragonal-to-monoclinic transformation as well, but it is accompanied by a significant decrease in surface area. Only 80% of the t-ZrO2 is transformed to m-ZrO2 at 298 K because of the apparent presence of some crystallites with lattice strain, which increases the temperature for the tetragonal-to-monoclinic phase transformation. The results of this study demonstrate that m-ZrO2 with high surface areas can be prepared by exposing t-ZrO2 to water vapor at room temperature.