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 ZrO 2 and then to monoclinic ZrO 2 in both the absence and presence of water vapor. In the absence of H 2 O vapor, t-ZrO 2 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-ZrO 2 . Exposure of t-ZrO 2 to 3 kPa of H 2 O or immersion in liquid water at 298 K results in its extensive (∼80%) transformation to m-ZrO 2 without a loss in surface area. This observation is attributed to a decrease in the difference between the surface free energy of m-ZrO 2 and that of t-ZrO 2 caused by the adsorption of H 2 O. 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-ZrO 2 is transformed to m-ZrO 2 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-ZrO 2 with high surface areas can be prepared by exposing t-ZrO 2 to water vapor at room temperature.