Transformation of tetragonal zirconia phase to monoclinic phase in the presence of Fe3+ ions as probes: an EPR study

EPR was mainly used to study the morphological, textural and structural behavior of zirconium hydroxide [ZrO(OH)2] with respect to calcination under air at different temperatures. For calcination temperatures less than 700°C, the tetragonal and monoclinic phases of the solid were present. In this range of temperatures an EPR signal with gxx=1.9755, gyy=1.9720 and gzz=1.9562 was observed and attributed to Zr3+ ions located in octahedral sites with strong tetragonal distortion. The dehydration of OH- groups from solids could be responsible for the Zr4+ reduction into Zr3+ ions. A second signal, centered at g=2.0018, was also observed and assigned to trapped single electrons located in oxygen vacancies of ZrO2. A third signal with gxx=2.0040, gyy=2.0082 and gzz=2.0334 was attributed to adsorbed O2- species. Finally, a fourth signal obtained at low magnetic field with different g values was attributed to Fe3+ ions located in sites with a purely rhombic field. For high calcination temperatures (>700°C), the tetragonal phase was completely transformed into monoclinic phase. In this phase, the trapped single electrons and the adsorbed O2- species disappeared whereas the number of Zr3+ ions increases when compared to that obtained at lower calcination temperatures. This increase could be related to the reduction of Zr4+ by the trapped single electrons and the formation of the monoclinic phase which stabilizes the Zr3+ ions. In this latter phase, the Fe3+ ions are located in sites which have the same environmental symmetry than in tetragonal phase but with specific EPR parameter values.