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

Abstract

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 g_xx=1.9755, g_yy=1.9720 and g_zz=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 g_xx=2.0040, g_yy=2.0082 and g_zz=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.