Paramagnetic defects in polycrystalline zirconia: An EPR and DFT study

The paramagnetic defects present in pristine zirconium dioxide and those formed upon reductive treatments (either annealing or UV irradiation in H2 ) are described and rationalized by the joint use of Electron Paramagnetic Resonance (EPR) and DFT supercell calculations. Three types of Zr3+ reduced sites have been examined both in the bulk of the solid (one centre) and at the surface (two centres). Trapping electron centers different from reduced Zr ions are also present, whose concentration increases upon annealing. A fraction of these sites are paramagnetic showing a symmetric signal at g = 2.0023 but the majority of them are EPR silent and are revealed by analysis of electron transfer from the reduced solid to oxygen. The presence of classic F type centres (electrons in bulk oxygen vacancies) is disregarded on the basis of the g tensor symmetry. This is expected, on the basis of theoretical calculations, to be anisotropic and thus incompatible with the observed signal. In general terms zirconium dioxide has some properties similar to typical reducible oxides such as TiO2 and CeO2 (excess electrons stabilized at cationic sites) but is much more resistant to reduction than this class of materials. While point defects in doped (Y3+, Ca2+) zirconium dioxide materials have been widely investigated for their role as ionic conductors, the defectivity of pristine ZrO2 is much less known and this paper presents a thorough analysis of this phenomenon.