Electron paramagnetic resonance study of monoclinic zirconium dioxide polycrystalline powders doped with paramagnetic transition-metal ions
Abstract
Samples of monoclinic zirconium dioxide have been prepared with loadings of either 200 ppm wt/wt Cr3+ or 200 ppm Fe3+ ions, and the effect of calcination on these powders has been observed. For Fe3+, at temperatures below 1170 °C, a broad signal at g≈ 2 is observed due to low-spin Fe3+ ions and another signal at g≈ 4.3 due to high-spin Fe3+ ions, both on the surface. At temperatures above the transition temperature of 1170 °C, migration into the ZrO2 bulk is observed, i.e. loss of the peak at g= 2 along with the appearance of other absorptions at ca. g= 4.
In the case of Cr3+ ions doped onto ZrO2, only absorptions due to Cr5+ ions situated on the surface are observed at g= 1.97. On heating, the intensity of this signal decreases. Calcination through the transition temperature (1170 °C) does not cause migration of either the Cr3+ ions or the Cr5+ ions into the bulk. Treatment with ethane-1,2-diol results in a nine-line EPR spectrum, demonstrating that the chromium ions undergo further oxidation to Cr6+ ions on the surface. The Cr3+ doped ZrO2 powder undergoes a colour change from white to grey at the transition temperature. Spectra recorded on samples at liquid-nitrogen temperatures suggest that Zr4+ ions have been reduced to Zr3+ ions.