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.