| Abstrakt: |
Electron paramagnetic resonance (EPR) measurements have been made at X-band and room temperature on monoclinic HfO2 and ZrO2 powders from several suppliers. They reveal the presence of eight main paramagnetic centers H1, H2, H3, H4, and Z1, Z2, Z3, and Z4. H1 and Z1 are analogous as H4 and Z4 and H2 and Z2 are similar as H3 and Z3. H1 and Z1 have axial symmetry with g∥⊥e, where ge is the free electron g value. H1 is found in all, and Z1 in all but one, of the samples in their as-received state but with a wide range of concentrations. However, annealing the samples in air up to 900 °C reduces the volume concentration range and the areal concentrations all become of order 1011 cm-2. Irradiation with γ-rays does not affect their concentration. The Z1 centers are found to be the same as those previously observed in ZrO2 powders that were attributed to Zr3+ ions in coordinatively unsaturated (cus) sites at and/or near the surface. Our results are consistent with this model for Z1 and with an analogous model of cus Hf3+ for H1. H4 and Z4 are centers of isotropic symmetry with g values that are both within ±0.0004 of 2.0027; they are produced in all HfO2 and ZrO2 samples, respectively, that are heated in vacuum at ≥300 °C. Their concentration reaches a maximum of order 1017 cm-3 or 1012 cm-2 in the range of 550–750 °C. They are also most likely to be mainly at and/or near the surface and to involve an electron trapped in an oxygen vacancy cluster. The EPR spectra of H2 and Z2 are consistent with those of S=1/2 centers of orthorhombic symmetry with principal g values about equal to or just less than ge suggesting that they are trapped electron centers. The electrons produced by γ-irradiation are trapped at... [ABSTRACT FROM AUTHOR] |
