Electron paramagnetic resonance characterization of defects in monoclinic HfO 2 and ZrO 2 powders

Electron paramagnetic resonance (EPR) measurements have been made at X-band and room temperature on monoclinic HfO 2 and ZrO 2 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 ∥ < g ⊥ < g e , where g e 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 10 11 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 ZrO 2 powders that were attributed to Zr 3 + 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 Hf 3 + 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 HfO 2 and ZrO 2 samples, respectively, that are heated in vacuum at ≥ 300 ° C . Their concentration reaches a maximum of order 10 17 cm − 3 or 10 12 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 g e suggesting that they are trapped electron centers. The electrons produced by γ -irradiation are trapped at precursors to H2 but are easily detrapped. Z2 centers also appear to be shallow electron traps. Their identity is uncertain; they have some characteristics of electrons trapped in oxygen vacancies and of CO 2 − radicals. H3 and Z3 are likely to involve holes trapped on oxygen, possibly as O − and O 2 − type centers, respectively, but their location in not known. Their concentration increases to an upper limit as the γ -ray dose is increased and this shows that their precursors are trapping charge generated by the γ -rays. Like the H2 and Z2 centers, even annealing at 100 ° C releases the charge but their precursors, at least in HfO 2 , are not destroyed. The significance of these centers is discussed.