Charge compensation in an irradiation-induced phase of δ-Sc4Zr3O12

Robust materials that are durable under irradiation are needed by a resurgent nuclear industry, and oxide ceramics are leading candidates for a number of nuclear applications, such as the storage of nuclear waste [1]. The delta (d) phase of Sc4Zr3O12 is an example of an oxide that has recently been shown to have excellent radiation tolerance/ amorphization resistance [2, 3]. It has been proposed that the origin of the pronounced radiation tolerance in such materials is linked to crystal structure characteristics [3]. The structure of d-Sc4Zr3O12 is similar to that of fluorite (CaF2), but with rhombohedral (not cubic) symmetry (space group R 3 [4]). Ion irradiation damage experiments on d-Sc4Zr3O12 using 300 keV Kr 2? ions, have revealed an order-to-disorder (O-D) phase transformation from an ordered d-phase to a disordered fluorite structure at low ion dose [2]. This O-D transformation is similar to that observed in other fluorite derivative oxides, especially pyrochlore compounds [5]. However, in d-Sc4Zr3O12 at high ion dose, a surprising second transformation, a disorder-to-order (D-O) transformation, was observed [6]. This D-O transformation was shown to involve a structural rearrangement from a disordered fluorite to an ordered bixbyite structure [6]. Sickafus et al. [7] found that the irradiation-induced bixbyite phase contained a high concentration of Zr. The Sc:Zr ratio in the bixbyite phase was found to be 4:3, just as it is in the d-phase (to within the error of the experimental measurements). This is unexpected because bixbyites are usually sesquioxide compositions with a trivalent cation. This said, we would propose to write the composition of the irradiation-induced bixbyite as ðSc4=7Zr3=7Þ2O3; where the compound is made solely from 3 cations. It is, however, well known that Zr strongly desires a 4? valence. Thus, the bixbyite formula, ðSc4=7Zr3=7Þ2O3; runs counter to intuition. Here we attempt to identify the charge compensation mechanism for incorporation of excess Zr in a Sc–Zr–O bixbyite. As mentioned previously, one of the possible charge compensation mechanisms for Zr incorporation in a Sc– Zr–O bixbyite involves Zr. This ion is a paramagnetic species, so we used EPR spectroscopy to determine the relative abundance of Zr in both a pristine d-Sc4Zr3O12 sample and in an irradiated d-Sc4Zr3O12 sample that contains the bixbyite phase. Figure 1 shows mass-corrected, first-derivative EPR spectra, measured at 5 K, for both pristine and irradiated d-Sc4Zr3O12. Both samples display a weak signal (approximately 10 spins/g) with a g-value of approximately 2.0027, calibrated against DPPH. This is close to the free electron value of 2.00238. Figure 1 also shows an EPR spectrum obtained from a zirconia sample that was annealed in a reducing environment to produce a nominal composition of *ZrO1.98 containing Zr 3? ions [2]. The EPR spectrum from this reduced compound is remarkably similar to that from the ion-irradiated d-Sc4 Zr3O12 sample containing the bixbyite phase. Similar studies [8, 9] on reduced ZrO2 samples have associated M. W. Blair (&) Earth and Environmental Science Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA e-mail: mblair@lanl.gov