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Accumulation and Recovery of Irradiation Effects in SrTiO3 W. J. Weber,(a,b) W. Jiang,(b,c) and S. Thevuthasan Supported by Office of Basic Energy Sciences. (a) EMSL User. (b) Environmental and Health Sciences Division. (c) Postdoctoral Research Fellow. Single crystals of SrTiO3 (Commercial Crystal Laboratories, Inc.), oriented along the [100] axis, were irradiated at 180 to 200 K with 410 keV He+, 400 keV O+ or 1.0 MeV Au2+ ions at an incident angle of 60° , which produced near-surface damage that could be probed by Rutherford backscattering spectroscopy in channeling geometry (RBS/C) methods. The accumulation of disorder and subsequent thermal recovery were followed in situ using 2 MeV He+ RBS/C. The irradiation and in situ ion-beam analysis were carried out using the accelerator facilities at the EMSL. Isochronal annealing on all these samples was performed in situ (i.e., in vacuum) at temperatures ranging from 300 to 870 K for time periods of 20 minutes. After each annealing step, the sample was cooled down, and the RBS/C measurements were performed at 300 K. The accumulation of relative disorder as a function of dose (dpa) in SrTiO3 irradiated at 200 K with 1.0 MeV Au2+ ions is shown in Figure 9.15. The relative disorder shows a very strong sigmoidal dependence on dose, and the results indicate that the random level (or amorphous state) is reached at a dose of about 1.0 dpa. A fit of the direct-impact/defect-stimulated (DI/DS) model for amorphization (Weber in press) is shown by the solid curve, and yields a value of about 300 for s s/s a, where s a and s s are effective cross-sections for direct-impact and defect-stimulated amorphization, respectively. The large value for s s/s a indicates the strong dominance of defect accumulation in the amorphization process. At low doses (<0.3 dpa), both amorphization and interstitial defects contribute to the relative disorder, which may explain why the relative disorder is slightly higher than that indicated by the model fit in this dose range. Figure 9.15. Relative disorder at the damage peak in SrTiO3 irradiated at 200 K with 1.0 MeV Au2+ ions. Single crystals of SrTiO3 were irradiated at 180 K with low doses of 410 keV He+ or 400 keV O+ to produce a partially damaged state (<25% disorder at the damage peak) consisting primarily of point defects. The recovery of defects in these samples was studied by isochronal annealing. RBS/C measurements after each isochronal annealing step were made at temperatures well below the annealing temperature to minimize the defect recombination during data acquisition. The recovery behavior for disorder on the oxygen sublattice at the damage peak is shown in Figure 9.16 for SrTiO3 irradiated with 400 keV O+ ions. The results indicate that thermal recovery processes on the oxygen sublattice are complete after annealing at 475 K for this sample and these irradiation conditions. The results also suggest the presence of two recovery stages on the oxygen sublattice, with peak recoveries at about 200 and 300 K, respectively. The experimentally measured activation energy for migration of oxygen vacancies in single crystal SrTiO3 is on the order of 0.98 to 1.3 eV (Yamaji 1975; Paladino et al. 1965). Computer simulations of defects in SrTiO3 indicates that the oxygen vacancy has a migration energy of 0.65 to 0.76 eV (Akhtar et al. 1995; Crawford and Jacobs 1999). Recent computer simulations indicate that the oxygen interstitial has a migration energy of about 0.2 eV (Williford and Weber submitted). These results are consistent with the recovery behavior observed in Figure 9.16. Figure 9.16. Isochronal recovery on the oxygen sublattice for SrTiO3 irradiated with 400 keV O+ ions at 180 K. Thermal recovery of the disorder on the cation sublattice at the damage peak is shown in Figure 9.17 for SrTiO3 single crystals irradiated with 410 keV He+ ions and with 400 keV O+ ions. The results show a single recovery stage between 200 and 400 K and gradual recovery at higher temperatures for both irradiation-induced damage states, which is similar to the recovery behavior previously observed in SrTiO3 irradiated with 1.0 MeV Au2+ (Thevuthasan in press) to low ion fluences. Complete recovery on the cation sublattice occurs at temperatures above 900 K. Based on computer simulations (Akhtar et al. 1995), the cation vacancy migration energies are 2.52 and 11.6 eV for Sr and Ti, respectively, which suggests that these defects are relatively immobile during irradiation and thermal annealing. Recent computer simulations (Crawford and Jacobs 1999; Williford and Weber submitted) indicate that the Ti interstitial has a relatively high migration energy of 2.6 to 3.6 eV and is thus immobile. However, these computer simulations (Crawford and Jacobs 1999; Williford and Weber submitted) indicate that the Sr interstitial has a migration energy of 0.76 to 1.2 eV, which is consistent with the observed recovery behavior in Figure 9.17. Figure 9.17. Isochronal recovery on the cation sublattice for SrTiO3 irradiated with 410 keV He+ or 400 keV O+ ions at 180 K. References Akhtar, M. J., Z.-U.-N. Aakhtar, R. A. Jackson, and C. R. A. Catlow, J. Amer. Ceramic Society 78, 421-428 (1995). Crawford, J., and P. Jacobs, J. Solid State Chem. 144, 423-429 (1999). Paladino, A. E., L. G. Rubin, and J. S. Waugh, J. Phys. Chem. Solids 26, 391-397 (1965). Thevuthasan, S., W. Jiang, J. S. Young, and W. J. Weber Nucl. Instrum. and Methods, in press. Weber, W. J., Nucl. Instrum. and Methods, in press. Williford, R. E., and W. J. Weber, J. Solid State Chem., submitted. Yamaji, A., J. Amer. Ceram. Society 58, 152-153 (1975). William R. 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