Micro-Scale Depth-Resolved Control of Elastic, Optical and Structural Inhomogeneities in Swift Heavy Ion Irradiated Spinel

Due to its high temperature properties and good resistance to fast neutrons and ions, magnesium aluminate spinel (MgAl₂O₄) is a promising ceramic candidate material for inert matrix nuclear fuel energy applications, as well as for photonics, electronics, crystal phosphors, and laser media in harsh radiation environment. However, to what extent spinel’s structural, mechanical and optical properties are tolerant to swift heavy ion (SHI)-based fission products, remains to be investigated. To emulate the radiation damage by SHIs, spinel in its crystalline form was irradiated by Bi ions with energy 710 MeV at the ion dosage varied over the range of 10¹⁰ - 6 x 10¹² ions/cm² derived from the high energy ion accelerator.<br/>Post-irradiation depth profiling of photolimunescence (PL) micro-spectral peak intensities revealed a sharp color center-driven emission enhancement at a depth of 24 μm, at which the highest radiation-induced <i>nuclear displacement damage</i> is expected according to SRIM profile calculations. On the other hand, the spatial profiling of compressional and shear acoustic wave velocities, probed by the Brillouin light scattering micro-spectroscopy, revealed a close correlation with depth-dependent <i>electron energy</i><i> losses</i>.<br/>With the rise of ion fluence there is a general decrease in the speed of sound, and therefore in the elastic properties. However, the irradiated spinel structure tends to get stiffer at depths approaching maximum nuclear damage. Due to radiation-induced stress the structure gets even more stiff than that of the pristine spinel at depths extending to 30 – 40 μm. Normalized photoelastic coefficients also demonstrate a similar depth-dependent behavior. Maximum losses in photo-elastic property are scaled with those in ionization energy, especially with the increase of irradiation fluence. This depth-resolved Brillouin spectral behavior is confirmed by high resolution transmission electron microscopy, indicating the existence of latent tracks with a diameter of 4-5 nm in an almost intact matrix of crystalline spinel for doses up to 10¹² ions/cm².<br/>At the maximum ion fluence of 6 x 10¹² ions/cm², a multimode Brillouin spectra are observed, corresponding to different crystalline and amorphous phases taking place at depths extending to 80 μm, at which the tracks tend to overlap and form a mixture of crystalline and amorphous phases with the latter ones having weaker elastic properties compared to those of the crystalline matrix. The interplay between nuclear and ionization losses are discussed in terms of micro-scale PL, elastic and photo-elastic inhomogeneities taking place in SHI irradiated spinel. Extreme SHI-driven ionization processes can be employed to sense and control sizeable opto-mechanical functionality in spinel similar to what was recently found in compound semiconductors [2].<br/>This work is supported by the grants from Nazarbayev University (11022021CRP1504 and NU 20122022FD4130) and Kazakhstan Ministry of Science & Higher Education (AP19679332).<br/><br/>[1] K.E. Sickafus, L. Minervini, R.W. Grimes, J.A. Valdez, M. Ishimaru, F. Li, K.J. McClellan, T. Hartmann. “Radiation tolerance of complex oxides”, Science, 289 (5480), 748 (2000)<br/>[2] J. Dong, Y. Li, Y. Zhou, A. Schwartzman, H. Xu, B. Azhar, J. Bennett, J. Li and R. Jaramillo “Giant and Controllable Photoplasticity and Photoelasticity in Compound Semiconductors” Phys Rev Lett. 129, 065501 (2022)