Accumulation of structural defects and modification of micromechanical properties of MgAl2O4 single crystals irradiated with swift heavy ions

Abstract

Single crystals of MgAl2O4 have been irradiated by 156-МeV 132Хе and 2.25-GeV 197Au ions with fluences between 6.6 × 1010 and 2 × 1012 ions/cm2. After isodose ion-irradiation, the spectra of radiation-induced optical absorption (absorption of a pristine sample is subtracted) for the (100), (110) and (111) oriented crystals coincide. The fluence dependence of the concentration of several defects responsible for optical absorption at 4.5–7.1 eV have been analyzed. The integral of all five Gaussians, obtained via spectra decompositions and served as a measure of relevant defects, continuously increases with the fluence without any sign of saturation. Thus, similar to oxygen-vacancy-related F+ and F centers (Gaussians peaked at 4.8 and 5.3 eV, respectively), the defects responsible for elementary bands at 5.9, 6.6 and ∼7.1 eV are also of radiation-induced origin. The thermal stability of all defects has been determined via stepwise annealing of the irradiated samples. The tentative origin of the 5.9, 6.6 and ∼7.1 eV bands (their decay occurs by about 200 K higher than that for the F-type centers) is considered. Nanoindentation measurements show an ion-induced increase of the sample hardness up to 30%. The hardening effect follows the evolution of the energy loss of the Xe and Au ions as a function of sample depth.