Optical absorption and Raman studies of neutron-irradiated Gd₃Ga₅O₁₂ single crystals

In this work we have performed a comparative analysis of optical absorption and micro-Raman spectra for series of Gd₃Ga₅O₁₂ (GGG) single crystals irradiated by fast neutrons with fluences from 10¹⁶ n/cm² to 10¹⁸ n/cm². It was found that the optical absorption spectra of non-irradiated Czochralski grown GGG consist of the relatively narrow lines in the UV spectral range related to the 4f–4f transitions in Gd³⁺. Transitions from the ⁶S_7/2 ground state to the ⁶P, ⁶J and ⁶D states in a Gd³⁺ cation are clearly detected. For a GGG crystal containing Ca impurity ions, additional absorption band at 350 nm is observed, and it is tentatively ascribed to oxygen vacancies associated with Ca impurities. Several neutron-induced optical absorption bands, including ones related to F⁺ centres, were observed in fast neutron-irradiated Gd₃Ga₅O₁₂ single crystals. Furthermore, clear evolution of the Raman spectra with neutron dose is also found. In particular, fine structure observed between 1350 and 1550 cm⁻¹ is completely disappeared at high fluences, while several significant changes and the emergence of new transitions have been ascertained in spectral range of 100–850 cm⁻¹.