Abil T. Zhalgas

This study presents a comprehensive Raman spectroscopic and mechanical investigation of Gd₃Ga₅O₁₂ (GGG) single crystals irradiated with 231 MeV ¹³¹Xe ions at fluences ranging from 1 × 10¹¹ to 3.3 × 10¹³ ions/cm². Raman analysis reveals that all fundamental vibrational modes of the garnet structure remain observable up to the highest fluence, with the preservation of garnet crystalline topology/absence of secondary crystalline phases. However, significant line broadening (FWHM increase by 20–100%) and low-frequency shifts indicate progressive lattice disorder and phonon-defect scattering. High-frequency Ga-O stretching modes (A_1g, T_2g ~740 cm⁻¹) remain the most resistant to irradiation, while low-energy translational modes involving Gd³⁺ ions exhibit pronounced degradation and partial disappearance at high fluence. Complementary nanoindentation measurements show radiation-induced softening: hardness decreases by up to ≈60% at 3.3 × 10¹³ ions/cm², consistent with amorphization and overlapping ion tracks (~10–12 μm deep). Raman spectroscopy shows that the garnet lattice remains as the only crystalline phase up to 3.3 × 10¹³ ions/cm², while significant line broadening, mode suppression and a strong hardness decrease indicate progressive structural disorder and partial amorphization of the near-surface region. These results demonstrate that GGG maintains crystalline integrity below the track-overlap threshold (~6 keV/nm) but undergoes strong structural relaxation and mechanical weakening once this limit is exceeded. A new analytical methodology has been developed to quantify radiation-induced structural degradation.

The intrinsic defect-related luminescence of as-grown Gd₃Ga₅O₁₂ (GGG) single crystals was investigated using synchrotron excitation in the vacuum-ultraviolet region. The photoluminescence spectrum at 7 K exhibits a broad emission band centered near ~2.5 eV (≈500 nm), accompanied by narrow 4f–4f transitions of uncontrolled Tb³⁺ impurity ions. Gaussian decomposition of the broad band reveals three components at approximately 3.05, 2.70, and 2.50 eV, attributed to self-trapped excitons (STE), antisite-associated recombination, and F/F⁺-type oxygen-vacancy centers, respectively. Temperature-dependent photoluminescence measurements show that the defect-related luminescence is governed by two thermally activated nonradiative channels with activation energies of ~5–8 meV and 42–56 meV. The shallow channel dominates at 20–50 K, while the main quenching occurs between 100 and 150 K, leading to nearly complete suppression of the broad emission at room temperature. The present results therefore characterize the emission behavior of intrinsic, shallow defect centers in as-grown GGG and contribute to understanding defect recombination pathways that are critical for optimizing garnet-based scintillation and photonic materials.