Radiation-Induced Disorder and Lattice Relaxation in Gd₃Ga₅O₁₂ Under Swift Xe Ion Irradiation

This study presents a comprehensive Raman spectroscopic and mechanical investigation of Gd3Ga5O12 (GGG) single crystals irradiated with 231 MeV Xe 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, confirming the structural stability of the lattice without formation 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 (A1g, T2g ~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). 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 method integrates Raman spectroscopic diagnostics with mechanical nanoindentation data through a unified normalization and logarithmic-slope formalism, enabling precise correlation between vibrational and mechanical responses.