Influence of Structural Disorder on Self-Trapped Exciton Luminescence in Natural Quartz

In this study, Radioluminescence (RL), Raman and Fourier Transform Infrared (FTIR) spectroscopy analyses were used to investigate the relationship between structural disorder and self-trapped exciton (STE) luminescence in natural quartz grains measured over a temperature range of 8-300 K. The investigated samples originate from diverse geological settings with crystallisation ages spanning from Ma to Ga and include two granites, one metamorphosed granite and two sedimentary samples. The RL spectra exhibit a dominant blue emission band centred at ~2.6 eV, attributed to STE emission, which disappears at higher temperatures (> 200 K). Analysis of RL intensity as a function of temperature (50-150 K) using an Arrhenius model yields quenching energies in the range of 22-46 meV. The variation of full width at half maximum (FWHM) with temperature was modelled to estimate phonon energies, which ranged from ~10 to 47 meV. Raman spectroscopy reveals systematic shifts and broadening of the 127 cm-1 and 203 cm-1 bands, indicating variations in lattice strain and structural disorder. FTIR measurements further demonstrate progressive changes in the structural organisation of the Si-O-Si framework through variations in structural order index derived from the 778 cm-1 and 692 cm-1 vibrational bands. Well-ordered natural quartz samples exhibited higher quenching and phonon energies, narrow RL, Raman and FTIR bands and higher FTIR structural order indexes, indicating more stable STEs than the structurally disordered natural quartz samples. Compared to synthetic quartz reported in the literature, natural quartz samples exhibit broader RL emission bands, lower quenching energies and earlier thermal quenching, reflecting a higher degree of structural disorder and defect concentration. The combined RL, Raman, and FTIR results demonstrate that STE emission in natural quartz is strongly controlled by lattice disorder and geological history, confirming the critical role of exciton-phonon interactions in determining STE luminescence behaviour.