Dry annealing of radiation-damaged zircon: single-crystal X-ray and Raman spectroscopy study

Structural reconstitution upon dry thermal annealing of mildly to strongly radiation-damaged, gem-quality zircon from Sri Lanka has been studied by single-crystal X-ray diffraction and Raman spectroscopy. Results of structure refinement of a strongly radiation-damaged zircon (sample GZ5, calculated alpha dose ~4 × 10¹⁸ g⁻¹) indicate the existence of an interstitial oxygen site that is sparsely occupied (about 4% of all O atoms). Annealing of this sample at T_a (annealing temperature) = 700 °C has resulted in nearly complete recrystallization of its amorphous volume fraction and significant decrease in the occupation of O-interstitial sites. For all samples studied, annealing up to T_a ≤ 650–700 °C is characterised by preferred recovery of Raman shifts (compared to Raman FWHMs; full width at half band maximum) and extensive contraction of the unit-cell volume, in particular along unit-cell dimension a. This low-T annealing is dominated by epitaxial growth of the crystalline volume fraction at the expense of the amorphous volume fraction, and general recovery of low-energy defects. During annealing at T_a = 700–1400 °C there is preferred recovery of Raman FWHMs (compared to Raman shifts) and only mild unit-cell contraction. High-T annealing is dominated by the recovery of high-energy defects such as recombination of cation Frenkel pairs. Here, unit-cell parameter a shows a remarkable behaviour (namely, mild re-increase at T_a = 700–1150 °C and mild final shrinking at T_a = 1000–1400 °C), which is attributed to enhanced contortion of ZrO₈ polyhedrons due to cation repulsion. The combined data set of Raman band and unit-cell parameter presented herein will help analysts to assign Raman spectra of annealed unknowns to certain recovery stages.