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 similar to 4 x 10(18) 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 degrees 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 degrees 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, inparticular 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 degrees 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 degrees C and mild final shrinking at T-a = 1000-1400 degrees 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.

Automated summary

The study focused on the structural reconstitution of gem-quality zircon from Sri Lanka after dry thermal annealing, revealing that annealing at different temperatures resulted in the recovery and changes of crystalline and amorphous fractions, accompanied by variations in unit-cell volume and dimensions.