Infrared spectroscopic analysis of zircon: Radiation damage and the metamict state

Radiation damage and the nature of the metamict state in natural zircons were studied and analysed using reflection and absorption infrared spectroscopy. IR bands of crystalline zircon in the far infrared region remain detectable in highly metamict samples. This suggests that some local order or short-range order persists around Zr atoms in the amorphized phase. Signals due to Si-O-Si linkages, which do not exist in crystalline zircon, were observed in the regions of 500-800 cm(-1) and 1000-1300 cm(-1). The results suggest the appearance of high-Q species and a complex polymerization state in the metamict state. The dielectric constant (epsilon = epsilon ' + i epsilon ") and energy loss function (-Im(1/epsilon)) were obtained through Kramers-Kronig analysis. Radiation damage leads to significant and continuous variations in epsilon and -Im(1/epsilon). IR spectra of damaged samples were analysed using the effective-medium approach. It was found that IR signals of damaged zircons consist of two principal components: one with broad spectral features from the amorphized/metamict material and the other with relatively sharp lines from crystalline material with various degrees of structural distortion. The former signals increase in intensity with increasing dose while the latter decrease in intensity. The signals from the amorphized phase have been detected in samples with radiation dose as low as 1.5 x 10(18) alpha -events g(-1). The dose dependence of the fraction of the amorphized phase was extracted. The result confirms earlier analysis in x-ray diffraction and NMR studies. The IR data show a feature that may be due to the existence of an intermediate phase in moderately damaged samples. An extra, sharp IR band near 796 cm(-1), which shows no detectable orientation dependence, is observed in all the samples with intermediate. degrees of damage and its intensity shows a systematic change with increasing dose-an increase followed by a decrease. This band is absent or very weak in both undamaged and very heavily damaged samples.