Effect of defects on the Raman-active modes in zircon (ZrSiO₄): a first-principles study

Zircon (ZrSiO₄) is a ubiquitous Earth’s crust mineral crucial for geochronology due to it’s natural inclusion of radioactive elements. For the same reason, zircon was suggested as a candidate host material for immobilization of radioactive waste. However, over the course of a zircon’s lifetime it’s crystal structure is being slowly destroyed due to internal radiation damage. This effect, known as metamictization, is focus of increased interest in the research community.
Here, we present a first-principles study on the effects of various substitutional defects, including radioactive elements like U and Th, on the Raman active modes in zircon. We find excellent quantitative agreement of the slopes ∂ω/∂c_U (where c_U is the molar concentration of uranium) with the experimental data of Geisler et al. (Eur. J. Mineral. 17, 883 (2005)) who performed Raman measurements in high-uranium zircon originating from the Chernobyl «lava». Based on this result, we predict the corresponding shift of Raman active modes in zircon when other extrinsic defects in the zircon lattice are considered. Our study provides the first quantum-mechanical atomistic understanding of the processes governing the changes in Raman signature in defective zircon crystals.