Effect of substitutional impurities on vibrational properties of zircon: A first-principles study

Density-functional theory was used to investigate the effect of atomic impurities on the structural and vibrational properties of zircon (tetragonal ZrSiO₄). Atomic impurities considered include radioactive elements U and Th, as well as Hf, Sn, and Ti, substituted on the Zr-site. Using the supercell approach to model a range of substitutional concentrations, impurities were found to cause changes in the volume of the host lattice. This effect was shown to be partially equivalent to the application of a lattice strain. This quantum-based finding is in excellent agreement with the heuristic lattice-strain model traditionally employed in the geosciences to account for the compatibility of impurities in host lattices. Vibrational properties of substituted zircon were also investigated in order to provide a quantum mechanical understanding of Raman spectroscopy measurements on natural zircon. The computational analysis reproduces existing experimental data reported for uranium-substituted zircon and provides general predictive trends for other impurities including Th, Hf, Sn, and Ti. The insights gained by this study regarding the Raman signature of the presence of substitutional impurities set the groundwork for future study of the more substantial lattice disruptions that characterize radiation damage due to alpha decay in zircon.