Revisiting EELS Fine Structure Analysis in Zircon as a Tool for Interpreting its Structural Evolution into a Disordered System

Zircon (ZrSiO4) is a mineral of singular importance in Earth science, the most widely used mineral in U-Pb geochronology in various geological contexts [1], including high-temperature and high-pressure events (e.g. impact events), and the oldest mineral ever found on Earth. However, the accumulation of radiation damage over time critically affects the physical and chemical properties of zircon crystals [2]. Experimental techniques capable of simultaneously providing structural and chemical information in these complex and tiny damaged areas are highly sought after. Fine chemical analysis using electron energy-loss spectroscopy (EELS) enables to link structure and chemistry in crystalline materials, at the high spatial resolution accessible in the TEM. The near-edge structures arising from core-level excitation in EELS contain such chemical information, since they are very sensitive to chemical bonding and local atomic environment. However, their robust interpretation in terms of orbital hybridizations between atoms requires first-principle simulations, and a correct understanding of the fine structures primarily depends on the agreement between experiments and calculations. In the energy range of EELS, the Si-L2,3 (99 eV) and O-K edges (532 eV) of zircon are easily accessible. Yet, thus far, only few studies have focused on the interpretation of the Si-L2,3 and O-K near-edge fine structures in zircon, using multiple scattering approaches [3-5]. These simulations have shown some agreement with experiments, but further improvements could enhance the understanding of the spectra, which depend on the knowledge of the crystal structure of the chosen sample. Here, instead of multiple scattering, we show that density functional theory reproduces the experimental peaks of the O-K fine structures remarkably well. This DFT approach allows to interpret each peak in terms of molecular orbitals, with a clear distinction between peaks linked to the O-Si and O-Zr hybridizations (see Fig. 1). Notably, the most intense peak is a signature of the O-Si bond. The discussion will be extended to the Si-L2,3 near-edge fine structures, while the low-loss part of the spectrum is also highly structured and rich in information. The experiments were performed using a monochromated probe-corrected FEI Titan and a probe-corrected C-FEG Jeol NeoARM, and the near-edge structure variations were enabled by direct electron detection using a Quantum Detector MerlinEELS. Advanced EELS processing was performed using statistical methods as implemented in Hyperspy [6], allowing spectral variations to be detected and mapped at the nanoscale. The potential and limitations of using EELS fine structure analysis to track zircon's structural evolution will be explored through examples that replicate Earth-like conditions [7]. Experimental and theoretical O-K (a) and Si-L23 (b) near-edge structures of zircon (QE: Quantum Espresso code), which crystallizes in a tetragonal structure with edge-sharing ZrO8 dodecahedra and SiO4 tetrahedra. The main hybridizations from DFT are indicated. (c) The modification of electron density due to core-hole screening highlights the higher degree of covalence of Si compared to Zr.

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