Density Effect on the Structure of Liquid SiO2 Compressed up to 10 Mbar

Abstract:

With the recent discovery of many exoplanets, modeling the interior of these celestial bodies is becoming a fascinating scientific challenge. In this context, it is crucial to accurately know the equations of state and the physical properties of the constituent materials. Among these constituents, complex silicates (i.e. (Mg,Fe)SiO₃ and (Mg,Fe)₂SiO₄) and the products of their dissociation (SiO₂, MgO) are of major interest. Their metallization, dissociation and structural properties as density and temperature increase are a central issue for the modeling of the mantle of terrestrial planets or the cores of giant and icy giant planets [1].

We present here an experimental and theoretical study of the electronic and ionic structural properties of fused silica compressed up to 10 Mbar using X-ray Absorption Near Edge Spectroscopy (XANES). The results were obtained in two experimental campaigns on the LULI2000 laser at the Ecole Polytechnique. With an approach previously tested on aluminium [2], we obtained high quality XANES data at different well-controlled temperature and density conditions in the Warm Dense Matter (WDM) regime. These conditions were obtained by using two different target’s geometries in order to probe either released matter either re-shocked matter. This last technique allows us to obtain higher pressures than on the Hugoniot for lower temperatures, more representatives of planetary conditions. With this process, we can compare XANES spectra on different isotherms between 1 eV and 4 eV on a large scale of densities (from r₀/2 to 4r₀). Coupled to ab initio calculations, these results provide us structural information on liquid SiO₂ such as the pair correlation functions or the associated Si-O bounding ordering of the system. The structural information obtained here allows us to improve the SiO₂ phase diagram particularly in the so-called bonded liquid region [3] and to confirm the loss of conductivity with the increase of density already observed in previous ab initio calculations [4].

REFERENCES

[1] K. Umemoto et al., Science 311, 983 (2006)

[2] A. Benuzzi-Mounaix et al., Phys. Rev. Lett, 107, 165006 (2011)

[3] D. G. Hicks et al, Phys. Rev. Lett. 97, 025502 (2006)

[4] S. Mazevet et al, submitted to PRL.