Mixed Coordination Silica at Megabar Pressure

Cong Liu, Jiuyang Shi, Hao Gao, Junjie Wang, Yu Han, Xiancai Lu, Hui-Tian Wang, Dingyu Xing, and Jian Sun

Phys. Rev. Lett. 126, 035701 – Published 20 January 2021

Abstract

Silica ( ${SiO}_{2}$ ), as a raw material of silicon, glass, ceramics, abrasive, and refractory substances, etc., is of significant importance in industrial applications and fundamental research such as electronics and planetary science. Here, using a crystal structure searching method and first-principles calculations, we predicted that a ground state crystalline phase of silica with $R \bar{3}$ symmetry is stable at around 645–890 GPa, which contains six-, eight-, and nine-coordinated silicon atoms and results in an average coordination number of eight. This mixed-coordination silica fills in the density, electronic band gap, and coordination number gaps between the previously known sixfold pyrite-type and ninefold ${Fe}_{2} P$ -type phases, and may appear in the core or mantle of super-Earth exoplanets, or even the solar giant planets such as the Neptune. In addition, we also found that some silicon superoxides, Cmcm ${SiO}_{3}$ and Ccce ${SiO}_{6}$ , are stable in this pressure range and may appear in an oxygen-rich environment. Our finding enriches the high-pressure phase diagram of silicon oxides and improves understanding of the interior structure of giant planets in our solar system.

Received 22 September 2020
Accepted 24 December 2020

DOI:https://doi.org/10.1103/PhysRevLett.126.035701

Physics Subject Headings (PhySH)

Crystal structure Pressure effects Structural phase transition

Core of giant planets

First-principles calculations

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Cong Liu¹, Jiuyang Shi¹, Hao Gao¹, Junjie Wang¹, Yu Han¹, Xiancai Lu², Hui-Tian Wang¹, Dingyu Xing¹, and Jian Sun ^1,*

¹National Laboratory of Solid State Microstructures, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
²State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China

^*Corresponding author. jiansun@nju.edu.cn

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Vol. 126, Iss. 3 — 22 January 2021

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Images

Figure 1
Energetics, dynamical stability, and crystal structures of the $R \bar{3}$ phase silica. (a) Enthalpies of the silica phases of interest relative to the pyrite-type structure in the pressure range between 500 and 1000 GPa. (b) Phonon spectra of the $R \bar{3}$ phase at 700 GPa. Red and orange lines represent the acoustic and optical branches, respectively. (c) Crystal structure of the predicted $R \bar{3}$ phase, where blue and red spheres represent Si and O atoms, respectively. There are three kinds of different polyhedra containing silicon atoms of sixfold (Si1, blue), eightfold (Si2, cyan), and ninefold (Si3, green), respectively, and all oxygen atoms are fourfold and form tetrahedra (white).
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Figure 2
Compression behaviors of the interested silica phases under high pressures, including (a) density, (b) band gap, and (c) effective coordination number analysis. Vertical dashed lines denote the silica phase transition pressures from the pyrite-type to the $R \bar{3}$ phase and from the $R \bar{3}$ phase to the ${Fe}_{2} P$ -type. Magenta (brown) line in (c) corresponds to the ideal sixfold (ninefold) of silicon atoms in the silica crystals.
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Figure 3
Energetics of the Si-O system and crystal structures of the most stable compounds at 400–1000 GPa. (a) Convex hulls for formation enthalpies ( $Δ H_{f}$ , relative to the most stable phases of ${SiO}_{2}$ [11, 12, 14, 15] and $O_{2}$ [31]) at different pressures. (b) Pressure-composition phase diagram of the Si-O compounds from 400 to 1000 GPa. The crystal structures of the Cmcm phase ${SiO}_{3}$ (c) and the Ccce phase ${SiO}_{6}$ (d) at 700 GPa, as well as corresponding coordination of silicon and oxygen atoms.
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Figure 4
Proposed pressure-temperature phase diagrams of (a) silica, (b) ${SiO}_{3}$ , and (c) ${SiO}_{6}$ at planetary core conditions. The melting curve (dark green dashed line) of silica is obtained from shock wave experiments [33]. The phase boundary lines are marked with black solid lines and dashed boundaries (indicate pressure-temperature conditions out of the QHA validity range). Small brown or blue squares represent the pressure-temperature conditions at the core-mantle boundary (CMB) in terrestrial or ocean type exoplanets by Sotin et al. [34], and green circles represent the CMB conditions in terrestrial type exoplanets by Hakim et al. [35] and van den Berg et al. [36], respectively. Numbers inside the small squares and circles represent the planet mass in units of Earth mass ( $M_{\oplus}$ ). Red squares and circles denote the estimated pressure-temperature conditions at CMB in the solar giant planets (e.g., Uranus, Neptune, and Saturn), respectively, by Guillot et al. [37] and Nettelmann et al. [38].
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