Unraveling crystalline structure of high-pressure phase of silicon carbonate

Although CO₂ and SiO₂ both belong to group-IV oxides, they exhibit remarkably different bonding characteristics and phase behavior at ambient conditions. At room temperature, CO₂ is a gas, whereas SiO₂ is a covalent solid with rich polymorphs. A recent successful synthesis of the silicon-carbonate solid from the reaction between CO₂ and SiO₂ under high pressure [M. Santoro et al., Proc. Natl. Acad. Sci. U.S.A. 108, 7689 (2011)] has resolved a long-standing puzzle regarding whether a Si_xC_1-xO₂ compound between CO₂ and SiO₂ exists in nature. Nevertheless, the detailed atomic structure of the Si_xC_1-xO₂ crystal is still unknown. Here, we report an extensive search for the high-pressure crystalline structures of the Si_xC_1-xO₂ compound with various stoichiometric ratios (SiO₂:CO₂) using an evolutionary algorithm. Based on the low-enthalpy structures obtained for each given stoichiometric ratio, several generic structural features and bonding characteristics of Si and C in the high-pressure phases are identified. The computed formation enthalpies show that the SiC2O6 compound with a multislab three-dimensional (3D) structure is energetically the most favorable at 20 GPa. Hence, a stable crystalline structure of the elusive Si_xC_1-xO₂ compound under high pressure is predicted and awaiting future experimental confirmation. The SiC₂O₆ crystal is an insulator with elastic constants comparable to typical hard solids, and it possesses nearly isotropic tensile strength as well as extremely low shear strength in the 2D plane, suggesting that the multislab 3D crystal is a promising solid lubricant. These valuable mechanical and electronic properties endow the SiC₂O₆ crystal for potential applications in tribology and nanoelectronic devices, or as a stable solid-state form for CO₂ sequestration.