Silicon carbonate phase formed from carbon dioxide and silica under pressure

Abstract

The discovery of nonmolecular carbon dioxide under high-pressure conditions shows that there are remarkable analogies between this important substance and other group IV oxides. A natural and long-standing question is whether compounds between CO 2 and SiO 2 are possible. Under ambient conditions, CO 2 and SiO 2 are thermodynamically stable and do not react with each other. We show that reactions occur at high pressures indicating that silica can behave in a manner similar to ionic metal oxides that form carbonates at room pressure. A silicon carbonate phase was synthesized by reacting silicalite, a microporous SiO 2 zeolite, and molecular CO 2 that fills the pores, in diamond anvil cells at 18-26 GPa and 600-980 K; the compound was then temperature quenched. The material was characterized by Raman and IR spectroscopy, and synchrotron X-ray diffraction. The experiments reveal unique oxide chemistry at high pressures and the potential for synthesis of a class of previously uncharacterized materials. There are also potential implications for CO 2 segregation in planetary interiors and for CO 2 storage.high-pressure chemistry | material science | optical spectroscopy C arbon dioxide and silicon dioxide are two archetypal, group IV oxides of paramount importance for fundamental and applied chemistry and planetary sciences. CO 2 is the dominant component of the atmosphere of Earth-like planets, exists in icy forms in outer planets and asteroids, plays an important role in volcanic and seismic processes, is used as a supercritical solvent for chemical reactions, and its anthropogenic production is a major environmental issue. SiO 2 is also one of the most abundant components of terrestrial planets, and an important technological material. The chemical relationship between CO 2 and SiO 2 , in particular their reactivity, is thus of interest. Although the two systems are both group IV oxides, they are remarkably different under ambient conditions, because CO 2 is molecular and is held together by C═O double bonds, whereas SiO 2 forms network structures involving single Si─O bonds. These bonding patterns radically change under pressure. Two nonmolecular CO 2 crystalline phases and a glassy form have been discovered above 30 GPa that bear similarities to SiO 2 (1-14). The crystalline phases contain carbon in fourfold coordination by oxygen (1-10, 13, 14), and mixtures of CO 4 and CO 3 units have been found in a glassy form that has been named carbonia (13).A possible approach to favor the chemical reaction between CO 2 and SiO 2 is to select a microporous silica polymorph, such as silicalite. At ambient conditions, silicalite is characterized by a framework of four-, five-, six-, and ten-membered rings of SiO 4 tetrahedra with 5.5-Å pores (15) (Fig. 1, Left Inset). Recently, it has been found that the pores can be completely filled by simple molecules such as CO 2 and Ar under pressure, which prevents pressure-induced amorphization (PIA) and stabilizes the crystalline framework up to at least 25 GPa at room temperature (1...

Unraveling Crystalline Structure of High-Pressure Phase of Silicon Carbonate

“…The structural search is carried out for all solids under strong compression, i.e., under a high pressure of 20 GPa. This pressure is close to the reactive pressure of SiO 2 and CO 2 implemented in the experiment [20].…”

“…However, the calculated formation energies for all the structures predicted are positive, indicating that the predicted structures are all metastable thus far. Recently, Santoro et al [20] successfully synthesized a silicon-carbonate phase through the reaction between CO 2 and SiO 2 under highpressure conditions, which demonstrates that it is possible to synthesize various crystalline phases of Si x C 1−x O 2 compounds in the laboratory.…”

Carbon substitution for oxygen in silicates in planetary interiors

“…The substitution of tetrahedral (sp 3 ) carbon for silicon in silicate minerals and melts has often been suggested as a possibility at high pressure (5-13), but there was little experimental evidence for it until recent phase equilibrium studies indicated that CO 2 can undergo transformation from a molecular gas to an extended solid at very high pressure and temperature to form a cristobalite-like structure consisting of a network of corner-sharing CO 4 tetrahedra (13,14). Recent studies have also indicated the possibility of a chemical reaction between SiO 2 and CO 2 to form a silicon carbonate phase under pressure (12) and the possibility of amorphous solid phases (7,11). However, the experimental results and their structural interpretations have remained somewhat controversial, and, in any event, such reactions appear to require pressures near a megabar, probably making them unimportant for most of the crust and mantle.…”

Stability of dense liquid carbon dioxide

“…Experimental (11) and theoretical (13) works suggest that CO 2 is produced at high pressure (P) and temperature (T) during decarbonating reactions with silica in subducted basalts and is subsequently released into the ocean and atmosphere during volcanic activity (8). Moreover, reactions between silica and free CO 2 may also take place under such conditions, leading to the formation of silicon carbonates (15). Whether free CO 2 is stable or decomposes into oxygen and diamond in the mantle is currently unclear (11,12,16,17).…”