Anhydrous Aluminum Carbonates and Isostructural Compounds

Abstract

We synthesized the inorganic anhydrous aluminum carbonates Al2[C2O5][CO3]2 and Al2[CO3]3 by reacting Al2O3 with CO2 at high pressures and temperatures and characterized them by Raman spectroscopy. Their structures were solved by X-ray diffraction. Al2[CO3]3 forms at around 24–28 GPa, while Al2[C2O5][CO3]2 forms above 38(3) GPa. The distinguishing feature of the new Al2[C2O5][CO3]2-structure type is the presence of pyrocarbonate [C2O5]2–-groups, trigonal [CO3]2groups, and octahedrally coordinated trivalent cations. Al2[CO3]3 has isolated [CO3]2–-groups. Both Al-carbonates can be recovered under ambient conditions. Density functional theory calculations predict that CO2 will react with Fe2O3, Ti2O3, Ga2O3, In2O3, and MgSiO3 at high pressures to form compounds which are isostructural to Al2[C2O5][CO3]2. MgSi[C2O5][CO3]2 is predicted to be stable at pressures relative to abundant mantle minerals in the presence of CO2. This structure type allows the incorporation of four elements (Mg, Si, Fe, and Al) abundant in the Earth’s mantle in octahedral coordination and provides an alternative phase with novel carbon speciation for carbon storage in the deep Earth.

Twisted [C2O5]2−-groups in Ba[C2O5] pyrocarbonate

“…It is characterized by the presence of twisted pyrocarbonate anions. This is in contrast to other pyrocarbonates, 1,2,18 to the proposed theoretical equilibrium geometry of the [C 2 O 5 ] 2− -species 10,11 and in disagreement with the recent prediction of a monoclinic Ba[C 2 O 5 ]. 19 The ability of the [C 2 O 5 ] 2− -group to rotate the constituting [CO 3 ] 2− -groups relative to each other while maintaining the shared oxygen atom provides structural flexibility.…”

“…1c). 18 The maximal distances between oxygen atoms in the [C 2 O 5 ] 2− -groups (O⋯O) are similar in these pyrocarbonates (≈4.3 Å). While the current manuscript was under evaluation, a structure prediction for Ba[C 2 O 5 ] was published.…”

High-pressure synthesis of acentric sodium pyrocarbonate, Na2[C2O5]

“…3 Very recently, the synthesis of the mixed carbonate–pyrocarbonate Al 2 [C 2 O 5 ][CO 3 ] 2 showed that pyrocarbonates containing trivalent cations may also be obtained. 4…”

“…It was found that the geometry of the pyrocarbonate group can vary significantly by a tilt of the two planar [CO 3 ] 2− -groups of the [C 2 O 5 ] 2− -complex anion around the common central bridging oxygen atom. 1–4…”

“…Raman modes occurring at these wavenumbers are characteristic for vibrations of a [C 2 O 5 ] 2− -group. 1–4 Hence, we mapped the phase distribution across the gasket hole in order to locate the new pyrocarbonate for the subsequent single crystal X-ray diffraction experiments (Fig. 1c, e and f).…”