Anhydrous Aluminum Carbonates and Isostructural Compounds.

We synthesized the inorganic anhydrous aluminum carbonates Al 2 [C 2 O 5 ][CO 3 ] 2 and Al 2 [CO 3 ] 3 by reacting Al 2 O 3 with CO 2 at high pressures and temperatures and characterized them by Raman spectroscopy. Their structures were solved by X-ray diffraction. Al 2 [CO 3 ] 3 forms at around 24-28 GPa, while Al 2 [C 2 O 5 ][CO 3 ] 2 forms above 38(3) GPa. The distinguishing feature of the new Al 2 [C 2 O 5 ][CO 3 ] 2 -structure type is the presence of pyrocarbonate [C 2 O 5 ] 2- -groups, trigonal [CO 3 ] 2─ groups, and octahedrally coordinated trivalent cations. Al 2 [CO 3 ] 3 has isolated [CO 3 ] 2- -groups. Both Al-carbonates can be recovered under ambient conditions. Density functional theory calculations predict that CO 2 will react with Fe 2 O 3 , Ti 2 O 3 , Ga 2 O 3 , In 2 O 3 , and MgSiO 3 at high pressures to form compounds which are isostructural to Al 2 [C 2 O 5 ][CO 3 ] 2 . MgSi[C 2 O 5 ][CO 3 ] 2 is predicted to be stable at pressures relative to abundant mantle minerals in the presence of CO 2 . 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.