Anhydrous Aluminum Carbonates and Isostructural Compounds

We synthesized the inorganic anhydrousaluminum carbonatesAl(2)[C2O5][CO3](2) andAl(2)[CO3](3) by reacting Al2O3 with CO2 at high pressures and temperaturesand characterized them by Raman spectroscopy. Their structures weresolved by X-ray diffraction. Al-2[CO3](3) forms at around 24-28 GPa, while Al-2[C2O5][CO3](2) forms above 38(3) GPa.The distinguishing feature of the new Al-2[C2O5][CO3](2)-structure type is thepresence of pyrocarbonate [C2O5](2-)-groups, trigonal [CO3](2 R)groups, andoctahedrally coordinated trivalent cations. Al-2[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 whichare isostructural to Al-2[C2O5][CO3](2). MgSi[C2O5][CO3](2) is predicted to be stable at pressures relative toabundant mantle minerals in the presence of CO2. This structuretype allows the incorporation of four elements (Mg, Si, Fe, and Al)abundant in the Earth's mantle in octahedral coordination andprovides an alternative phase with novel carbon speciation for carbonstorage in the deep Earth. Twonew aluminum carbonates (Al-2[C2O5][CO3](2) and Al-2[CO3](3)) were synthesized in a laser-heated diamondanvil cell at high pressures and temperatures from Al2O3 and CO2. Al-2[C2O5][CO3](2) contains [C2O5](2-)-, [CO3](2 R)groups,and octahedrally coordinated trivalent cations. Al-2[CO3](3) has isolated [CO3](2-)-groups. Both Al-carbonates can be recovered under ambient conditions.DFT calculations predict that pyrocarbonates with Fe3+,Ti3+, Ga3+, In3+, or Mg2+/Si4+ instead of Al3+ may be stable as high-pressurephases.