New High-Pressure Structures of Transition Metal Carbonates with O3C–CO3 Orthooxalate Groups

Based on the density functional theory and crystal structure prediction approaches, we found a novel high-pressure structure of Fe2CO4-P1¯. It is characterized by the presence of ethane-like O3C–CO3 groups or so-called orthooxalate groups. The formation of such O3C–CO3 groups has been proposed earlier in melts and aqueous carbonate solutions, but no such examples were known in inorganic crystalline materials. We found that this structure is dynamically and thermally stable at pressures of 50 GPa. Similar structures were also predicted to be dynamically stable for Mn2CO4, Ni2CO4, and Co2CO4. In addition, FeCO3 was found to transform into a similar structure with O3C–CO3 orthooxalate groups at a pressure above 275 GPa. Additionally, for the first time, we describe the self-diffusion of metal atoms in carbonates at high pressure and at high temperatures. The prediction of novel carbonate structures extends the crystal chemistry of inorganic carbonates beyond the established ones with [CO3] triangles, [C2O5] pyro-groups, and [CO4] tetrahedra

Novel Calcium sp 3 Carbonate CaC2_2O5_5-I4ˉI\bar{4}2d May Be a Carbon Host in Earth’s Lower Mantle

CaC$_2$O$_5$-$I\bar{4}$2d was obtained by reacting CO$_2$ and CaCO$_3$ at lower Earth mantle pressures and temperatures ranging between 34 and 45 GPa and between 2000 and 3000 K, respectively. The crystal structure was solved by single-crystal X-ray diffraction and contains carbon atoms tetrahedrally coordinated by oxygen. The tetrahedral CO$_4$$^{4−}$groups form pyramidal [C$_4$O10]$^{4−}$complex anions by corner sharing. Raman spectroscopy allows an unambiguous identification of this compound, and the experimentally determined spectra are in excellent agreement with Raman spectra obtained from density functional theory calculations. CaC$_2$O$_5$-$I\bar{4}$2d persists on pressure release down to ∼18 GPa at ambient temperature, where it decomposes into calcite and, presumably, CO$_2$ under ambient conditions. As polymorphs of CaCO$_3$ and CO$_2$ are believed to be present in the vicinity of subducting slabs within Earth’s lower mantle, they would react to give CaC$_2$O$_5$-$I\bar{4}$2d, which therefore needs to be considered instead of end-member CaCO$_3$ in models of the mantle mineralogy

First Finding of High-Pressure Modifications of Na₂CO₃ and K₂CO₃ with sp³‑Hybridized Carbon Atoms

The transition from structures with classical [CO3] triangles to structures with [CO4] tetrahedra, corresponding to the transition from sp2 to sp3 hybridization of carbon atoms, is quite well established for alkaline earth carbonates CaCO3 and MgCO3. Here, using a crystal structure prediction technique, we show that alkali carbonates Na2CO3 and K2CO3 follow the same trend. Both compounds form isostructural sp3-hybridized phases, Na2CO3–C2/m and K2CO3–C2/m, which became thermodynamically stable at pressures above 125 and 150 GPa, respectively. The automated topological search through ICSD has shown that the found C2/m structures, as well as sp3-structures of CaCO3 and MgCO3 do not have topological analogs among silicates and phosphates. Transitions of Na2CO3 and K2CO3 to C2/m structures are realized without sufficient perturbation of the initial Na2CO3–P21/m and K2CO3–P1̅ structures and require relatively small atomic displacements of carbon and oxygen atoms. These transitions are realized through simple energy optimization. This indicates the absence or low height of the energy barrier. In the wide interval of pressures before the transition to the sp3 structures, carbon atoms of [CO3] triangles are gradually displaced from the plane defined by three oxygen atoms due to the interaction with the fourth oxygen atom. In the case of Na2CO3, the dihedral angle C–O–O–O describing the degree of this displacement increases from 5 to 12°, when the pressure increases from 60 to 127 GPa. At pressures above 130 GPa, the angle abruptly increases to the value of 31°, which corresponds to the formation of the sp3-hybridized phase Na2CO3–C2/m. Based on the examples of alkali and alkaline earth carbonates, we show that the transition from a sp2-hybridized [CO3] triangle to a sp3-hybridized [CO4] tetrahedron is realized when the fourth oxygen atom approaches the carbon atom at a distance less than 2.0 Å, which is usually realized at pressures of around 100 GPa. The stable structures with sp3-hybridized carbon atoms have not been found for Li2CO3 in the considered pressure range up to 200 GPa, and we show that the P63/mcm structure of this compound is stable in sp2 form up to a pressure of 700 GPa or even higher. This indicates that not all the structures of carbonates adopt sp3 form even at extreme pressures