First Finding of High-Pressure Modifications of Na2CO3and K2CO3with sp3-Hybridized Carbon Atoms

The transition from structures with classical [CO3] triangles to structures with [CO4] tetrahedra, corresponding to the transition from sp2to sp3hybridization of carbon atoms, is quite well established for alkaline earth carbonates CaCO3and MgCO3. Here, using a crystal structure prediction technique, we show that alkali carbonates Na2CO3and K2CO3follow the same trend. Both compounds form isostructural sp3-hybridized phases, Na2CO3–C2/mand 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/mstructures, as well as sp3-structures of CaCO3and MgCO3do not have topological analogs among silicates and phosphates. Transitions of Na2CO3and K2CO3to C2/mstructures are realized without sufficient perturbation of the initial Na2CO3–P21/mand 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 sp3structures, 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 Li2CO3in the considered pressure range up to 200 GPa, and we show that the P63/mcmstructure of this compound is stable in sp2form up to a pressure of 700 GPa or even higher. This indicates that not all the structures of carbonates adopt sp3form even at extreme pressures.