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.