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

Abstract

The transition from structures with classical [CO 3 ] triangles to structures with [CO 4 ] tetrahedra, corresponding to the transition from sp 2 to sp 3 hybridization of carbon atoms, is quite well established for alkaline earth carbonates CaCO 3 and MgCO 3 . Here, using a crystal structure prediction technique, we show that alkali carbonates Na 2 CO 3 and K 2 CO 3 follow the same trend. Both compounds form isostructural sp 3 -hybridized phases, Na 2 CO 3 −C2/m and K 2 CO 3 −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 sp 3 -structures of CaCO 3 and MgCO 3 do not have topological analogs among silicates and phosphates. Transitions of Na 2 CO 3 and K 2 CO 3 to C2/m structures are realized without sufficient perturbation of the initial Na 2 CO 3 −P2 1 /m and K 2 CO 3 −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 sp 3 structures, carbon atoms of [CO 3 ] 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 Na 2 CO 3 , 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 sp 3hybridized phase Na 2 CO 3 −C2/m. Based on the examples of alkali and alkaline earth carbonates, we show that the transition from a sp 2 -hybridized [CO 3 ] triangle to a sp 3 -hybridized [CO 4 ] 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 sp 3 -hybridized carbon atoms have not been found for Li 2 CO 3 in the considered pressure range up to 200 GPa, and we show that the P6 3 /mcm structure of this compound is stable in sp 2 form up to a pressure of 700 GPa or even higher. This indicates that not all the structures of carbonates adopt sp 3 form even at extreme pressures.

Pb2[C2O6]-P3̄m1: new insights into the high-pressure behavior of carbonates

“…Previous studies of the behavior of alkaline-earth and alkali metal carbonates at high pressures have revealed several unexpected crystal chemical features. The high-pressure modifications of calcium, 12,37 magnesium, 12,13,38 sodium and potassium 14 carbonates contain carbon atoms in tetrahedral coordination. Moreover, it was shown that [CO 4 ] tetrahedra in these structures polymerize.…”

“…For instance, in the case of CaCO 3 , the structure with sp 3 -hybridyzed carbons occurs above 75 GPa, 12 while in the case of MgCO 3 , it occurs above 80 GPa 12,13 and in the case of Na 2 CO 3 and K 2 CO 3 , it appears above 100 GPa. 14 However, in some cases, such as SrCO 3 , 15 BaCO 3 15 and Li 2 CO 3 , 14 carbon remains in the sp 2 -hybridized state and [CO 4 ] tetrahedra are not observed up to 200 GPa. The present study aims to investigate whether the transition of carbon atoms to the sp 3 -hybridized state will be observed in the case of lead carbonate in the unexplored high-pressure region.…”