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

Wepredict new stable sp(3)-hybridized structuresof Na2CO3 and K2CO3, appearingat pressures above 100 GPa. The transition to sp(3) structuresis prevailed by the gradual deformation of [CO3] triangles,observed in the wide interval of pressures. The transition from structures withclassical [CO3]triangles to structures with [CO4] tetrahedra, correspondingto the transition from sp(2) to sp(3) hybridizationof carbon atoms, is quite well established for alkaline earth carbonatesCaCO(3) and MgCO3. Here, using a crystal structureprediction technique, we show that alkali carbonates Na2CO3 and K2CO3 follow the same trend.Both compounds form isostructural sp(3)-hybridized phases,Na2CO3-C2/m and K2CO3-C2/m, which became thermodynamically stable at pressures above125 and 150 GPa, respectively. The automated topological search throughICSD has shown that the found C2/m structures, as well as sp(3)-structures of CaCO3 and MgCO3 do not have topological analogs among silicatesand phosphates. Transitions of Na2CO3 and K2CO3 to C2/m structuresare realized without sufficient perturbation of the initial Na2CO3-P2(1)/m and K2CO3-P1 structures and require relatively small atomic displacementsof carbon and oxygen atoms. These transitions are realized throughsimple energy optimization. This indicates the absence or low heightof the energy barrier. In the wide interval of pressures before thetransition to the sp(3) structures, carbon atoms of [CO3] triangles are gradually displaced from the plane definedby three oxygen atoms due to the interaction with the fourth oxygenatom. In the case of Na2CO3, the dihedral angleC-O-O-O describing the degree of this displacementincreases from 5 to 12 & DEG;, when the pressure increases from 60to 127 GPa. At pressures above 130 GPa, the angle abruptly increasesto the value of 31 & DEG;, which corresponds to the formation of thesp(3)-hybridized phase Na2CO3-C2/m. Based on the examples of alkali andalkaline earth carbonates, we show that the transition from a sp(2)-hybridized [CO3] triangle to a sp(3)-hybridized[CO4] tetrahedron is realized when the fourth oxygen atomapproaches the carbon atom at a distance less than 2.0 & ANGS;, whichis usually realized at pressures of around 100 GPa. The stable structureswith sp(3)-hybridized carbon atoms have not been found forLi(2)CO(3) in the considered pressure range up to200 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 indicatesthat not all the structures of carbonates adopt sp(3) formeven at extreme pressures.