1 research outputs found
First Finding of High-Pressure Modifications of Na<sub>2</sub>CO<sub>3</sub> and K<sub>2</sub>CO<sub>3</sub> with sp<sup>3</sup>‑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 Ã…, 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