Fig. 5: Geometry of a protonated \({[{{\rm{CO}}}_{4}]}^{4-}\)-tetrahedron and eigenvectors of the atomic displacements of selected characteristic Raman modes.

a Geometry of one protonated \({[{{\rm{CO}}}_{4}]}^{4-}\)-tetrahedron in the crystal structure of H2CO3-Cmc21 from single crystal structure refinement at 40(2) GPa. Eigenvector of the atomic displacements in polymeric carbonic acid for the characteristic Raman mode b at ≈820 cm−1 and c at ≈910 cm−1 from DFPT calculations.