Fig. 4: Crystal structure of H2CO3-Cmc21 and polymerized protonated \({[{{\rm{CO}}}_{4}]}^{4-}\)-tetrahedra.

a Orthorhombic crystal structure (Cmc21, Z = 4) of polymeric carbonic acid (H2CO3) at 40(2) GPa from single crystal structure refinement. b Polymerized protonated \({[{{\rm{CO}}}_{4}]}^{4-}\)-tetrahedra in the crystal structure of H2CO3-Cmc21 forming chains along the c-axes.