The known metal–C2O4 structures may be divided into two modifications, α and β. The α-modification has an order–disorder struxture, revealing one-dimensional disordering of the metal–oxalate chains, and the β-modification is ordered. The crystal structures of orthorhombic γ-MnC2O4 {poly[μ-oxalato-manganese(II)]; space group Pmna, a = 7.1333 (1), b = 5.8787 (1), c = 9.0186 (2) Å, V = 378.19 (1) Å3, Z = 4 and Dx
= 2.511 Mg m−3} and γ-CdC2O4 {poly[μ-oxalato-cadmium(II)]; space group Pmna, a = 7.3218 (1), b = 6.0231 (1), c = 9.2546 (2) Å, V = 408.13 (1) Å3, Z = 4 and Dx
= 3.262 Mg m−3} have been obtained from powder diffraction patterns. The structures are isostructural. Each metal atom in each structure is coordinated by seven O atoms which belong to five oxalate ions. The crystal packing, which contains noticeable cavities in the [101] and [001] directions, is not close packed and essentially differs from the known disordered α- and ordered β-modifications of transition metal oxalates. This modification seems to be metastable. It was found that a spontaneous γ→β phase transition takes place for γ-CdC2O4.
A Review on Transition-metal Oxalate Based Electrode for Supercapacitors