Structure disordering and thermal decomposition of manganese oxalate dihydrate, MnC₂O₄·2H₂O

It is found that the known regular structures of MnC₂O₄·2H₂O (I) do not allow to refine the powder X-ray pattern of (I) properly using the Rietveld method. Implementation of order-disorder scheme [28] via the including of appropriate displacement vector improves the refinement results. Also it is found that in the case of (I) the similar improvement may be achieved using the data on two phases of (I) obtained as result of decomposition MnC₂O₄·3H₂O single crystal in the mother solution after growth. Thermal decomposition of (I) produce the anhydrous γ-MnC₂O₄ (II) the structure of which is differ from the known α- and β-modifications of VIIIb transition metal oxalates. The solved ab initio from the powder pattern structure (II) (space group Pmna, a = 7.1333 (1), b = 5.8787 (1), c = 9.0186 (2) Å, V = 378.19 (1) ų, Z = 4 and D_x = 2.511 Mg m⁻³) contains seven-coordinated Mn atoms with Mn–O distances of 2.110–2.358 Å, and is not close-packed. Thermal decomposition of (II) in air flows via forming of amorphous MnO, the heating of which up to 723 K is accompanied by oxidation of MnO to Mn₂O₃ and further recrystallization of the latter.