Refinement of the substructure and superstructure of romanechite

The substructure and superstructure of romanechite, (Ba,H₂O)₂Mn₅O₁₀, were refined using a crystal from Schneeburg, Germany. The subcell is monoclinic, space group C2/m, with a = 13.929(1) Å, b = 2.8459(4) Å, c = 9.678(1) Å, β = 92.39(1)°, Z = 2, and it refined to R = 0.036 for anisotropic temperature factors using 598 reflections. The Mn(2) octa-hedra of the structure show significant distortion consistent with the concentration of Mn³⁺ on the Mn(2) site.

The supercell, consisting of a tripling along b (i.e., the tunnel axis), results from ordering of Ba and H₂O along the tunnel lengths. Further, long-exposure diffraction patterns show modulated streaks along [201]*, implying only short-range ordering between the tunnels in one direction. A simplest monoclinic unit cell was chosen with space group C2/m and Z = 6. It was refined to R = 0.041 for anisotropic temperature factors using 954 reflections. Ba preferentially occupies the Ba(1) site, and correspondingly the closest framework oc-tahedra [Mn(2) site] are preferentially occupied by Mn³⁺.