Acta Crystallographica Section C

Crystal Structure Communications

Volume 64, Part 3 (March 2008)

inorganic compounds

Acta Cryst. (2008). C64, i21-i26 [ doi:10.1107/S0108270107068011 ]

Synthetic aenigmatite analog Na₂(Mn_5.26Na_0.74)Ge₆O₂₀: structure and crystal chemical considerations

G. J. Redhammer, G. Roth, D. Topa and G. Amthauer

Abstract: Disodium hexamanganese(II,III) germanate is the first aenigmatite-type compound with significant amounts of manganese. Na₂(Mn_5.26Na_0.74)Ge₆O₂₀ is triclinic and contains two different Na positions, six Ge positions and 20 O positions (all with site symmetry 1 on general position 2i of space group P $\overline{1}$ ). Five out of the seven M positions are also on general position 2i, while the remaining two have site symmetry $\overline{1}$ (Wyckoff positions 1f and 1c). The structure can be described in terms of two different layers, A and B, stacked along the [011] direction. Layer A contains pyroxene-like chains and isolated octahedra, while layer B is built up by slabs of edge-sharing octahedra connected to one another by bands of Na polyhedra. The GeO₄ tetrahedra show slight polyhedral distortion and are among the most regular found so far in germanate compounds. The M sites of layer A are occupied by highly charged (trivalent) cations, while in layer B a central pyroxene-like zigzag chain can be identified, which contains divalent (or low-charged) cations. This applies to the aenigmatite-type compounds in general and to the title compound in particular.

Formula: Na₂(Mn_5.26Na_0.74)Ge₆O₂₀