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3.4 i. Cu2+ substitution in braunite: Evidence of symmetry breaking resulting from Jahn-Teller induced distortions and cation ordering (R. Miletich, T.G. Sharp and F. Brunet)

Substitution of the eightfold-coordinated M2+ cations in braunite-type crystal structures, M[8]Mn6O6SiO4, are expected to modify the crystal structure and influence the high-pressure stability of the braunite structure type. Non-isostructural changes can be expected for Cu2+ substitution owing to the Jahn-Teller distortion of the Cu2+ cations. In order to investigate the influence of the Jahn-Teller distortion on the [4+4]-coordinated M2+ site geometry, the growth of CuMn6O8SiO4 (abswurmbachite) single crystals and their characterization by X-ray diffraction has been undertaken.

Synthesis experiments based on transport reactions of chlorine complexes were performed at T = 760° - 880 °C and P(Cl2) ≤ 20 bar in quartz-glass tubes, yielding braunite-abswurmbachite solid solutions, (Mn,Cu)Mn6O8SiO4, containing to 30 mol% of the Cu-end-member. High-pressure synthesis experiments carried out in the same temperature range at P = 9 - 12 kbar in a piston-cylinder apparatus increased the degree of Cu substitution to approximately 75 - 80 mol% which indicates that Cu-substitution is related to pressure. The pure endmember has not been obtained yet, therefore experiments at higher pressures (15 - 18 kbar) are planned.

X-ray diffraction studies on the synthesized solid-solutions indicate symmetry breaking: the ideal tetragonal I41/acd space-group symmetry of braunite is reduced to orthorhombic Ibca and Pcca symmetries. The actual coordination of the M = Cu2+ cations show a change from [4+4] in non-distorted braunite to [4+2(+2)] coordination polyhedra. This change is accompanied by a point-symmetry reduction from 222 to a simple twofold . . 2 symmetry. The distortion mechansim is based mainly on positional displacements of the M cation along [001] (by approximately 0.10 Å) off the ideal position in non-distorted braunite. Also the independent shifts of the coordinating oxygen atoms allowed by the increased degrees of freedom resulting from the point-symmetry reduction accommodate the changed bonding requirements of the Cu2+ cations. Thus the symmetry breaking appears to arise from the need to accommodate the Cu2+ cations in a Jahn-Teller distorted [4+2(+2)] environment which is more likely than the high-symmetry [4+4] coordination.

The distortional mechansim for the Cu cations explains the orthorhombic Ibca symmetry. On the other hand, Pcca symmetry was observed for solid solutions in the 50 mol% region. Moreover, an isomorphic Pcca superstructure with b' = 2b was observed for 75 % braunite/ 25 % abswurmbachite compositions. These additional reductions in symmetry can be attributed to cation ordering, as would be expected from the differences in the copper and manganese stereochemistry.

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