[SrF0.8(OH)0.2]2.526[Mn6O12]:: Columnar rock-salt fragments inside the todorokite-type tunnel structure

A new type of composite structure is described consisting of the todorokite-type [Mn6O12] framework with large square tunnels accommodating columnar fragments of the rock-salt structure. The crystal structure of the new todorokite-type [SrF0.8(OH)(0.2)](2.526)[Mn6O12] compound is solved from transmission electron microscopy, and the structure of its anhydrous form [SrF0.8O0.1](2.526)[Mn6O12] is refined from X-ray powder diffraction data. The [Mn6O12] framework consists of mutually perpendicular walls built of three edge-sharing rutile-type strings of MnO6 octahedra delimiting large square tunnels with the size of 3 x 3 octahedra. The interior space in the tunnels is filled with rock-salt type [Sr(F,OH)](4) columns. The structure can be interpreted as being an incommensurate composite structure with the modulation vector q(1) = gamma c(1)* (gamma = 0.63157(3)) parallel to the direction of tunnel propagation. The octahedral tunnel walls compose subsystem I with a [Mn6O12] composition and a periodicity c(1) = 2.84 angstrom, whereas the [Sr(F,OH)](4) columns belong to subsystem II with a periodicity c(1)/gamma = 4.49 angstrom, resulting in a [Sr(F,OH)](4 gamma)[Mn6O12] composition. [SrF0.8(OH)(0.2)](2.526)[Mn6O12] demonstrates a much larger number of cations inside the tunnels in comparison with the known synthetic and natural marine todorokites. The [SrF0.8(OH)(0.2)](2.526)[Mn6O12] compound shows a spin-glass behavior below T* approximate to 26 K with a dominant antiferromagnetic correlation.