SrF0.8(OH)0.2]2.526[Mn6O12]: Columnar Rock-Salt Fragments Inside the Todorokite-Type Tunnel Structure

Abstract

A new type of composite structure is described consisting of the todorokite-type [Mn 6 O 12 ] framework with large square tunnels accommodating columnar fragments of the rock-salt structure. The crystal structure of the new todorokite-type [SrF 0.8 (OH) 0.2 ] 2.526 [Mn 6 O 12 ] compound is solved from transmission electron microscopy, and the structure of its anhydrous form [SrF 0.8 O 0.1 ] 2.526 [Mn 6 O 12 ] is refined from X-ray powder diffraction data. The [Mn 6 O 12 ] framework consists of mutually perpendicular walls built of three edge-sharing rutile-type strings of MnO 6 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 = γc 1 * (y = 0.63157(3)) parallel to the direction of tunnel propagation. The octahedral tunnel walls compose subsystem I with a [Mn 6 Ο 12 ] composition and a periodicity c 1 = 2.84 A, whereas the [Sr(F,OH)] 4 columns belong to subsystem II with a periodicity c 1 /γ = 4.49 A, resulting in a [Sr(F,-OH)] 4γ [Mn 6 O 12 ] composition. [SrF 0.8 (OH) 0.2 ] 2.526 [Mn 6 O 12 ] demonstrates a much larger number of cations inside the tunnels in comparison with the known synthetic and natural marine todorokites. The [SrF 0.8 -(OH) 0.2 ] 2.526 [Mn 6 O 12 ] compound shows a spin-glass behavior below T* ≈ 26 K with a dominant antiferromagnetic correlation.