[SrF 0.8 (OH) 0.2 ] 2.526 [Mn 6 O 12 ]: Columnar Rock-Salt Fragments Inside the Todorokite-Type Tunnel Structure Artem M. Abakumov,* ,² Joke Hadermann, ‡ Gustaaf Van Tendeloo, ‡ Maksim L. Kovba, ² Yuri Ya. Skolis, ² Svetlana N. Mudretsova, ² Evgeny V. Antipov, ² Olga S. Volkova, § Aleksandr N. Vasiliev, § Natalia Tristan, ⊥ Ru ¨diger Klingeler, ⊥ and Bernd Bu ¨chner ⊥ Department of Chemistry and Low Temperature Physics Department, Moscow State UniVersity, 119992 Moscow, Russia, EMAT, UniVersity of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium, and Leibniz Institute for Solid State and Materials Research (IFW) Dresden, 01171 Dresden, Germany ReceiVed October 20, 2006. ReVised Manuscript ReceiVed December 30, 2006 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 × 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 *(γ ) 0.63157(3)) parallel to the direction of tunnel propagation. The octahedral tunnel walls compose subsystem I with a [Mn 6 O 12 ] composition and a periodicity c 1 ) 2.84 Å, whereas the [Sr(F,OH)] 4 columns belong to subsystem II with a periodicity c 1 /γ ) 4.49 Å, 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. 1. Introduction Natural todorokites (Na, Ca, K, Ba, Sr) 0.3-0.7 (Mn, Mg, Al) 6 O 12 ×3.2-4.5H 2 O belong to the dominant components of ocean Mn nodules and possess some of the largest tunnels among the variety of A x MnO 2 tunnel manganites (after woodruffite). 1,2 The synthetic analogues of these microporous Mn oxides exhibit a range of potential applications as octahedral molecular sieves, catalysts, matrices for cation intercalation/deintercalation for secondary power sources, and ion-exchange materials for the removal of radionuclides from aqueous radioactive wastes. Until high-resolution electron microscopy and Rietveld refinement were performed, the todorokite structure was a subject of controversy for many years, mainly due to the absence of single crystals and the poor crystallinity of the powder samples. 3-8 The todorokite structure is based on a [Mn 6 O 12 ] framework, consisting of rutile-type strings of MnO 6 octahedra. Three edge-sharing strings form a wall; mutually perpendicular walls share corners delimiting square tunnels of the size of 3 × 3 octahedra (Figure 1). The space in the tunnels is occupied by [M +1,+2 (H 2 O) 6 ] octahedral strings with water molecules at the corners of the octahedra and the M +1,+2 alkali and/or alkali-earth cations at the centers of these octahedra. The water molecules form hydrogen bonds with the oxygen atoms of the tunnel walls. The tunnel interior in the A x MnO 2 manganites is filled with cationic species, such as alkali-earth cations, compen- sating the negative charge of the [Mn +4-δ O 2 ] octahedral framework. Square hollandite-type tunnels and the six-sided tunnels of the CaM 2 O 4 (M ) Ti, Mn, Fe) 9-11 and CaMn 3 O 6 12 structures incorporate one column of A-cations. The larger “figure-of-eight” and S-shaped tunnels of the SrMn 3 O 6 , 13 Ba 6 - Mn 24 O 48 , 14 CaMn 4 O 8 , 15 and Na 4 Mn 9 O 18 16,17 structures can * Corresponding author. E-mail: abakumov@icr.chem.msu.ru. Tel: (095) 939-33-75. Fax: (095) 939-47-88. ² Department of Chemistry, Moscow State University. ‡ University of Antwerp. § Low Temperature Physics Department, Moscow State University. ⊥ Leibniz Institute for Solid State and Materials Research. (1) Post, J. E.; Heaney, P. J.; Cahill, C. L.; Finger, L. W. Am. Mineral. 2003, 88, 1697. (2) Lei, G. Mar. Geol. 1996, 133, 103. (3) Turner, S.; Buseck, P. S. Science 1981, 212, 1024. (4) Chukhrov, F. V.; Gorshkov, A. I.; Sivtsov, A. V.; Beresovskaya, V. V. Nature 1979, 278, 631. (5) Miura, H. J. Hokkaido UniV., Fac. Sci., Ser. 4: Geol. Mineral. 1991, 23, 41. (6) Post, J. E.; Bish, D. L. Am. Mineral. 1988, 73, 861. (7) Post, J. E.; Heaney, P. J.; Hanson, J. Am. Mineral. 2003, 88, 142. (8) Burns, R. G.; Burns, V. M.; Stockman, H. W. Am. Mineral. 1983, 68, 972. (9) Bertaut, F.; Blum, P. J. Phys. 1956, 17, 517. (10) Giesber, H. G.; Pennington, W. T.; Kolis, J. W. Acta Crystallogr., Sect. C 2001, 57, 329. 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