Title: Hydrogen Bonding and Jahn–Teller Distortion in Groutite,α-MnOOH, and Manganite,γ-MnOOH, and Their Relations to the Manganese Dioxides Ramsdellite and Pyrolusite
Abstract: The crystal structures of groutite,α-MnOOH (space groupPnma,a=10.667(1),b=2.871(1),c=4.554(1) Å,Z=4), and manganite,γ-MnOOH (space groupP21/c,a=5.304(1),b=5.277(1),c=5.304(1) Å,β=114.38(2)°,Z=4), both from the Kalahari manganese field (South Africa), were refined including hydrogen positions from room temperature X-ray single-crystal data. The refinements converged toRvalues of 1.5% for 479 (groutite) and of 2.0% for 821 (manganite) unique reflections, respectively. A (101) twin refinement based onF2obswas applied for manganite leading to a twin contribution of ca. 0.9:0.1. The structures of groutite and manganite are distorted derivatives of the MnO2polymorphs ramsdellite and pyrolusite (rutile structure), respectively. The structural distortions of the oxyhydroxides are caused by an interaction of Jahn–Teller distortion of octahedrally coordinated Mn3+(four short and two long Mn–O distances) and hydrogen bonding. In both structures two symmetrically distinct O sites (O1 and O2) are three-coordinated by Mn3+. The choice which O site forms an OH group is governed by the orientation of the Jahn–Teller distortion and space constraints dictated by the octahedral framework topology. The two long Mn–O bonds are formed by both O1 and O2 thus the Jahn–Teller distortion alone does not determine the preference of the OH group. In groutite, the OMn3coordination fragment which shows the strongest deviation from planarity toward a trigonal pyramid bonds to H where the O–H vector points perpendicular to the Mn3plane. In manganite the coordinations of O1 and O2 are very similar, thus H shows long range disorder as observed by twinning. Both, groutite and manganite have short O–H···O distances (2.6 Å) giving rise to peculiar IR absorption features between 3200 and 1800 cm−1. Oriented single-crystal slabs of manganite and diaspore (isostructural to groutite) were studied by polarized FTIR spectroscopy at 82 and 298 K, and all H-bonding related IR-absorptions were assigned. Manganite,γ-MnOOH, and groutite,α-MnOOH, both transform in air above ca. 300°C toβ-MnO2(pyrolusite) which was studied byin situtemperature dependent single-crystal X-ray diffraction. The topotactic relation is preserved during the transformation.
Publication Year: 1997
Publication Date: 1997-11-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 219
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