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Title:
Single-crystal X-ray diffraction and spectroscopic studies on humboldtine and lindbergite: weak Jahn-Teller effect of Fe2+ ion
Authors:
Echigo, Takuya; Kimata, Mitsuyoshi
Affiliation:
AA(Earth Evolution Sciences, Life and Environmental Sciences, University of Tsukuba; , Japan International Research Center for Agricultural Sciences), AB(Earth Evolution Sciences, Life and Environmental Sciences, University of Tsukuba)
Publication:
Physics and Chemistry of Minerals, Volume 35, Issue 8, pp.467-475 (PCM Homepage)
Publication Date:
09/2008
Origin:
SPRINGER
Keywords:
Humboldtine, Lindbergite, Crystal structure, Bond-valence theory, Jahn&ndash, Teller effect, Hydrogen bond, Jahn–Teller effect
DOI:
10.1007/s00269-008-0241-7
Bibliographic Code:
2008PCM....35..467E

Abstract

The single-crystal of humboldtine [Fe2+(C2O4) · 2H2O] was first synthesized and the crystal structure has been refined. Single-crystal X-ray diffraction data were collected using an imaging-plate diffractometer system and graphite-monochromatized MoK α radiation. The crystal structure of humboldtine was refined to an agreement index (R1) of 3.22% calculated for 595 unique observed reflections. The mineral crystallizes in the monoclinic system, space group C2/ c, with unit cell dimensions of a = 12.011 (11), b = 5.557 (5), c = 9.920 (9) Å, β = 128.53 (3)˚, V = 518.0 (8) Å3, and Z = 4. In this crystal structure, the alternation of oxalate anions [(C2O4)2-] and Fe2+ ions forms one-dimensional chain structure parallel to [010]; water molecules (H2O)0 create hydrogen bonds to link the chains, where (H2O)0 is essentially part of the crystal structure. The water molecules with the two lone electron pairs (LEPs) on their oxygen atom are tied obliquely to the chains, because the one lone electron pair is considered to participate in the chemical bonds with Fe2+ ions. Humboldtine including hydrogen bonds is isotypic with lindbergite [Mn2+(C2O4) · 2H2O]. The donor-acceptor separations of the hydrogen bonds in humboldtine are slightly shorter than those in lindbergite, which suggests that the hydrogen bonds in the former are stronger than those in the latter. The infrared and Raman spectra of single-crystals of humboldtine and lindbergite confirmed the differences in hydrogen-bond geometry. In addition, Fe2+-O stretching band of humboldtine was split and broadened in the observed Raman spectrum, owing to the Jahn-Teller effect of Fe2+ ion. These interpretations were also discussed in terms of bond-valence theory.
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