- Time-of-flight neutron diffraction study on the low temperature phases of IF7
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A neutron diffraction study on iodine heptafluorine has been performed using the high resolution powder diffractometer (HRPD) at the spallation source ISIS, England.The previously unknown structure of the low temperature phase IF7 (III) was solved.IF7 (III) crystallizes in the orthorhombic space group Pbab (No. 54), a=852.160(4) pm, b=884.096(4) pm, c=599.089(2) pm with 4 formula units per unit cell.The IF7 molecule is found to exhibit an almost perfect pentagonal bipyramid with an axial bond length of 179.5(2) pm and an average equatorial bond length of 184.9 pm.The I(Feq)5 unit is slightly puckered with puckering displacements of 2.7o for two of the five Feq atoms leading to a c2 symmetry for the molecule.Two order-disorder processes lead from the plastic phase IF7 (I) to the ordered phase IF7 (III).At the first phase transition at 150 K IF7 molecules align with respect to their Fax-I-Fax units.Within the second transition at 100 K they order with respect to their I(Feq)5 units.The latter can be grouped into two sets of five-membered rings with vertices pointing up and down the orthorhombic c axis, respectively.There is evidence suggesting a temperature dependence to this second ordering process; the amount of residual disorder is 5percent at 77 K and 1.4percent at 5 K.
- Marx, R.,Mahjoub, A. R.,Seppelt, K.,Ibberson, R. M.
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- Apparently regular octahedral coordination of Ag(II) in If 6[Ag(SbF6)3]
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The crystal structure of IF6[Ag(SbF6)3] has been determined by X-ray single crystal and X-ray powder diffraction analysis. The structure consists from [Ag(SbF6)3] - chains and [IF6]+ cations placed between them. Ag2+ is found in regular octahedral coordination of six fluorine atoms [Ag-F = 6 × 2.170(12) A]. EXAFS analysis of JahnTeller distortions in IF6[Ag(SbF6)3] shows that Ag2+ surrounding is elongated octahedral at room temperature. EXAFS signals are fitted with four short and two long Ag-F distances [4 × 2.11(1) A 2 × 2.40(2) A]. This result is in apparent contradiction with X-ray diffraction studies where the [AgF6] moiety appears to be a regular octahedra. The Raman spectrum of IF6[Ag(SbF 6)3] is in agreement with the presence of regular IF 6 and deformed SbF6 groups.
- Mazej, Zoran,Goreshnik, Evgeny,Arcon, Iztok,Logar, Natasa Zabukovec,Kaucic, Venceslav
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p. 224 - 229
(2010/06/12)
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