- Infrared Study of the Surface Properties of HTB-Type Al-, Cr-, Fe-Hydroxyfluorides
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The surface properties of iron, chromium, and aluminum fluorides in their hexagonal tungsten bronze (HTB) form have been investigated by infrared spectroscopy. The presence of hydroxyls is clearly observed. H/D exchange experiments with different deuterated probe molecules having various molecular sizes show that they are localized inside of the channels of the structure. The v(OH) bands in the infrared spectra of these materials are downward shifted compared to those of corresponding metal oxides, whereas the δ(OH) in-plane bending mode presents an unusual high wavenumber. These spectral features are compared to those observed on zeolites, for which hydroxyl environment and bridged conformation are similar. HTB compounds exhibit both strong Lewis and Br?nsted acid sites. The use of two basic probes with a different size (pyridine and ammonia) allows one to localize and to quantify these two kinds of acidity. Br?nsted acidity is related to the presence of hydroxy groups into the microporous channels and to chemisorbed HF, whereas Lewis acidity is due to defect sites both on the outer surface of crystallites and in the channels. The strength of acid sites is unambiguously found stronger than that reported for Al, Cr, and Fe oxides. This result is discussed taking into account the electronegativity of fluorine but also the bridged conformation of OH groups, as in the case of zeolites.
- Vimont, Alexandre,Lavalley, Jean-Claude,Francke, Lo?c,Demourgues, Alain,Tressaud, Alain,Daturi, Marco
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- Crystal structure and thermal behaviour of K2[CrF5 · H2O]
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K2[CrF5 · H2O] is monoclinic: a = 9.6835(3) A, b = 7.7359(2) A, c = 7.9564(3) A, β = 95.94(1)°, Z = 4, space group C2/c (no 15). Its crystal structure was solved from its X-ray powder pattern recorded on a powder diffractometer, using for the refinement the Rietveld method. It is built up from isolated octahedral [CrF5 · OH2]2- anions separated by potassium cations. The dehydration of K2[CrF5 · H2O] leads to anhydrous orthorhombic K2 -CrF5: a = 7.334(2) A, b = 12.804(4) A, c = 20.151(5) A, Z = 16, space group Pbcn (no 60), isostructural with K2FeF5.
- Sassoye, Capucine,De Kozak, Ariel
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- Synthesis and crystal structure of CuIIMoIVF6 and CrIINbIVF6 (LT form)
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The fluorides CuMoF6 and CrNbF6 are triclinic, isostructural with CuSnF6.Their crystal structures were solved in the space group P1, by the Rietveld method using the data of their X-ray powder patterns. Both crystal structures can be described as a three-dimensional framework of alternating 6 corner-sharing [M11F6] and [M′IVF6] octahedra (MII = Cr, Cu; M′IV = Nb, Mo) with ferrodistortive Jahn-Teller ordering.
- Llorente,Goubard,Gredin,Bizot,Chassaing,Quarton
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p. 1538 - 1542
(2008/10/09)
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- Thermodynamic Properties of CrF2 and CaCrF4
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The thermodynamic properties of CrF2 and CaCrF4 at 800 and 298 K have been studied by the e.m.f. method with an F- ion solid electrolyte.The most reliable values of the thermodynamic quantities for CrF2 are recommended.
- Rezukhina, T. N.,Gorshkova, T. I.,Tsvetkov, A. A.
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p. 1149 - 1152
(2007/10/02)
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