175 research outputs found
Lernlust, eine Comenius-Fibel : für den zeitgemäss vereinigten Sach-, Sprach- und Schreibunterricht nach einem vollständigen Lehrgang der kombinierten Laut- und Normalwortmethode
No full textbearb. von L. F. Göbelbecker ; mit neuen Bildern von Otto Kubel und neuer Schreibschrift ... von A. Stocke
Structural Phase Transition and Magnetic Anisotropy of La substituted M-Type Sr Hexaferrite
No full textPowder diffraction refinement of cubic and rhombohedral iron iodine boracite, Fe3B7O13I
No full textStructural parameters of iron iodine boracite, Fe3B7O13I, [cubic with space group F[image omitted]3c at 373 K, lattice parameters acub = 12.22869(18) Å; rhombohedral with space group R3c at 10 K, lattice parameters ahex = 8.64534(21) and chex = 21.2035(8) Å] was refined from X-ray powder diffraction data; Rprofile = 2.4% (373 K) and 3.0% (10 K). For precise lattice parameter determination, the structure at low temperature was refined by using a sample with silicon as reference. The refinements show that the (averaged) difference between the two iron iodine distances increases from cubic → orthorhombic → rhombohedral structure from 0 Å → 0.20(1) ↠ 0.32(4) Å, respectively
Synthesis, structure and physical properties of single crystalline K₃Fe₅F₁₅, a tetragonal tungsten bronze type and mixed valency fluoride
No full textLe travail comporte la synthèse de monocristaux stœchiométriques de K₃Fe₅F₁₅ par une méthode de flux, élaborée à l'aide de mesures thermoanalytiques, optiques et par rayons-X. La structure orthorhombique à température ambiante a été affinée par diffraction aux rayons-X sur un monodomaine ferroélastique en symétrie centrosymétrique, excluant la possibilité de ferroélectricité, prédite antérieurement. Un modèle d'ordonnancement des ions Fe²⁺ et Fe³⁺ a été élaboré. La corrélation entre les orientations des paramètres de la maille orthorhombique, des indices de réfraction principaux et des coefficients d'extinction principaux a été établie. Une transition structurale à 290K à une phase monoclinique a été décelée par voie optique et en dessous de 120K une phase à ordre antiferromagnétique avec ferromagnétisme dit "faible" a été mise en évidence par diffraction neutronique, aux rayons-X et par des mesures optiques et magnétiques. Des considérations de symétrie conduisent à la constatation que la phase en dessous de 120K permet nécessairement à la fois la ferroélectricité et le ferromagnétisme
Structure of the fully ferroelectric/fully ferroelastic orthorhombic room-temperature phase of iron iodine boracite, Fe3B7O13I
No full textFe3B7O13I, Mr=578.13, orthorhombic, Pca21, a=8.6540(3), b=8.6481(3), c=12.2336(5) A, v=915.57(6) A3, Z=4, Dx=4.19Mg m-3, A(Mo Ka)=0.7107A, u08.31 mm-1, F(000)=1080, T=298K, R=0.03 for 1936 reflexions from an optically controlled single-domain crystal. The metal surroundings of the three Fe atoms are analysed in detail; two of three metal sites (Fe2, Fe3) have similar chemical environments and equal displacement parameters. The averaged difference between the two Fe-I distances is compared with those of other non-cubic boracites. It decreases linearly as the ionic radius of the halogen increases, except for boracites containing Cr and Cu atoms with degenerate electronic states (showing Jahn-Teller effect)
Metastable NAT in Ice Clouds
No full textMetastable NAT in Ice-Clouds
Fabian Weiss, Frank Kubel, Óscar Gálvez, Markus Hoelzel, Stewart F. Parker, Riccardo Iannarelli, Michel J. Rossi, Hinrich Grothe
Polar Stratospheric Clouds and Cirrus Clouds contain, besides pure water ice, a rather large fraction of various hydrates. These are very important for the formation of the cloud, which is a yet not well understood process. We recently solved the structure of a metastable NAT phase (alpha-NAT), we believe to not only be present, but playing a major role in the formation of clouds.
On the basis of previous work on this phase by Grothe et al. [1], we enhanced the production of alpha-NAT to the point, where we could produce enough sample to do neutron diffraction. This enabled us to solve the structure.
Our quantum mechanical calculations, using this newly found structure, show a large affinity towards water-ice. With this in mind, we interlaced our results with the experiments of R. Iannarelli [2] to derive a new 3-step NAT-formation mechanism in ice-clouds, which could explain some of the observed kinetics better than the mechanism postulated in Zondlo et al. [3].
1. Grothe, H., Tizek, H., Waller, D. & Stokes, D. The crystallization kinetics and morphology of nitric acid trihydrate. Phys. Chem. Chem. Phys., 8, 2232-2239 (2006)
2. Iannarelli, R. Multidiagnostic Observations on HCl and HNO3 Hydrate Films in the Temperature Range 170-205K: A Kinetic Study. PhD Thesis 21791, ETH Zürich, (2013).
3. Zondlo, M.A., Hudson, P.K., Prenni A.J. & Tolbert, M.A. Chemistry and microphysics of polar stratospheric clouds and Cirrus clouds. Ann. Rev. Phys. Chem., 51, 473-499 (2000)
ChemInform Abstract: The Crystal Structures of SrAlF5 (I) and Ba0.43(1)Sr0.57(1)AlF5 (II).
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