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191.
R. Gerardin E. Millon J.F. Brice O. Evrard G. Le Caer 《Journal of Physics and Chemistry of Solids》1985,46(10):1163-1171
As in magnetite Fe3O4, calcium ferrite CaFe3O5 is an oxide in which electron transfer occurs between the iron ions (). This intervalence exchange process has been studied by 57Fe Mössbauer spectroscopy and by electrical conductivity measurements. In CaFe3O5, the Fe3+ and Fe2+ ions occupy different crystallographic sites and have a deformed octahedral coordination. Each Fe2+O6 octahedron shares an edge with two Fe3+O6 octahedra. In the antiferromagnetic region (TN = 282 ± 2 K), the Fe3+ and Fe2+ ions are well differentiated. Thermally-activated electron transfer is observed above tn, in the paramagnetic region and is well characterized by the Mössbauer spectra. These are analyzed using the hypothesis of an electron jump limited to a trimer Fe3+Fe2+Fe3+ which leads to a relaxation time of 180 ns at 298 K and 80 ns at 400 K. Within this temperature interval, the process follows the Arrhenius law with an activation energy of 0.10 eV. Electrical conductivity measurements lead to similar results with an activation energy of 0.09 ± 0.02 eV. 相似文献
192.
Crystals of brittle materials are easily cracked during manufacture and processing. This paper examines the thermal strains existing in crystals during and after pulling and the strains which can be built-in by the facet effect. It is shown that during the growth of a crystal with a radius R, there is a maximum acceptable axial gradient roughly proportional to R?1.5 and that to prevent cracking after growth there is a maximum rate of cooling proportional to R?2. Results obtained with bismuth silicon oxide for which all the relevant parameters (breaking strain, expansion coefficient, thermal diffusivity and cooling constant) have been measured are in accord with these relations at least for 4 <R<23 mm. The analysis seems valid for other materials (e.g. lithium niobate) with small breaking strains and negligible plastic ranges. For bismuth silicon oxide, (110) and (100) facets have lattice constants larger than the bulk of the crystal by about 4 and 6 parts in 105 respectively. The edges of these facets are thus regions which are strained by amounts corresponding to 20 or 30% of the breaking strain. Under normal growth conditions, this strain is not particularly significant, but it does affect post-growth handling and during rapid growth, interface instabilities develop first at the facet edges. 相似文献
193.
The mesoporous H-bonded organic-inorganic hybrid material is built up from the assembly of the isopolymolybdate [Mo36O112(OH2)16]8- and H2bipy2+, which displays large interconnected cavities and exhibits a reversible water sorption behaviour while maintaining its striking crystal integrity. 相似文献
194.
A direct sample fraction deposition method was developed for off-line size-exclusion chromatography (SEC)/matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. By using electrospray, the SEC eluent, together with a suitable matrix solution added coaxially, was directly deposited on the MALDI plate. Owing to the formation of very small droplets in electrospray, solvent evaporation is much faster. The fractionation volume in narrow-bore SEC, which can directly be collected in one MALDI spot, can easily be optimized in the range of a few microlitres. In addition, fairly homogeneous sample spots were obtained. The possible influence of composition variation of the SEC effluent on the analytical results using direct fraction deposition was investigated; no substantial effects were observed. The applicability of the method was demonstrated by characterizing a broad poly(methyl methacrylate) sample. Copyright 2000 John Wiley & Sons, Ltd. 相似文献
195.
Dr. Chongwei Zhu Dr. Albert Poater Dr. Carine Duhayon Dr. Brice Kauffmann Dr. Alix Saquet Valérie Maraval Prof. Dr. Remi Chauvin 《Angewandte Chemie (International ed. in English)》2018,57(20):5640-5644
Ring carbo‐mers of oligo(phenylene ethynylene)s (OPEn, n=0–2), made of C2‐catenated C18 carbo‐benzene rings, have been synthesized and characterized by NMR and UV‐vis spectroscopy, crystallography and voltammetry. Analyses of crystal and DFT‐optimized structures show that the C18 rings preserve their individual aromatic character according to structural and magnetic criteria (NICS indices). Carbo‐terphenyls (n=2) are reversibly reduced at ca. ?0.42 V/SCE, i.e. 0.41 V more readily than the corresponding carbo‐benzene (?0.83 V/SCE), thus revealing efficient inter‐ring π‐conjugation. An accurate linear fit of E1/2red1 vs. the DFT LUMO energy suggests a notably higher value (?0.30 V/SCE) for a carbo‐quaterphenyl congener (n=3). Increase with n of the effective π‐conjugation is also evidenced by a red shift of two of the three main visible light absorption bands, all being assigned to TDDFT‐calculated excited states, one of them restricting to a HOMO→LUMO main one‐electron transition. 相似文献
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The solid solution Li8−2xCaxCeO6 (0 < x ≤ 0,5) and the definite phase Li6CaCeO6 have been obtained at 800°C through a study of Li---Ca---Ce---O system. Electrical measurements on the doped phases Litetr.6 [Li2-2xCaxCe□]oct.O6 show that the conductivity varies slightly with the creation of vacancies in the octahedral layers. This result unambiguously confirms the following diffusion mechanism: the conduction is assumed essentially by lithium ions located in the tetrahedral layers. The compound Li6CaCeO6 is isostructural with Li6In2O6. The cell is trigonal,
Å, c = 10,603 Å, c/a = 1,0587, and Z = 6. This new quaternary phase, which belongs to the same structural family of oxides of the type Li8MO6, either pure or doped with calcium, may be represented by the formula Litetr.6[Ca Ce□]oct.O6. Electrical and structural data are correlated for this compound. 相似文献