共查询到20条相似文献,搜索用时 15 毫秒
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J. Kelber A. H. Reis Jr. A. T. Aldred M. H. Mueller O. MassenetG. DePasqualiG. Stucky 《Journal of solid state chemistry》1979,30(3)
BaVSe3 has been synthesized and its crystal structure determined at 293(2)°K. The structure was solved in the hexagonal space group P63/mmc (D46h), with a = 6.9990(11) and c = 5.8621(13) Å. Scans (2 Θ) of a polycrystalline sample revealed that BaVSe3 undergoes a transition to an orthorhombic unit cell (b′ 31/2 a, a′ a, c′ c) at 303(5)°K. Magnetic susceptibility measurements between 4 and 300°K indicate that BaVSe3 is paramagnetic down to 41(1)°K, where magnetic ordering occurs, with a magnetic moment in the ordered phase of 0.2 μB per vanadium atom. The orthorhombic lattice distortion may be caused by the “freezing in” of “soft” vibrational modes. 相似文献
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O. Phaovibul J. Loboda-a
kovi R. Hosemann F. J. Balt-Calleja 《Journal of Polymer Science.Polymer Physics》1973,11(11):2273-2282
The relevance of diamagnetic susceptibility as a tool for the structure analysis of solid high polymers is stressed in the light of some new examples. The present results complement previous data and offer new aspects on the diamagnetic investigations of longchain hydrocarbons, especially polyethylene (PE). The molecular susceptibility is proportional to the average number of repeat units in the chain. The proportionality factor defines an intermolecular constant μk which characterizes different physical states. This was found to be 2.5 × 10?6 for the liquid and 3.5 × 10?6 cgs for the crystalline state of paraffins and polyethylene (solution-crystallized). For melt-crystallized material, μk, approaches the typical value of the liquid paraffin in agreement with previous results. Such a low μk is probably related to the increased disorder of the paracrystalline lattice domains, in contrast to the more ordered microparacrystallites in the so-called “single crystals,” where μk = 3.5 × 10?6. In single crystals of branched PE, μk approaches 2.5 × 10?6 with increasing branching ratio. Like paraffins in the gaseous state, molten PE, with chains longer than 1000 Å, has μk = 0. If the solution-crystallized material is molten for 10 min and thereafter cooled, μk retains the original value 3.5 × 10?6 cgs characteristic of the crystalline state. Hence, solution-crystallized polyethylene apparently possesses a kind of “memory.” Such a “memory” can, nevertheless, be partly destroyed when molten PE is stirred for 10 min and then quenched. Aggregates of solution-precipitated crystals with 3% branching concentration give μk = 2.9 ± 0.2 × 10?6 in good agreement with x-ray diffraction data. Finally, experimental details on the magnetic measurements are critically discussed, and various aspects of improvements for further investigations are also described. 相似文献
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An exact method based on Green's equation is used to find the diffusion-controlled faradaic current for certain electrode geometries that incorporate edges and vertices. Thereby the magnitudes of the time-independent current density associated with angled electrode/electrode and electrode/insulator junctions are calculated. As well, the square-root-of-time-dependent currents associated with vertices, receive attention. These terms extend to longer times, the Cottrell formulation appropriate for short times. Though most of the problems solved here have been tackled previously, the novel Green function approach is shown to be straightforward and intuitive. 相似文献
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W. M. Prest 《Journal of Polymer Science.Polymer Physics》1970,8(11):1897-1908
The effect of molecular weight distribution on the viscoelastic properties of “entangled” polymers has been examined with blends of narrowly distributed polystyrene and broadly distributed polydimethylsiloxane. It is shown that blending laws established for nonentangled polymers do not apply to high molecular weight systems. The steady-state shear compliance of a blend is examined as a function of its molecular weight and the molecular weight of its components, and an approximation is given for the longtime viscoelastic response of entangled blends. 相似文献
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T. Daniel. Crawford 《Theoretical chemistry accounts》2006,115(4):227-245
This review describes the first-principles calculation of chiroptical properties such as optical rotation, electronic and
vibrational circular dichroism, and Raman optical activity. Recent years have witnessed a flurry of activity in this area,
especially in the advancement of density-functional and coupled cluster methods, with two ultimate goals: the elucidation
of the fundamental relationship between chiroptical properties and detailed molecular structure, and the development of a
suite of computational tools for the assignment of the absolute configurations of chiral molecules. The underlying theory
and the basic principles of such calculations are given for each property, and a number of representative applications are
discussed. 相似文献
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