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Russian Physics Journal - A kinetic model of the formation of pyramid-like bulges (pedestals) at the bases of vertical nanowires is proposed. The formation of the pedestals at the early stage of... 相似文献
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The substitution of small contents of trivalent cations for manganese in the oxides Ln0.57Ca0.43Mn1−xMxO3 (Ln=Pr, Na) has been explored for M=Al, Ga, Fe, Cr, Sc, In. It is shown that similarly to Ba-doping, the M-doped Pr-manganites exhibit a great predisposition to ferromagnetism in a low magnetic field of 0.25 T, reaching ferromagnetic (FM) fractions of 85% for M=Ga and 100% for m=Cr, whereas in contrast only small FM fractions (∼8%) can be reached for the M-doped Nd-manganites. The great ability of both Pr and Nd manganites to exhibit ultrasharp steps at 2.5 K, at increasing magnetic field is also demonstrated. The different behaviors of the so-doped manganites are interpreted in terms of geometric effects (size of the A-sites cations and of the doping elements) and of electronic configurations and magnetic properties of the dopants. 相似文献
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Mme Maryvonne L. Martin M. Franois Lefevre Mme Denise Lapeyre M. Grard J. Martin M. Richard Mantione 《Magnetic resonance in chemistry : MRC》1969,1(1):19-26
Magnetic non-equivalence of methylenic protons in A? CH2? CH3 groups were studied for several types of compounds. Results are compared with respect (1) to the nature of heteroatom A:A = O and A = S and (2) to the structure of the local dissymmetry element: asymmetric carbon, ketal group and allenic system. Influence of substitution is discussed in relation to conformational problems. Furthermore a variation of the methylene geminal coupling constant with substituents of carbon α to OEt is observed. 相似文献
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Grard J. Martin Marie-Thrse Quemeneur Maryvonne L. Martin 《Magnetic resonance in chemistry : MRC》1969,1(3):191-202
The accuracy of several methods for deuterium content determination in organic molecules are tabulated. The sample-substitution (using one or two NMR sample tubes) and internal standard (inter- or intra-molecular) processes are discussed and compared. These methods are applied to some examples and the NMR results are confirmed by mass spectrometry. 相似文献
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A. Barnabé A. Maignan M. Hervieu B. Raveau 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(2):145-150
Doping of Mn-site by chromium, cobalt and nickel has been investigated in the calcium rich manganites and
. Whatever the nature of the doping element, a rapid disappearance of the charge ordered (CO) state is observed, decreasing as the doping rate increases. But the most important result concerns the Cr-doped compounds for which a re-entrant insulator to metal transition at 90-120 K is observed for x
=0.10-0.12, in a zero magnetic field. The possibility to induce colossal magnetoresistance (CMR) properties for high (IV) contents ((IV)/) for x
=0.07-0.12 is shown for the first time, the resistance ratios reaching at 30 K. This study also shows differently that the small size of the A-site cation (Pr, Ca) is not a redhibitory obstacle
to the appearance of CMR properties in manganites, in contrast with previously established phase diagrams.
Received: 18 June 1997 / Revised: 4 August 1997 / Accepted: 10 November 1997 相似文献
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Maryvonne Hervieu Bernadette Domengès Bernard Raveau 《Journal of solid state chemistry》1985,58(2):233-242
During the investigation of the phosphate bronzes (PO2)4(WO3)2m [MPTBP] and Kx(P2O4)2WO3)2m [DPTBH] crystals of a new type were observed. HREM images of these crystals showed twinned ReO3-type slabs the junction of which was parallel to the (102)ReO3 plane. The proposed model identified the twin boundary as built from P2O7 groups involving the formation of pentagonal tunnels. The structure of this new type of extended defects is quite original: it corresponds to a new structural type named “diphosphate tungsten bronzes with pentagonal tunnels” [DPTBP], for which no regular member could be synthesized. Image calculations were performed to confirm the junction model. Apart from the disordered stacking of the ReO3-type slabs, very few defects were observed and shear planes were only obtained in reduced samples. This new structural type takes its place in the large family of phosphate tungsten bronzes where all members (DPTBH, MPTBH, MPTBP) are very closely related. 相似文献
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The reduction of various 2,3-ferroceno-1-indanones and 1,2-(α-oxotetramethylene)-methylcymantrenes by metal hydrides or Grignard reagents is stereo-specific, except for LiAlH4, the observed stereoselectivity being in all uses more than 95%. The endo configuration of the dominant alcohol is established from TLC, IR, RMN and mass data. These criteria for the assignment of the structure of these alcohols are shown to be general. 相似文献