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11.
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
12.
13.
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
14.
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
15.
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
16.
17.
The first members of the series AxP4O8(WO3)2m were studied by means of electron microscopy. These bronzes can be classified into two groups on the basis of ReO3-type block composition: even- and odd-m members. High-resolution lattice images of tungstophosphate crystals (m ≤ 10) allow us to establish a correlation between the image contrast and the framework of the structure. The structural mechanism proposed for this series is discussed and compared to the possibility of intergrowth, and to the crystallographic shear phenomena observed in tungsten and molybden oxides. 相似文献
18.
Rouquette J Haines J Al-Zein A Papet P Damay F Bourgeois J Hammouda T Doré F Maignan A Hervieu M Martin C 《Physical review letters》2010,105(23):237203
The electronic ferroelectric lutetium ferrite (LuFe(2)O(4)) was studied by x-ray diffraction as a function of pressure. Pressure is shown to induce an irreversible rhombohedral to orthorhombic transition leading to a supercell determined by the combination of electron and synchrotron x-ray diffraction. This new configuration is proposed to be charge ordered in agreement with the results of resistivity measurements. 相似文献
19.
A. Maignan C. Martin M. Hervieu B. Raveau 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(1):41-45
The substitution of manganese for cobalt in the perovskite La
0.8
Sr
0.2
CoO
3
has been studied. A significant increase of the magnetoresistance (MR) is obtained, reaching 60% at 5 K under 7 T for . This behavior originates from a spectacular increase of the resistivity correlated to a significant decrease of ferromagnetism
by Mn doping. This enhancement of magnetoresistance can be interpreted by the growth of ferromagnetic clusters in the insulating
matrix, by applying a magnetic field.
Received 7 May 1999 相似文献
20.
The study of the structural and magnetic phase diagram of the manganites La1−xAgxMnO3 shows similarity with the La1−x′Srx′MnO3 series, involving a metallic ferromagnetic domain at relatively high temperature (≈300 K). The Ag-system differs from the Sr-one by a much smaller homogeneity range (x≤1/6) and the absence of charge ordering. But the most important feature of the Ag-manganites deals with the exceptionally high magnetoresistance (−25%) at room temperature under 1.2 T, that appears for the composition x=1/6. The latter is interpreted as the coincidence of the optimal double exchange condition (Mn3+:Mn4+=2) with Tmax=300 K (maximum of the ρ(T) curve in zero field). 相似文献