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1.
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
2.
C. AutretC. Martin A. MaignanM. Hervieu B. RaveauG. André F. Bourée 《Journal of solid state chemistry》2002,165(1):65-73
The Ho0.5Sr0.5MnO3 perovskite, synthesized in air, has been studied by combining neutron powder and electron diffraction techniques. The Pnma-type structure exhibits a strong tilting of the MnO6 octahedra. This octahedra tilting and microtwinning involve a complex strained structure. No structural transition is observed down to 1.4 K, but short-range A-type antiferromagnetism running over only a few perovskite subcells is evidenced below ≈90 K. The different behavior of this perovskite compared to other Ln0.5Sr0.5MnO3 perovskites is discussed in terms of A-site cationic mismatch. 相似文献
3.
Close-packed structures with formula AB2O4 were studied in terms of polyhedra arrangement. The junction between octahedral layers (kagome) and mixed (octahedral and tetrahedral) layers were analyzed; this association allows one to determine two types of double layers whose packing leads to three closely related structural types: spinel, double-hexagonal LiFeSnO4, and a hypothetical hexagonal structure. Olivine structure shows a polyhedral arrangement closely related to those of these three structures and can be described alike in terms of mixed layers and double layers. AB2O4 close-packed oxides can exhibit polymorphism; from this analysis a mechanism, involving geometrical operations applied to the double layers, is proposed for the different transitions, really observed as (DH) LiFeSnO4 spinel and olivine-spinel or theoretical as (DH) LiFeSnO4—hypothetical hexagonal and olivine—hypothetical hexagonal. 相似文献
4.
Battle PD Blundell SJ Brooks ML Hervieu M Kapusta C Lancaster T Nair SP Oates CJ Pratt FL Rosseinsky MJ Ruiz-Bustos R Sikora M Steer CA 《Journal of the American Chemical Society》2004,126(39):12517-12527
The temperature dependence of the crystal structure and electronic properties of brownmillerite-like Ca(2.5)Sr(0.5)GaMn(2)O(8) has been studied by neutron powder diffraction and muSR spectroscopy. The results show that short-range 2D magnetic order begins to develop within the perovskite-like bilayers of MnO(6) octahedra approximately 50 K above the 3D Néel temperature of approximately 150 K. The bilayers show a structural response to the onset of magnetism throughout this temperature range whereas the GaO(4) layers that separate the bilayers only respond below the 3D ordering temperature. XANES spectroscopy shows that the sample contains Mn(3+) and Mn(4+) cations in a 1:1 ratio, and the behavior in the region of the Néel transition is interpreted as a local charge ordering. Electron diffraction and high-resolution electron microscopy have been used to show that the local microstructure is more complex than the average structure revealed by neutron diffraction, and that microdomains exist in which the GaO(4) tetrahedra show different orientations. It is argued that the bonding requirements of diamagnetic gallium control the electronic behavior within the perovskite-like bilayers. 相似文献
5.
Sylvie Malo Antoine Maignan Sylvain Marinel Maryvonne Hervieu Kenneth R. Poeppelmeier Bernard Raveau 《Solid State Sciences》2005,7(12):1492
Recently, the ferroelectromagnet YMnO3 has been the focus of interest because it exhibits both antiferromagnetism (Néel temperature 80 K) and ferroelectricity (Curie temperature 914 K). There have been no reports of complete YMn1−xMxO3 solid solutions in which substitution of the foreign M cation preserves the hexagonal P63cm structure. In contrast there exist several homeotypic phases with the general formula, Ln1+nCunMO3+3n (n=1 (M=Ti), 2 (M=V) and 3 (M=Mo); Ln: lanthanide). Several YMn1−x(Cu3/4Mo1/4)xO3 compounds have been synthesized. The solid solution, from YMnO3 (x=0) to YCu3/4Mo1/4O3 (x=1) has been characterized by X-ray diffraction and transmission electron microscopy study. For 0<x<0.9, the compounds are found to crystallize in the non-centrosymmetric structure, space group P63cm, of YMnO3. The Mn-free end member, x=1, crystallizes in a complex multiple cell, the superstructure being associated to Cu3+/Mo6+ cationic ordering. Dilution of the Mn3+ magnetic array by the paramagnetic (Cu2+) and diamagnetic (Mo6+) cations is found to decrease the antiferromagnetic ordering temperature and it becomes undetectable for x0.5 compositions. 相似文献
6.
C. Yaicle F. Fauth R. Retoux M. Hervieu A. Maignan 《Journal of solid state chemistry》2005,178(5):1652-1660
Combining low-temperature electron (ED) and synchrotron powder diffraction (SPD) techniques, we have precisely determined the phase-separated crystal structure of Pr0.5Ca0.5Mn0.97Ga0.03O3. We demonstrate that the phase separation is associated with the onset of charge/orbital ordering at ∼230 K and that two ordered phases coexist at low temperature. In order to account for the significant anisotropic linewidth broadening observed in the SPD patterns, we had to include a specific strain model in the refinements. One of the phases, the most severely strained, is suggested to result from an imperfect charge ordering. 相似文献
7.
A novel mixed valent tellurium oxide, SrTe3O8, has been synthesized and its crystal structure was determined ab initio from powder X-ray diffraction data. This oxide, which crystallizes in a tetragonal unit-cell, P42/m space group, with very close a and c cell parameters (6.8257(1) and 6.7603(1) Å, respectively), exhibits a very original structure built up of corner-sharing TeO6 (Te6+) octahedra and Te2O8 (Te4+) twin-pyramidal units. The latter ones form [Te3O8]∞ chains running along the [001] and the [110] directions. Besides the four sided tunnels where the Sr2+ cations are located, there are very large four sided tunnels running along the c-axis which are obstructed by the electronic lone pairs of the Te4+ cations. 相似文献
8.
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
9.
10.
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