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制备了一系列磁铅石型复合氧化物CaNiyAl12-yO19-δ(y=0.3,0.6,0.9,1.0),并用XRD、XPS、TPR和TGA技术对其结构和性能进行了表征。结果表明,该系列复合氧化物具有相同的晶体结构和相似的还原稳定性。当Ni调变量既0〈y≤1,可以获得磁铅石型复合氧化物CaNiyAl12-yO19-δ晶相。在780℃二氧化碳重整甲烷制合成气反应过程中,还原态复合氧化物CaNiyAl12  相似文献   
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BaV6O11 was synthesized under high pressures and crystallizes in a structure closely related to magnetoplumbite. [V(1)O6]-octahedra share common edges and form a Kagomé lattice normal to the hexagonal [0 0 1] direction. The layers are connected in the direction of c via trigonal [V(3)O5]-bipyramids and [V(2)O6]-octahedra, which share common faces. The Ba-atoms are incorporated into cavities of the vanadium oxide framework and are coordinated by 12 oxygen atoms in the shape of a dodecahedron.Three magnetic anomalies at approximately 250, 115 and 75 K were detected in this compound. All of them are accompanied by anomalies in the specific heat measurement. To characterize possible structural transitions and determine the response of the structure to the magnetic anomalies, single crystal X-ray diffraction studies were carried out in the temperature range from 293 to 80 K. At 250 K the compound undergoes a structural phase transition. The space group above the transition temperature is P63/mmc, at lower temperature the symmetry reduces to P63mc. For the refinements in P63mc an inversion twin model was used, this way accounting for the loss of the center of symmetry. The structural phase transition is characterized by a small displacement of the V(1)-atom (forming the Kagomé lattice) out of its central position in the octahedra. As a consequence part of the octahedral edges/angles are increased, while the opposite ones are decreased. One limiting surface of the octahedral sheet is corrugated, while the other one is smoothened with respect to the high-temperature structure. This deformation of the octahedral sheets leads to the corresponding geometrical changes in the other coordination polyhedra.The structural response to the magnetic anomaly at 115 K is weak and mainly observable in the geometric parameters concerning the [V(1)O6]-octahedra and [V(3)O5]-bipyramids. This may serve as a first indication that the corresponding central atoms play an important role in the mechanism of the magnetic phase transition.  相似文献   
3.
High quality PbFe12O19 single crystals were grown by a solution-melt crystallization technique and characterized by synchrotron radiation diffraction structural studies, Mössbauer, Raman and infrared spectral measurements. In the terahertz and sub-terahertz range two absorption bands are observed, which are associated with the dynamics of structurally disordered lead and iron ions. We develop a structural model that comprises Fe2+ ions at the (1/2 × 4e) (0, 0, z) position with bipyramidal oxygen environment, and Pb ions at the (24l) (x, y, z) position with generic strongly distorted 12 vertex polyhedron of oxygen. The atomic displacement ellipsoids of the oxygen surrounding the lead ion are stretched towards the Pb ion within the plane perpendicular to the hexagonal axis. The suggested model is in full agreement with the local symmetry of the Fe2 and Pb ions and with the chemical composition of the sample determined by X-ray analysis.  相似文献   
4.
The title compounds (Sr0.96−xBa0.04)Al12−yMgyO19:Tbx (0<x<0.4; 0<y<0.18) are single-phase magnetoplumbite determined by X-ray powder diffraction analysis. The characteristic emission lines of 5D37Fj (j=2, 3, 4, 5) and 5D47Fj (j=4, 5, 6) of Tb3+ are recorded under the VUV excitation. The intensive luminescence mainly comes from 5D37Fj transition when the concentration of Tb3+ is low. However, when the concentration of Tb3+ starts to increase from very low concentration, 5D47Fj transition is becoming dominant. Three broad excitation bands at 165, 193 and 233 nm have been observed. The band at 165 nm originates from the overlap between the host absorption and the charge transfer of Tb3+-O2−. The other two broad bands are the first spin-allowed and the spin-forbidden of 4f-5d transition, respectively. The experimental observation of the 4f-5d transition of Tb3+ is consistent well with the theoretical expectations.  相似文献   
5.
制备了一系列磁铅石型复合氧化物CaNiyAl12-Yo19-δy=0.3,0.6,0.9,1.0),并用XRD、XPS、TPR和TGA技术对其结构和性能进行了表征。结果表明,该系列复合氧化物具有相同的晶体结构和相似的还原稳定性。当Ni调变量既0相似文献   
6.
A new magnetoplumbite-type hexagonal cobalt oxide, SrCo12O19, has been synthesized at a high pressure of 3 GPa and its structural and magnetic properties have been studied using single crystalline samples. Bond-valence calculations for five crystallographically independent Co sites show a rather wide charge distribution from +2.15 to +3.50. Magnetic measurements have revealed anisotropic magnetic behavior below a magnetic ordering temperature of 80 K. Comparative discussions on the structure-property relationship for SrCo12O19 and related compounds suggest the presence of localized spins with a uniaxial anisotropy at the trigonal bipyramidal site.  相似文献   
7.
Single crystals of the novel compound BaSn6Co6O19 with maximum width 1 mm and thickness around 0.05 mm were grown from a barium chloride flux. The composition was determined from refinements of single crystal X-ray diffraction data and microprobe analysis. BaSn6Co6O19 crystallizes in the magnetoplumbite type structure (hexagonal, space group P63/mmc, a=6.0940(1) Å, c=23.9633(5) Å, V=770.69 Å3, Z=2). A significant disorder is generated by random occupation of two octahedrally coordinated crystallographic sites with Co2+ and Sn4+ ions, while further sites are exclusively occupied by either Co2+ (tetrahedrally coordinated) or Sn4+ (octahedrally coordinated). One site with mixed occupation realizes the topology of a kagome net. The temperature dependence of the magnetic susceptibility for a single crystal BaSn6Co6O19 reveals a low temperature antiferromagnetic order at TN=14 K. A relatively large value of frustration factor f//=|ΘW//|/TN≈26 and f=|ΘW|/TN≈12 implies a frustrated antiferromagnetism.  相似文献   
8.
Eu-doped lanthanum aluminum oxynitride (LaAl12(O,N)19) with magnetoplumbite structure was prepared by nitridation of the oxide precursor obtained from aluminum glycine gel and subsequent post-annealing. Eu-doped lanthanum aluminum oxynitride exhibited blue light emission at 440 nm with a shoulder at 464 nm under excitation at 254 nm. Isostructural Eu-doped calcium aluminum oxide (CaAl12O19) exhibited a single emission peak at 415 nm. Structural refinement using neutron powder diffraction indicated that the lanthanum site occupied partially by Eu2+ splits into 2d and 6h sites in the aluminum oxynitride. The longer emission and the shoulder peak in the former aluminum oxynitride were observed in relation to the increasing covalency as well as crystal field splitting around doped Eu2+ induced by site splitting involved with the two kinds of anions.  相似文献   
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