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排序方式: 共有70条查询结果,搜索用时 31 毫秒
41.
M. D. Marcos J. P. Attfield 《Physica C: Superconductivity and its Applications》1996,270(3-4):267-273
The crystal structure including the cation distribution, of a polycrystalline sample of nominal composition Tl0.5Pb0.5Sr2Ca2Cu3O9 with Tc = 118.2 K has been determined using resonant synchrotron X-ray diffraction data collected at the Cu K, Tl LIII and Sr K edges and time-of-flight powder neutron diffraction data. No oxygen deficiency was observed, but cation disorder at all the non copper sites according to the formula (Tl0.60Pb0.40)(Sr1.60Ca0.40)(Ca1.93Tl0.07) Cu3O9 gives a mean hole concentration of 0.18(1) per Cu atom for the three CuO2 planes, consistent with the high Tc for this material. Analysis of five time-of-flight powder neutron diffraction data between 80 and 150 K have revealed a possible discontinuity in the variation of the c lattice parameter at Tc, due to an anomaly in the position of the apical oxygen atoms. 相似文献
42.
The structural and electronic transport properties of La1−x
Ce
x
MnO3 (x=0.0–1.0) have been studied. All the samples exhibit orthorhombic crystal symmetry and the unit cell volume decreases with
Ce doping. They also make a metal-insulator transition (MIT) and transition temperature increases with increase in Ce concentration
up to 50% doping. The system La0.5Ce0.5MnO3 also exhibits MIT instead of charge-ordered state as observed in the hole doped systems of the same composition. 相似文献
43.
Two novel gallium diphosphonates, (C5H5N)Ga3F(O3PC2H4PO3)2 (I) (triclinic, P-1, a=8.2880(12) Å, b=11.7197(16) Å, c=11.7601(17) Å, α=71.589(3)°, β=70.577(3)°, γ=77.313(3)°, V=1013.7(2) Å3, Z=2, R1=0.0352, wR2=0.0980) and (C5H5NH)2[Ga4F2(O3PC2H4PO3)3] (II) (triclinic, P-1, a=8.670(4) Å, b=9.742(3) Å, c=10.406(2) Å, α=81.44(3)°, β=65.83(5)°, γ=67.16(3)°, V=739.0(4) Å3, Z=2, R1=0.0600, wR2=0.1495) have been synthesised by solvothermal methods in the presence of pyridine and their structures determined using single-crystal X-ray diffraction data. Both compounds I and II are composed of various Ga-centered polyhedra and ethylenediphosphonate groups that link together to form framework materials with one- and two-dimensional channel systems, respectively. The two structures are formed in the presence of structure-directing pyridine molecules that are directly bound to some of the Ga atoms in I, and are protonated as pyridinium cations in II. Compounds I has a charge-neutral framework, while compound II has an anionic framework. Both materials provide rare examples of organically structure-directed framework metal diphosphonate materials. 相似文献
44.
Harvey HG Slater B Attfield MP 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(13):3270-3278
We report here on the solvothermal synthesis and crystal structure of the hybrid organic-inorganic framework material Al(2)[O(3)PC(3)H(6)PO(3)](H(2)O)(2)F(2).H(2)O (orthorhombic, Pmmn, a = 12.0591(2) A, b = 19.1647(5) A, c = 4.91142(7) A, Z = 4), the second member of the Al(2)[O(3)PC(n)H(2n)PO(3)](H(2)O)(2)F(2).H(2)O series. The structure consists of corrugated chains of corner-sharing AlO(4)F(2) octahedra in which alternating AlO(4)F(2) octahedra contain two fluorine atoms in a trans or a cis configuration. The diphosphonate groups link the chains together through Al-O-P-O-Al bridges and through the propylene groups to form a three-dimensional framework structure containing a one-dimensional channel system. The linkage of the corrugated inorganic Al-O-P layers within the structure results in the formation of two types of channel that differ in size, shape and composition. The smaller channel is unoccupied; the larger channel is more elongated and contains two extra-framework water molecules per unit cell. A computational investigation into the driving force that controls the stacking arrangement of the Al-O-P inorganic layers within this series of compounds reveals that the stacking is found to be controlled by thermodynamic factors, arising chiefly from the conformation of the organic linker molecule used to connect the inorganic sheets. It is found that the registration of the inorganic layers can be engineered by selecting an appropriate, simple organic spacer or linker alkyl chain, where an even number of carbon atoms in the alkyl chain directs formation of aligned, stacked, inorganic sheets (AAAAAA), and an odd number directs formation of unaligned, stacked sheets (ABABAB) and the formation of one or two channel types in the resultant structure, respectively. This combination of alkyl-chain linkers in conjunction with corrugated inorganic layers is an effective tool to rationally design the pore system of hybrid framework materials. 相似文献
45.
46.
47.
A.C. Mclaughlin J.P. Attfield R.S. Liu W.Z. Zhou 《Journal of solid state chemistry》2004,177(3):834-838
Ruthenocuprate microstructures and Ru valences have been studied. Electron microscopy reveals short-range order of the RuO6 octahedra rotations into a √2a×√2a×c supercell in Pb2RuSr2Cu2O8Cl. However, reanalysis of neutron diffraction data gives no significant difference between the populations of the rotation states, showing that the coherence length is very short (<100 Å). The Ru valence estimated from the XANES spectrum of Pb2RuSr2Cu2O8Cl is ∼5, in keeping with the physical properties of this material which show that there is essentially no Ru-Cu charge transfer. The Ru valence in doped Ru1−xMxSr2GdCu2O8 (M=Sn, Nb) is ∼4.8 in all samples, verifying a previous rigid band analysis of the charge distribution in these materials. 相似文献
48.
Ortega-San-Martin L Williams AJ Rodgers J Attfield JP Heymann G Huppertz H 《Physical review letters》2007,99(25):255701
An orbital ordering transition and electronic phase coexistence have been discovered in SrCrO3. This cubic, orbitally-degenerate perovskite transforms to a tetragonal phase with partial orbital order. The tetragonal phase is antiferromagnetic below 35-40 K, whereas the cubic phase remains paramagnetic at low temperatures. The orbital ordering temperature (35-70 K) and coexistence of the two electronic phases are very sensitive to lattice strain. X-ray measurements show a preferential conversion of the most strained regions in the cubic phase. This reveals that small fluctuations in microstrain are sufficient to drive long range separation of competing electronic phases even in undoped cubic oxides. 相似文献
49.
Large Magnetization and Frustration Switching of Magnetoresistance in the Double‐Perovskite Ferrimagnet Mn2FeReO6 下载免费PDF全文
Dr. Angel M. Arévalo‐López Graham M. McNally Prof. J. Paul Attfield 《Angewandte Chemie (International ed. in English)》2015,54(41):12074-12077
Ferrimagnetic A2BB′O6 double perovskites, such as Sr2FeMoO6, are important spin‐polarized conductors. Introducing transition metals at the A‐sites offers new possibilities to increase magnetization and tune magnetoresistance. Herein we report a ferrimagnetic double perovskite, Mn2FeReO6, synthesized at high pressure which has a high Curie temperature of 520 K and magnetizations of up to 5.0 μB which greatly exceed those for other double perovskite ferrimagnets. A novel switching transition is discovered at 75 K where magnetoresistance changes from conventional negative tunneling behavior to large positive values, up to 265 % at 7 T and 20 K. Neutron diffraction shows that the switch is driven by magnetic frustration from antiferromagnetic Mn2+ spin ordering which cants Fe3+ and Re5+ spins and reduces spin‐polarization. Ferrimagnetic double perovskites based on A‐site Mn2+ thus offer new opportunities to enhance magnetization and control magnetoresistance in spintronic materials. 相似文献
50.