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Non-resonant microwave absorption (NRMA) studies of superconducting MgB2 and a sample containing ∼10% by weight of MgO in MgB2 are reported. The NRMA results indicate near absence of intergranular weak links in the pure MgB2 sample. A linear temperature dependence of the lower critical field H
c1 is observed indicating a non-s wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d wave symmetry is also not observed. In the MgB2 + MgO sample, much larger low field dependent absorption is observed indicating the presence of intergranular weak links.
The hysteretic behavior of NRMA is compared and contrasted in the two samples. In the pure MgB2 sample, a large hysteresis is observed between the forward and the reverse scans of the magnetic field indicating strong
pinning of flux lines. This hysteresis saturates a few degrees below T
c while in the MgB2 + MgO sample, a much slower increase of hysteresis with decreasing temperature is observed, a signature of weaker pinning. 相似文献
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Two first uranyl chromate nitrates, K[(UO2)(CrO4)(NO3)] ( 1 ) and Rb[(UO2)(CrO4)(NO3)] ( 2 ), were prepared by solid‐state reactions and characterized by electron microprobe analysis and single‐crystal X‐ray diffraction. The compounds are isotypic [ 1 : monoclinic, P21/c, a = 9.881(5), b = 7.215(4), c = 14.226(6) Å, β = 124.85(3)°, V = 832.3(7) Å3; 2 : monoclinic, P21/c, a = 9.804(1), b = 7.359(1), c = 14.269(1) Å, β = 122.048(4), V = 872.6(1) Å3]. The structures of 1 and 2 are based upon the complex [(UO2)(CrO4)(NO3)]– layers with unprecedented structural topology, which consist of the UrO3NO3 units linked through CrO4 tetrahedra. The resulted kinked layer can be divided into chains arranged in the ladder fashion. The layers are parallel to (100) and are linked by A+ (A = K, Rb) cations located between the layers. 相似文献
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The crystal structure of the new mineral urusovite Cu[AlAsO5] has been determined (monoclinic system, sp. gr. P21/c, a = 7.335(1) Å, b = 10.255(1) Å, c = 5.599(1) Å, β = 99.79(1)°, V = 415.0(1) Å3, Z = 4). The structure is solved by direct methods and refined to R = 0.048 (wR = 0.103). The structure is built by open-branched {uB, ∞ 2 } [AlAsO5]2? layers parallel to the (100) plane. The layers consist of two-link chains of ∞ 1 [Al2O6] tetrahedra, which are parallel to the z-axis and are connected through the [AsO4]-groups. The sharing-edges [CuO5] tetragonal pyramids link the aluminoarsenate layers to form a three-dimensional framework. The established layer is the second example of an tetrahedral aluminoarsenate polyanion. 相似文献
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Sergey V. Krivovichev 《Journal of solid state chemistry》2003,170(1):106-117
Two new mixed organic-inorganic uranyl molybdates, (C6H14N2)3[(UO2)5(MoO4)8](H2O)4 (1) and (C2H10N2)[(UO2)(MoO4)2] (2), have been obtained by hydrothermal methods. The structure of 1 [triclinic, , Z=1, a=11.8557(9), b=11.8702(9), c=12.6746(9) Å, α=96.734(2)°, β=91.107(2)°, γ=110.193(2)°, V=1659.1(2) Å] has been solved by direct methods and refined on the basis of F2 for all unique reflections to R1=0.058, which was calculated for the 5642 unique observed reflections (|Fo|?4σF). The structure contains topologically novel sheets of uranyl square bipyramids, uranyl pentagonal bipyramids, and MoO4 tetrahedra, with composition [(UO2)5(MoO4)8]6−, that are parallel to (−101). H2O groups and 1,4-diazabicyclo [2.2.2]-octane (DABCO) molecules are located in the interlayer, where they provide linkage of the sheets. The structure of 2 [triclinic, , Z=2, a=8.4004(4), b=11.2600(5), c=13.1239(6) Å, α=86.112(1)°, β=86.434(1)°, γ=76.544(1)°, V=1203.14(10) Å] has been solved by direct methods and refined on the basis of F2 for all unique reflections to R1=0.043, which was calculated for 5491 unique observed reflections (|Fo|?4σF). The structure contains topologically novel sheets of uranyl pentagonal bipyramids and MoO4 tetrahedra, with composition [(UO2)(MoO4)2]2−, that are parallel to (110). Ethylenediamine molecules are located in the interlayer, where they provide linkage of the sheets. All known topologies of uranyl molybdate sheets of corner-sharing U and Mo polyhedra can be described by their nodal representations (representations as graphs in which U and Mo polyhedra are given as black and white vertices, respectively). Each topology can be derived from a simple black-and-white graph of six-connected black vertices and three-connected white vertices by deleting some of its segments and white vertices. 相似文献
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Ilya V. Kornyakov Victoria A. Vladimirova Oleg I. Siidra Sergey V. Krivovichev 《Molecules (Basel, Switzerland)》2021,26(7)
Averievite-type compounds with the general formula (MX)[Cu5O2(TO4)], where M = alkali metal, X = halogen and T = P, V, have been synthesized by crystallization from gases and structurally characterized for six different compositions: 1 (M = Cs; X = Cl; T = P), 2 (M = Cs; X = Cl; T = V), 3 (M = Rb; X = Cl; T = P), 4 (M = K; X = Br; T = P), 5 (M = K; X = Cl; T = P) and 6 (M = Cu; X = Cl; T = V). The crystal structures of the compounds are based upon the same structural unit, the layer consisting of a kagome lattice of Cu2+ ions and are composed from corner-sharing (OCu4) anion-centered tetrahedra. Each tetrahedron shares common corners with three neighboring tetrahedra, forming hexagonal rings, linked into the two-dimensional [O2Cu5]6+ sheets parallel to (001). The layers are interlinked by (T5+O4) tetrahedra (T5+ = V, P) attached to the bases of the oxocentered tetrahedra in a “face-to-face” manner. The resulting electroneutral 3D framework {[O2Cu5](T5+O4)2}0 possesses channels occupied by monovalent metal cations M+ and halide ions X−. The halide ions are located at the centers of the hexagonal rings of the kagome nets, whereas the metal cations are in the interlayer space. There are at least four different structure types of the averievite-type compounds: the P-3m1 archetype, the 2 × 2 × 1 superstructure with the P-3 space group, the monoclinically distorted 1 × 1 × 2 superstructure with the C2/c symmetry and the low-temperature P21/c superstructure with a doubled unit cell relative to the high-temperature archetype. The formation of a particular structure type is controlled by the interplay of the chemical composition and temperature. Changing the chemical composition may lead to modification of the structure type, which opens up the possibility to tune the geometrical parameters of the kagome net of Cu2+ ions. 相似文献
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Evgeny V. Alekseev Sergey V. Krivovichev Prof. Dr. Wulf Depmeier Karsten Knorr 《无机化学与普通化学杂志》2008,634(9):1527-1532
Three new uranyl polyphosphates, α‐K[(UO2)(P3O9)] ( 1 ), β‐K[(UO2)(P3O9)] ( 2 ), and K[(UO2)2(P3O10)] ( 3 ), were prepared by high‐temperature solid‐state reactions. The crystal structures of the compounds have been solved by direct methods: 1 – monoclinic, P21/m, a = 8.497(1), b = 15.1150(1), c = 14.7890(1) Å, β = 91.911(5)°, V = 1898.3(3) Å3, Z = 4, R1 = 0.0734 for 4181 unique reflections with |F0| ≥ 4σF; 2 – monoclinic, P21/n, a = 8.607(1), b = 14.842(2), c = 14.951(1) Å, β = 95.829(5)°, V = 1900.0(4) Å3, Z = 4, R1 = 0.0787 for 3185 unique reflections with |F0| ≥ 4σF; 3 – Pbcn, a = 10.632(1), b = 10.325(1), c = 11.209(1) Å, V = 1230.5(2) Å3, Z = 4, R1 = 0.0364 for 1338 unique reflections with |F0| ≥ 4σF. In the structures of 1 and 2 , phosphate tetrahedra share corners to form infinite [PO3]? chains, whereas, in the structure of 3 , tetrahedra form linear [P3O10]5? trimers. The structures are based upon 3‐D frameworks of U and P polyhedra linked by sharing common O corners. The infinite [PO3]? chains in the structures of 1 and 2 are parallel to [100] and [–101], respectively. The uranyl polyphosphate frameworks are occupied by host K+ cations. 相似文献
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Chernyatieva A. P. Aksenov S. M. Krivovichev S. V. Yamnova N. A. Burns P. C. 《Crystallography Reports》2019,64(2):239-246
Crystallography Reports - A new compound of the Rb1.5(NH4)0.5{Cu(P2O7)} composition, obtained by the high-temperature reaction method from a mixture of RbNO3, Cu(NO3)2, and (NH4)4P2O7, has been... 相似文献
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