Crystallographic and magnetic structures of highT c related (Bi/Pb)2Sr2FeO6.25 determined by neutron powder diffraction

The structure of (Bi/Pb)₂Sr₂FeO_6.25 was refined from high resolution neutron powder diffraction data taken at 300 K and 5 K. The average structure is well described in the orthorhombic spacegroup Amaa witha=5.4254 Å,b=5.4909 Å andc=23.2245 Å. The FeO₆ octahedra are tilted by 2.7° towards (0 1 0). The oxygen positions in the (Bi/Pb)O-layers are consistent with the occurrence of (Bi/Pb)–O–(Bi/Pb) chains. Excess oxygen was located between these chains. Antiferromagnetic 3-dim ordering of the Fe-moments withM=(2, 1, 2) _B was observed at 5 K with the wavevector pointing along (1 0 0).     

Superconductivity,crystallization, and structural relaxation of the new metallic glass system La1−x Si x (0.15≤x≤0.27)

New metallic glass alloys have been prepared by melt spinning of La–Si samples in a pumped system. The superconducting transition temperature,T _c , increases linearly with the La concentration, from 3.00 K at 73 at. % La to 3.80 K at 85 at. %. Three new metastable phases: -, -, and -La₃Si, were formed by annealing and crystallizing amorphous La₃Si. Their crystal structures are orthorhombic (a=6.32 Å,b=8.06 Å,c=9.96 Å), hcp (a=10.55 Å,c=5.05 Å), and tetragonal (a=6.92 Å,c=5.05 Å) resp.T _c increased to 3.75 K, 6.00 K and 6.80 K, resp. During low temperature anneals of an amorphous La₃Si alloy,T _c changed logarithmically with time.On leave from Institute of Physics, Academica Sinica, Beijing, China     

The crystal structure of the prototypic ceramic superconductor BaPbO3: An X-ray and neutron diffraction study

The structure of the distorted perovskite BaPbO₃ was studied with high-resolution X-ray diffraction at 300 K and 26 K and with neutron diffraction at 300 K. Simultaneous refinement of the neutron and X-ray data sets (300 K) using the Rietveld method yields a monoclinic structure with the space groupI 2/m and lattice parametersa=6.0278 (1) Å,b=6.0664(1) Å,c=8.5109(1) Å, and =90.083 (2)^o. The tilting of the oxygen octahedra is given asa ^– a ^– c ⁰ in Glazer's notation [11]. The monoclinic angle corresponds to the angle between the cubic directions [110] _c and _c . This is in contrast to the observations in BaBiO₃. The structure of BaBiO₃ has the same space groupI 2/m, the similar dimensions of the unit cell and the same tilt system, but = ([110] _c , [001] _c ) as the monoclinic angle. As a consequence there is only one type and size of, PbO₆ octahedra but two types of octahedra in BaBiO₃. This fact may influence the occurrence of superconductivity in solid solutions (Ba(Pb_1–x Bi _x )O₃ containing a large fraction of lead by enhancing valence fluctuations.     

Electrical characteristics and low-temperature sintering of BiFeO3-modified Pb(Zr,Ti)O3–Pb(Fe2/3W1/3)O3–Pb(Mn1/3Nb2/3)O3 ceramics with ZnO addition

Effects of ZnO addition on electrical properties and low-temperature sintering of BiFeO₃-modified Pb(Zr,Ti)O₃–Pb(Fe_2/3W_1/3)O₃–Pb(Mn_1/3Nb_2/3)O₃ were investigated. The investigations revealed that the sintering temperature can be decreased to 950 °C, and the favorable properties were obtained with 0.10 wt% ZnO added ceramics. The electrical properties were as follows: d₃₃ = 313 pC/N, K_p = 0.56, tan δ = 0.0053, ε_r = 1407 and T_c = 295 °C, which showed that this system was a promising material for the multilayer devices application.     

The reaction process of the Bi-Sr-Ca-Cu-O system and the forming mechanism of the 2212 superconducting phase

The reaction process and the reaction behavior of each component in the Bi-Sr-Ca-Cu-O system are presented in this paper. It reveals that the reaction is carried out in three different stages: forming of an insulating interphase at 680°C–790°C, forming of the 2212 superconducting phase at 790°C–860°C and forming often semiconducting phases in the presence of the liquid phase at 860°C–970°C. It is also confirmed that the 2212 superconducting phase (T _c=85 K) is formed by the reaction of a trinary interphase together with CuO, SrO and CaO. A new two-step method is presented to prepare the 2212 superconducting phase by a presynthesized interphase.     

Roles of the blocking layer in high temperature copper-oxide superconductors

Roles of the blocking layer that we named the layer which separates Cu–O₂ layers more than 6Å have been studied by comparing two kinds of highT _c copperoxide superconductors such as Bi(2212) and La(2126) compound. These following results have been obtained. The hole concentration decreases when Sr is substituted by La and increases when Bi is substituted by Pb in Bi(2212), and it can be optimized by these substitution. In La(2126)T _c becomes up to 43K and the hole concentration (p;[Cu–O]^+p) increases to 0.09 by the substitution of Ca for La and heat treatment under high oxygen partial pressure. The distance between Cu–O₂ layers in both Bi(2212) and La(2126) are not changed by these substitution and heat treatment. We have found that the blocking layer has not direct roles for the maximumT _c value of the material though by supplying carriers to Cu–O₂ layers, it affects the actualT _c value.     

Crystal structure of a new superconducting high-pressure phase in the YBaCuO-System

A superconducting (T _c=40K) high-pressure phase recently discovered in the system of perovskite type YBaCuO structures is investigated by high-resolution transmission electron microscopy and electron diffraction in order to find its crystal structure. A structure model is proposed on the basis of a comparison between the observed images and image simulations based on crystal chemical considerations. The new phase has aB-centered orthorhombic cell with a monoclinic primitive cell. The primitive cell is composed of two subunits. The first of these is identical with the unit cell of orthorhombic YBa₂Cu₃O_7–x (1-2-3 structure) withT _c=92K. In contrast to the first subunit, the second one contains two adjacent Cu–O chains but is identical otherwise. This second subunit has been observed as one type of planar defect in the 1-2-3 structure. It is therefore concluded, that other stacking polytypes composed of these two different units could exist. The structure of the new phase is compared to the structures of the other known high-T _c superconductors. The chemical formula for the new phase can be written as Y₂Ba₄Cu₇O_14+x, with x0.5±0.2.     

Phase transformation and microtwinning in crystals of the high-T c superconductor YBa2Cu3O8−x,x≅1.0

We employed transmission electron microscopy (TEM) and single crystal X-ray diffraction techniques, supplemented by differential thermal and thermo-gravimetric analysis (DTA, TG) and high temperature powder X-ray diffraction, to study microtwinning in orthorhombic crystals of the high-T _c superconductor YBa₂Cu₃O_8–x (x1.0). This twinning is associated with a structural phase transition at 750°C from a tetragonal high temperature phase (s.g.P4/mmm) to the orthorhombic ambient temperature phase (s.g.Pmmm) and seems to be inherent to virtually all orthorhombic crystals of YBa₂Cu₃O_8–x . The domain size ranges from typically 100 Å to 1000 Å. All our observations are compatible with a twin law where the tetragonal (110)-mirror plane is the twin element.     

Structural phase transitions at high-temperature in double perovskite Sr2GdRuO6

The crystal structure evolution of the Sr₂GdRuO₆ complex perovskite at high-temperature has been investigated over a wide temperature range between 298 K≤T≤1273 K. Powder X-ray diffraction measurements at room temperature and Rietveld analysis show that this compounds crystallizes in a monoclinic perovskite-type structure with P2₁/n (#14) space group and the 1:1 ordered arrangement of Ru⁵⁺ and Gd³⁺ cations over the six-coordinate M sites, with lattice parameters a=5.81032(8) Å, b=5.82341(4) Å, c=8.21939(7) Å, V=278.11(6) Å³ and angle β=90.311(2)^o. The high-temperature analysis shows that this material suffers two-phase transitions. At 373 K it adopts a monoclinic perovskite structure with I2/m space group, and lattice parameters a=5.81383(2) Å, b=5.82526(4) Å, c=8.22486(1) Å, V=278.56(2) Å³ and angle β=90.28(2)^o. Above of 773 K, it suffers a phase transition from monoclinic I2/m to tetragonal I4/m, with lattice parameters a=5.84779(1) Å, c=8.27261(1) Å, V=282.89(5) Å³ and angle β=90.02(9)^o. The high-temperature phase transition from monoclinic I2/m to tetragonal I4/m is characterized by strongly anisotropic displacements of the anions.     

39K quadrupole coupling and spin-lattice relaxation in the high temperature phases in KLiSO4

³⁹K quadrupole perturbed nuclear magnetic resonance spectra show that in KLiSO₄ atT _c =743 K a phase transition from a room temperature hexagonal to a high temperature orthorhombic phase takes place. The high temperature phase is definitely not incommensurately modulated. The huge shortening of the³⁹K spin-lattice relaxation time on approachingT _c from below demonstrates that KLiSO₄ becomes a superionic conductor above 743 K. The self-diffusion coefficient of the Li-ions is estimated asD=10^–6 cm²/s at 780 K.     

The effect of Sb concentration on the superconducting properties and phase composition of Bi-Pb-Sb-Sr-Ca-Cu oxide materials

The effect of Sb superconducting materials with nominal composition Bi_1.6Pb_0.4–x Sb _x Sr₂Ca₂Cu₃O _y (x=0.01–0.14) and Bi_1.5Pb_0.5–x Sb _x Sr₂Ca₂Cu₃O _y (x=0.025–0.12) has been investigated. These materials were subjected to DTA, ICP emission spectrometry, SEM, XRD and electric resistance measurements. It was found that the introduction of Sb raises the melting temperature of the material, it also raisesT _c (up to 110K) at several defined stoichiometric ratios. A certain lack of correspondence between the 2223 phase volume and theT _c value was noticed and a possible explanation has been proposed.     

Superconductivity and processing of single Bi−O layered cuprates in the (Bi,Pb)−Sr−(Ca,Y)−Cu−O system

The effects of improved materials processing on single Bi–O layered cuprates in the (Bi, Pb)–Sr–(Ca, Y)–Cu–O system have been investigatged. For Bi-1212 we have improvedT _c to 102 K. The bulk nature of superconductivity is confirmed by the presence of superconducting volume fractions ( ZFC) around 30–40%. The critical current density is 2×10⁶ Acm^–2 at 5 K and 0 T. Moreover, indications for the presence of a second phase probably Bi-1223 with a transition to superconductivity in the range of 115–150 K have been found.     

Superconductivity and normal electrical conductivity of amorphous lead films nucleated with molybdenum

Lead films vapor quenched onto nucleating monolayers of Mo or W exhibit strong lattice disorder and can be considered to be amorphous. The amorphous-to-crystalline transformation temperatureT _tr is indicated by a sharp drop of the electrical resistivity in the course of annealing.T _tr is found to be proportional tod ^–2 for Pb thicknessd smaller than 30 nm. The superconducting transition temperatureT _c is by 0.6 to 1 K smaller in the amorphous state than after crystallization. In both states,T _c is proportional tod ^–1. Prenucleation with about half a monolayer of Mo leads to quite the sameT _c depression as observed earlier by Strongin et al. on Pb films vapor quenched onto predeposited films of SiO, Ge or Al₂O₃. For comparison, experiments have been carried out with 2.5 nm Ge predeposits. As with Mo prenucleation, a well defined transformation temperatureT _tr of about the same value has been observed.T _c of bulk amorphous Pb can be extrapolated to be about 6.6 K.     

Monoclinic symmetry in LaSrCu0.5Co0.5O4−z found by neutron,Guinier-and synchrotron-X-ray powder structure analysis

High resolution X-ray and neutron powder diffraction data of semiconducting LaSrCu_0.5Co_0.5O_3.7 have been collected in the temperature range of 5 to 300 K. A monoclinic structure with space groupI 2/m a=3.7952(1) Å,b=3.7902(1) Å,c=12.6507(1) Å, =90.074(1) Deg was refined with the Rietveld method for individual data sets as well as for the collection of all data simultaneously. The structureI 2/m results from an internal shearing of the octahedra and-unlikeCmca in superconducting La_1.85Sr_0.15CuO₄-not from a second order phase transition inI4/mmm. An alternative model with two tetragonal latticesa ₁=3.7902,a ₂=3.7952,c=12.6507 Å is discussed.     

Magnetic transitions and magnetoresistance of -type () compounds

Magnetic transitions and magnetotransport properties of polycrystalline Tb_1−xGd_xMn₆Ge₆ (x=0.2–1.0) compounds have been investigated by magnetic property and resistivity measurements in an applied magnetic field up to 50 kOe. The cell parameter a,c and cell volume V of compounds (x=0.2–1.0) increase with an increasing Gd content. The compounds (x=0.2–1.0) show a rich variety of magnetic behavior, such as antiferromagnetic, ferrimagnetic and paramagnetic state with increasing temperature. Their Curie temperatures increase almost linearly with an increasing Gd content from 460 K for x=0.2 to 484 K for x=1.0. The compounds (x=0.2–1.0) display the field induced metamagnetic transitions, and the threshold fields first increase and then decrease with an increasing Gd content. The magnetoresistance curves of the Tb_0.4Gd_0.6Mn₆Ge₆ compounds in an applied magnetic field up to 50 kOe are presented and the magnetoresistance effects are related to the metamagnetic transitions.     

Formation of a cubic Sr2MnWO6 phase at elevated temperature; a neutron powder diffraction study

In a temperature dependent neutron powder diffraction (NPD) study we observed the high temperature cubic phase at 973 K in the polycrystalline double perovskite Sr₂MnWO₆. Rietveld analysis of the NPD data shows that the room temperature tetragonal phase exists up to 573 K (space group P4₂/n, a=8.0119 (4) Å, c=8.0141(8) Å). At 773 K, the primitive tetragonal symmetry change to body-centred tetragonal (space group I4/m, a=5.6935(5) Å, c=8.077(1) Å) and finally at 973 K it becomes face-centred cubic (space group Fm-3m, a=8.0864(8) Å). The changes in the structural symmetry are connected to the small distortion of the B-site octahedra, which are insensitive to the Differential Thermal Analysis (DTA) signal.     

Effects of nano-carbon doping and sintering temperature on microstructure and properties of MgB2

We fabricated nano-carbon (NC) doped MgB₂ bulks using an in situ process in order to improve the critical current density (J_c) under a high magnetic field and evaluated the correlated effects of the doped carbon content and sintering temperature on the phase formation, microstructure and critical properties. MgB_2−xC_x bulks with x = 0 and 0.05 were fabricated by pressing the powder into pellets and sintering at 800 °C, 900 °C, or 1000 °C for 30 min.We observed that NC was an effective dopant for MgB₂ and that part of it was incorporated into the MgB₂ while the other part remained (undoped), which reduced the grain size. The actual C content was estimated to be 68–90% of the nominal content. The NC doped samples exhibited lower T_c values and better J_c(B) behavior than the undoped samples. The doped sample sintered at 900 °C showed the highest J_c value due to its high doping level, small amount of second phase, and fine grains. On the other hand, the J_c was decreased at a sintering temperature of 1000 °C as a result of the formation of MgB₄ phase.     

Synthesis of Co2N by a simple direct nitriding reaction between nitrogen and cobalt under 10 GPa and 1800 K using diamond anvil cell and YAG laser heating

Synthesis of cobalt nitrides has been tried in a supercritical nitrogen fluid at high pressure (about 10 GPa) and high temperature (about 1800 K) using diamond anvil cell and YAG laser heating system. We have succeeded to synthesize a single phase of the CFe₂-type Co₂N easily in a short time. This is the first synthesis by a simple reaction between the pure cobalt and pure nitrogen (supercritical fluid nitrogen). The cell parameters of the synthesized Co₂N are a=4.662(9) Å, b=4.332(5) Å and c=2.749(9) Å, respectively.     

Chiral superconducting phase transition in 3-K phase of Sr2RuO4

We have investigated the transports of micro-fabricated sample of 3-K phase superconductivity (T_c  3 K) in Sr₂RuO₄–Ru eutectic system in order to clarify the pairing symmetry. Up to now, pure Sr₂RuO₄ (T_c = 1.5 K: 1.5-K phase) is widely recognized to be a spin-triplet odd-parity superconductor. However, the enhancement mechanism of T_c up to 3 K and the pairing symmetry of the 3-K phase have not been cleared yet. By using micro fabrication technique with focused ion beam, we have succeeded to extract individual superconducting channels for the 3-K phase in which only a few pieces of Ru inclusions are contained. Multiple kink structures observed in differential resistance–current (dV/dI − I) characteristics indicate serially connected superconducting filaments in the 3-K phase. We confirm that the 3-K phase is an odd-parity superconductor similar to pure Sr₂RuO₄ from the monotonous temperature dependence of the critical currents. In addition, we observed a quite unusual hysteresis in dV/dI − I below 2 K, which suggests the internal degrees of freedom in the superconducting state: the most probably the chiral p-wave state.