Preparation and Characterization of HgO and AgO Added La2CaBa2Cu5O z Superconductors

We have prepared a series of La₂CaBa₂Cu₅O _z (La2125) compounds by the standard solid-state reaction method. The oxygen stoichiometry in this R-123 type compound has been homogenized by two different methods. The first method is heating the sample in flowing oxygen at 500 °C for 6 hours and then slowly cooling to room temperature. The second method is to provide internal oxygen while sintering the sample by adding HgO or AgO. The addition of HgO or AgO in preparing oxide superconductors is advantageous when preparing them in bulk quantities, as HgO or AgO provide internal oxygen which is a much essential ingredient of the structure of oxide superconductors. Such a method of synthesis eliminates the need of additional oxygen annealing, thus enabling the synthesis in huge quantities at once. The composites referred to as La2125+xHgO and La2125+xAgO respectively, have been characterized by XRD, SEM, resistivity and magnetic measurements for their structural and superconducting properties. We compare the properties of oxygen-annealed and HgO/AgO added compounds and discuss the results in the light of applicability of such method in synthesizing bulk quantities of high-T _c oxide superconductors.      

Effect of Ca Substitution on the Superconductivity of La2.5Y0.5CaBa3(Cu0.88Fe0.12)7O z

The structural and superconducting properties of single-phase La_2.5?y Y_0.5Ca_1+y Ba₃ (Cu_0.88Fe_0.12)₇O _z (LYCaBCuFe) (y= 0.0?1.0) compounds with triple perovskite structure are investigated using X-ray diffraction, resistivity, a.c. susceptibility, and oxygen content measurements. Increasing Ca substitution for La resulted in a decrease in unit cell axes and volume. T _c ^R=0 shows a marginal increase from 31 K to 37 K for y = 0.0?0.21 and thereafter it decreases with increasing y leading to zero T _c ^R=0 at y ≥ 0.84. This shows that the suppression of T _c from 80 K to 31 K by Fe doping at x = 0.12 La_2.5Y_0.5CaBa₃(Cu_1?x Fe _x )₇O _z cannot be compensated by appropriate hole doping with Ca in LaYCaBCuFe.     

Properties of RBaCuFeO5+δ (R = Y, La, Pr, Nd, Sm-Lu)

The structural properties, thermal stability, thermal expansion, and electrical properties of the RBaCuFeO_5+δ (R = Y, La, Pr, Nd, Sm-Lu) layered ferrocuprates have been studied systematically. The linear thermal expansion coefficients of the RBaCuFeO_5+δ phases are ? 13 × 10^?6 and ? 13 × 10^?6 K^?1 for R = La-Eu and R = Gd-Lu, respectively. Their electrical conductivity varies widely, from σ = 2.0 × 10¹ S/cm for LaBaCuFeO_5.47 to σ = 8.4 × 10^?8 S/cm for TmBaCuFeO_5.06 at 300 K. The lattice parameters, thermal expansion, and conductivity of RBaCuFeO_5+δ vary nonmonotonically with the ionic radius of R³⁺ (atomic number of the rare earth), which is due to the fact that the 4f electrons of the rare-earth ions play a significant role in determining the structural and transport properties of these phases.     

Superconducting YBa2Cu3O7-δ thick films on Ba2RETaO6 (RE = Pr5 Nd, Eu, and Dy) substrates

Thick films of YBa₂Cu₃O_7-δ fabricated on polycrystalline Ba₂RETaO₆ (where RE = Pr, Nd. Eu, and Dy) substrates by dip-coating and partial melting techniques are textured and oaxis oriented, showing predominantly (00/) orientation. All the thick films show a superconducting zero resistance transition of 90 K. SEM studies clearly indicate platelike and needlelike grain growth over a wide area of the thick films. The values of the critical current density for these thick films are ∼10⁴ A/cm² at 77 K as determined by the nonresonant R.F. absorption method. Various processing conditions that affect the critical current density of thick films are also discussed.     

Superconductivity behaviour of screen-printed LnBa2Cu3O7 (Ln=Eu,Y) films

Thick films of the highT _csuperconducting oxides, LnBa₂Cu₃O₇, Ln = Eu, Y, have been fabricated by screen printing on alumina and SrTiO₃ substrates. Conditions for optimum superconductivity behaviour of the films have been established.T _c ^onset varies from 90–94 K for all the films but zero resistance was observed only in a few cases.     

Structural, Transport, and Magnetic Properties of (Ru0.9Nb0.1)Sr2(Gd0.67R0.67Ce0.66)Cu2O z (R = Nd, Gd, and Tb)

Samples with nominal compositions of (Ru_0.9Nb_0.1)Sr₂(Gd_0.67R_0.67Ce_0.66)Cu₂O _z (R = Nd, Gd, Tb) were prepared, and the influence of doped rare-earth element on the structural, magnetic and superconducting properties of these samples has been investigated. Resistivity and room-temperature thermoelectric power measurements showed that the superconductivity is mainly affected by the change in the hole concentration induced by doping of the rare-earth element. All of these samples exhibited weak ferromagnetic behavior at temperatures below approximately 90 K with the branching of zero-field-cooled (ZFC) and field-cooled (FC) magnetization. In the ZFC magnetization measurements, diamagnetic transition was observed at temperatures below approximately 15 K for the R=Gd and Nd samples. Magnetization measurements also revealed that the partial substitution of Tb for Gd results in a significant enhancement of weak-ferromagnetic component of the FC magnetization, as well as an increase in the magnetic ordering temperature up to 20 K. A quite opposite behavior was observed in the FC magnetization for the case of the R=Nd sample. The experimental results are discussed in conjunction with the local structural changes in the Ru sublattice, which are induced by doping of the rare-earth element, based on the results of Rietveld refinement of the X-ray diffraction data.     

Composites Ag5Pb2O6/M (M=CuO, Cu2O) and Exciton Models of Superconductivity

Resistive properties of composites Ag₅Pb₂O₆/M (M=CuO, Cu₂O) have been studied in the vicinity of the percolation threshold (concentration w/w～0.48). Metallic particles of the Byström-Evers oxide Ag₅Pb₂O₆ are covered by an overlayer of Cu₂O (～8 nm). Measurements suggest the exciton type superconductivity originally proposed by Little 1D model and extended to the 2D configuration by Ginzburg. An application of the Ginzburg model to the Ag₅Pb₂O₆ particles dressed in the Cu₂O overlayer involves monovalent Ag₂ dimers moving in the hexagonal channels of the Ag₅Pb₂O₆ lattice. Dimers are considered be unsaturated on the grain surface. It is applied to the Swiss cheese model of the percolation, and dielectric holes between metallic particles are assumed to act as resonant microcavities driving the strong coupling of the photon field to excitons in the Cu₂O overlayer. Accordingly, polariton states may be created and possible polariton mechanism of the superconductivity is discussed in terms of the model put forward by Laussy and coworkers.     

Synthesis of New Pb-based 1222 Layered Cuprates in the (Pb,S)Sr2(RE,Ce)2Cu2O z (RE=Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Y) System

A new family of the Pb-based layered cuprate with the 1222 structure has been synthesized in the (Pb,S)Sr₂(RE,Ce)₂Cu₂O _z (RE=Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Y) system. The almost-single phase samples are obtained for x=0.25 and y=1.0 in the composition of (Pb_1?x S _x )Sr₂(RE_2?y Ce _y )Cu₂O _z . Each of the samples has the crystal structure with a tetragonal symmetry. Both of the lattice parameters a and c are decreasing with the decrease of the ionic radius of RE element. Among these samples, the sample with RE=Er has the smallest resistivity in the whole temperature range and the slope of the resistivity versus temperature is the most gentle. Despite annealing under O₂ atmosphere at high-pressure of 121 atm, this sample is a semiconductor with the transport process characteristics of three-dimensional variable range hopping conduction. Then the samples of RE=Er with smaller y are investigated for the composition of (Pb_0.75S_0.25)Sr₂(RE_2?y Ce _y )Cu₂O _z , the sample with y=0.4 is found to show an onset of resistivity drop phenomenon at about 11 K with the decrease of temperature. Moreover, we also discover that the samples of RE=Ho with y=0.4 show an onset of the resistivity dropping at over 20 K and zero resistivity at 13 K. From these results, the present (Pb_0.75S_0.25)Sr₂(RE_2?y Ce _y )Cu₂O _z system is considered to be a new family of the Pb-based superconducting cuprates with the 1222 structure.     

Phonon Raman scattering of R2CuO4 (R = Pr,Nd, Sm,and Gd)

For Gd₂CuO₄, a new symmetry-forbidden phonon for the tetragonal T′ structure is observed. This implies that oxygen in the CuO₂ plane is locally distorted along the CuO₂ plane. Such distortion has never been observed for other T′-type superconductors, which show superconductivity by Ce doping. The disappearance of superconductivity for Gd can be correlated with the appearance of the distortion due to oxygen in the CuO₂ plane.     

The crystal structures of Ba2R2/3V2O8 (R = La, Nd) and Sr2La2/3V2O8; palmierite derivatives

The title phases Ba₂R_2/3V₂O₈ (R = La, Nd) and Sr₂La_2/3V₂O₈, synthesised by solid state reaction of oxides at 1350°C, have structures derived from that of the palmierite-type of Ba₃V₂O₈; the stoichiometry is often written as A₃RV₃O₁₂. Ba₂La_2/3V₂O₈ is hexagonal, spacegroup R3m, a = 5.75271(7), c = 21.0473(5) Å, Z = 3; cation ordering was determined by joint Rietveld refinement using X-ray and neutron powder diffraction data, R _wp = 4.45%, R _p = 6.33%, ² = 6.847. In the Ba₃V₂O₈ structure, Ba occupies two sites: 3a and 6c. In Ba₂La_2/3V₂O₈, Ba wholly occupies the 3a site; the 6c site contains both Ba, La and vacancies. Bond valence analysis was inconclusive, but tends to support the presence of Ba on the 3a site. Sr₂La_2/3V₂O₈ and Ba₂Nd_2/3V₂O₈ are isostructural, as confirmed by Rietveld refinement using X-ray powder diffraction data. Ba₂R_2/3V₂O₈ phases could not be synthesised for lanthanides, R, smaller than Sm.     

Mössbauer effect of SrLnFeO4 (Ln=La,Pr, Nd,Sm, Eu,Gd)

The Mössbauer effect has been studied in SrLnFeO₄ compounds (Ln=La, Pr, Nd, Sm, Eu, Gd) with K₂NiF₄ structure in the temperature range between 80K and 459K. The N?el temperture was determined to be 330±5K for SrLaFeO₄, 250±5K for SrPrFeO₄, 270±5K for SrNdFeO₄ and 400±10K for SrSmFeO₄. The results of the combined magnetic and quadrupole interactions indicated that the direction of internal magnetic field is perpendicular to the Z-axis of the EFG tensor and parallel to the c-axis of the crystal structure.     

K_2Ln(NO_3)_5·2H_2O(Ln=La,Ce,Pr,Nd,Sm)的晶体生长和热重-差热分析研究

本文生长出了K2Ln（NO3）5.2H2O（Ln=La；Ce；Pr；Nd；Sm）的单晶，并对其进行了晶体结构及差热-热重分析研究．结果表明，K2Ln（NO3）5.2H2O（Ln=La；Ce；Pr；Nd）的晶体属正交晶系，Fdd2空间群．首次生长出KPrN单晶并用直接法解出其晶体结构．解得KPrN的晶胞参数为：a=11．2210（10）,b=21．411（3），c=12．208（2）,Z=6；R=0．0240．对KLnN加热，则依次出现脱水、熔化、不可逆相变和NO的分解过程（K2Ce（NO3）5·2H2O除外）K2Ln（NO3）5·2H2O（Ln＝La；Nd；Sm）的NO分三步分解，K2Ln（NO3）5·2H。O（Ln=Ce；Pr）的NO分两步分解·KNO3和Ln（NO3）3·nH2O的混合物在225℃左右生成K2Ln（NO3）5     

Oxygen Configuration in the Superconductor (Hg0.7Mo0.3)Sr2(Ca1−xRx)Cu2Oz (R = Nd and Pr)

Two series of Hg-based oxides (Hg_0.7Mo_0.3)Sr₂(Ca_1?x R _x )Cu₂O _z (R = Nd and Pr, 0.2 ≤ x ≤ 0.7) have been synthesized. Electrical-resistivity measurements show that these compounds are superconductors with maximum onset T _c of 107 and 102 K for Nd- and Pr-containing samples, respectively. The neutron powder diffraction experiments on both as-prepared and O₂-annealed samples of R = Nd revealed that the O(3) site at the HgO_δ sheets are fully occupied and shifted towards the Hg/Mo site to form Mo–O bonds.     

MVO4-Ta2O5-Nb2O5 (M = Y, La) solid solutions

The subsolidus phase relations in the MVO₄-Ta₂O₅-Nb₂O₅ (M = Y, La) systems were studied. MNb_2-2xTa_2xVO₉ (I) and MTa_2-2xNb_2xVO₉ (II) solid solutions were prepared and characterized by x-ray diffraction, vibrational spectroscopy, and electron spin resonance. The range of solid solutions in I isx = 0–0.58 for M = Y and 0–0.4 for M = La; in II,x = 0–0.09 for M = Y and 0–0.1 for M = La. All of the solid solutions were found to contain V(IV) centers whose nearest neighbor environment depended on composition. The structure of LaTa_2-2xNb_2xVO₉ was shown to contain fragments characteristic of LaNb₂VO₉. The lattice parameters of these solid solutions correlate with the V(IV) content.     

Superconductivity and Magnetism in RuSr2GdCu2O8

The ruthenocuprate RuSr₂GdCu₂O₈ orders magnetically at 135 K and then becomes superconducting at 45 K. These transitions have been observed by several groups, but the intrinsically complex nature of the compound, as well as the unexpected coexistence of magnetism with high-temperature superconductivity, makes it uncertain what the magnetic, electronic, and structural character of its ground state is. We find that density functional-based evaluations give a magnetic and structural character that is consistent with the latest data: the RuO₆ octahedra rotate (by 7° around the c^ axis) causing a doubling of the cell, and an antiferromagnetic structure that has this same cell doubling is favored over a ferromagnetic alignment of Ru spins. The minority Ru d _xy states are partially occupied, leaving a metallic RuO₂ layer that dopes the CuO₂ bilayer and may lead to canting of the Ru spins.     

Average Co magnetic moment of R-Co-B (R = Y,Pr and Nd) compounds

We estimate the average Co magnetic moment and account for the moment variation by a model in Y_{n + 1}Co_{3n + 5}B_2n (n = 0, 1, 2, 3 and ∞), Pr_{m + n}Co_{5m + 3n}B_2n (m = 2, n = 1 and m = 2, n = 3), Nd_{m + n}Co_{5m + 3n}B_2n (m = 3, n = 2) and Y₂Co₁₄B compounds. And we obtain the magnetic properties of hypothetical Y_{m + n}Co_{5m + 3n}B_2n (n = 1, m = 2, 3, 4 and 5) compounds.     

New Ru-Based Layered Cuprates with 1232 Structure in the RuSr2(LnCe2−x Sr x )Cu2O z (Ln = Sm, Eu, and Gd) System

New Ru-based layered cuprates with 1232 structure have been synthesized in the RuSr₂(LnCe_2?x Sr _x )Cu₂O _z (Ln = Sm, Eu, and Gd) system. Nearly single-phase samples are obtained in the composition range from x = 0.0 to 0.2 for Ln = Sm and Eu, and at the composition of x = 0.2 for Ln = Gd. Each of the samples of Ln = Sm, Eu, and Gd with x = 0.2 crystallizes in a tetragonal lattice and lattice parameters a and c are 0.3845 and 1.700 nm, 0.3842 and 1.699 nm, and then 0.3839 and 1.698 nm, respectively. Each of them shows some anomalies at a low temperature in addition to a semiconductor-like temperature dependence of the resistivity. Especially, the sample with Ln = Gd shows two resistivity dropping phenomena at about 16 and 22 K. Then, all of nearly single-phase samples show diamagnetic signals originating from superconductivity, and also rapid increase of magnetization due to a transition from paramagnetic state to ferromagnetic one below 80 K.