Zero resistance states in modified Y-Ba-Cu-O system with transition temperatures above 135 K

Superconducting transition of minor-dispersed phase in modified YBa₂Cu₃O_7?δ samples is found to occur at around 140K. The amount of this minor phase is enough to provide zero resistivity above 135 K. The measurements of the electrical resistivity indicated that the material is stable, thermally recyclable and reproducible. X-ray analysis of the sample with the highestT _c shows a major phase with perovskite-like structure witha=3·820(1) Å;b=3·873(1) Å andc=11·659(2) Å along with several unidentified weak peaks. Magnetic measurements confirmed the mixed-phase nature with diamagnetic transition temperatures at 137,91 and 86 K. The minor phase responsible for superconductivity with zero resistivity above 135 K is about 0·4% of the bulk and its nature is still unidentified. The details of the preparation and chemical modification process and the results are presented.     

YBa2SnO5·5, a novel ceramic substrate for YBCO and BiSCCO superconductors

YBa₂SnO_5·5 has been synthesized and sintered as single phase material for its use as substrate for both YBCO and BiSCCO superconductors. YBa₂SnO_5·5 has a complex cubic perovskite (A₂BB’O₆) structure with the lattice constanta = 8·430 Å. The dielectric constant and loss factor of YBa₂SnO_5·5 are in a range suitable for its use as substrate for microwave applications. YBa₂SnO_5·5 is found to be chemically compatible with both YBCO and BiSCCO superconductors. The thick film of YBCO screen printed on polycrystalline YBa₂SnO_5·5 substrate gave aT _c(0) of 92 K and a critical current density (J _c) of 4 × 10⁴ A/cm² at 77 K. A screen printed BiSCCO thick film on YBa₂SnO_5·5 substrate gaveT _c(0) = 110 K and current density 3 × 10³ A/cm² at 77 K.     

Single-crystal neutron diffraction study of ferroelectric Ba0·92Ca0·08TiO3

Crystal structure of BaTiO₃ doped with 8% Ca²⁺ is refined using single-crystal neutron diffraction data and it is shown that the doped Ca²⁺ ion substitutes only at the Ba sites. The refined cell (P4 mm) parameters area=b=3·982(3) Å,c=4·003(3) Å with a finalR value of 0·02 (onF). Existence of multiple domains in the crystal is ruled out based on refinement with multidomain model.     

151Eu Mössbauer studies on Zn-doped EuBa2Cu3O7−y

¹⁵¹Eu Mössbauer studies have been performed on the compounds EuBa₂(Cu_1?x Zn _x )₃O_7?y withx=0·0, 0·025, 0·05, 0·075 and 0·1. The parent compound, EuBa₂Cu₃O_7?y is superconducting with a transition temperature (T _c ) of 88 K.T _c is depressed as Zn is substituted for Cu in this system and the compounds withx>0·05 do not show superconductivity down to 12 K.¹⁵¹Eu Mössbauer studies at 295 K show a single Mössbauer line in all the compounds (whether superconducting or not) with isomer shift value typical of Eu^{3 +} ion. Further, the isomer shift values are nearly independent ofx and the temperatures down to 10 K. These observations imply that the Cu-O network responsible for superconductivity is very weakly coupled to the Eu sublattice.     

X-ray powder diffraction study of ZnLi0·5Mn0·5SnO4

X-ray crystallographic investigation of the title complex oxide composition reveals the presence of two phases. The first is a spinel phase which is somewhat consistent with Zn₂SnO₄ witha ₀=8·646 Å. The other phase has been identified as a non-spinel phase of LiMnSnO₄, the presence of which was confirmed by synthesizing it and comparing its XRD pattern with that of the title composition. The exact composition of the two phases has been determined by intensity calculations. It seems that while cooperative J-T effect is responsible for the observed symmetry of LiMnSnO₄, the distribution of phase along with separation of phases is attributed to the solubility limit of Sn⁴⁺ in such quaternary compositions.     

Thermoelectric power in Gd3+-substituted Cu-Cd ferrites

Polycrystalline Cd _x Cu_1−x Fe_2−y Gd _y O₄ ferrites fory=0·0 and 0·1 were prepared by ceramic technique. X-ray diffractograms of powder samples show cubic symmetry withx⩾0·2 fory=0·0 and 0·1, while compositions withx=0·0 fory = 0·0 and 0·1 are tetragonal. The thermopower measurements for Gd³⁺-undoped ferrites in the temperature range 300 K to 788 K shown-type conductivity forx⩾0·2. The substitution of Gd³⁺ changedn-type conductivity of the compositions top-type. The mobilities calculated show decreasing trend on Gd³⁺ substitution. The values of activation energy ΔE and drift mobilityE _d suggest polaron formation in substituted samples. The conduction mechanism is explained on the basis of localized model and formation of Gd³⁺+Fe²⁺ stable pairs at B site and Cu¹⁺+Fe³⁺ at A site.     

Studies of structural and dielectric properties of Ba5BiTi3Nb7O30 ceramics

The polycrystalline samples of Ba₅BiTi₃Nb₇O₃₀ (hereafter BBTN) belonging to ferroelectric oxide family of tungsten bronze structure were prepared by high temperature solid-state reaction method. Preliminary X-ray analysis of the samples provided the lattice parametersa=11·9331 Å,b=14·9684 Å, andc=7·0193 Å, and also formation of a single-phase orthorhombic structure at room temperature (303 K). Detailed studies of dielectric constant (ε) and loss (tanδ) as a function of frequency (500 Hz to 10 KHz) at room temperature and also as a function of temperature (liquid nitrogen to 160°C) show the dielectric anomaly and structural phase transition at 16·8°C.     

Single-Step Synthesis of Sr4V2O6Fe2As2: The Blocking Layer Based Potential Future Superconductor

We report synthesis, structural details and transport measurements on Sr₄V₂O₆Fe₂As₂. Namely, the stoichiometric amounts of V₂O₅+1/2×SrO₂+7/2×Sr+2×FeAs are weighed mixed, ground thoroughly and palletized in rectangular form in a glove box in high purity Ar atmosphere. The pellet is further sealed in an evacuated (10^?5 torr) quartz tube and put for heat treatments at 750 and 1150°C in a single step for 12 and 36 hours respectively. Finally the quartz ampoule is allowed to cool naturally to room temperature. The as-synthesized sample is black in color. The compound crystallized in P4/nmm space group with lattice parameters a=b=3.925 Å and c=15.870 Å. Also seen are some small impurity lines. The compound did not exhibit superconductivity but instead a spin density wave (SDW) like metallic step at around 175 K is seen in R(T) measurements. Principally in [FeAs]^?1{Sr₄V₂O₆}^C[FeAs]^?1 the net value of blocking layer charge C must be either less or more than 2, to let it be electron or hole type superconductor respectively. Efforts are under way to achieve superconductivity in the studied system.     

Magnetic properties of the mixed spinel Co1+x Si x Fe2−2x O4

The structural and magnetic properties of the mixed spinel Co_1+x Si _x Fe_2?2x O₄ system for 0·1≤x≤0·6 have been studied by means of X-ray diffraction, magnetization, and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Si⁴⁺ ions occupy only tetrahedral (A) sites replacing Fe³⁺ ions, and the added Co²⁺ ions substitute for (B) site Fe³⁺ ions. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at the A and B sites, forx≤0·3. The Mössbauer intensity data shows that Si possesses a preference for the A site of the spinel. The variation of the saturation magnetic moment per formula unit measured at 300 K with the Si content, is explained on the basis of Neel’s collinear spin ordering model forx≤0·3 which is supported by Mössbauer, and X-ray data. The Curie temperature decreases nearly linearly with increase of the Si content, forx=0·1–0·6.     

Oxygen-enrichment of YBa2Cu3 YBa2Cu3O7-δ using the fluidization techniqueusing the fluidization technique

The oxygen-deficient phase of the highT _c superconductor, YBa₂Cu₃O₇, was oxygen-enriched using the fluidization technique to give good superconducting properties. The normal method of oxygen treatment at 900°C for 24 h and at 600°C for 24 h has been reduced to just one treatment at 600°C for 12 h by the fluidization technique to achieve almost the same strength of superconducting signal for the YBa₂Cu₃O₇ powder, which establishes the attractiveness of the latter route for the large-scale preparation of superconducting material. The particle sizes were in the range 0–90, 90–180 and 180–420 μm. The fluidized particles were crystalline with orthorhombic distortion.T _c ^onset , estimated using the a.c. magnetic susceptibility method, was 91·3 K. The volume fraction of superconducting material in the product was 83·7–85·3%, one of the highest values reported so far for YBa₂Cu₃O₇.     

Effect of praseodymium substitution on the growth and properties of Y1−x Pr x Ba2Cu3O7−δ (o <x < 0·4) single crystals

In the present paper, a modified self-flux technique has been successfully employed for the growth of pure and praseodymium substituted (partially) large single crystals of high temperature superconducting Y_1−x Pr _x Ba₂Cu₃O_7−δ (x = 0·0,0·2,0·4). Typical sizes of the platy and bulky crystals of pure YBCO(123) material are ≈ 2 × 2 × 0·1 mm³ and 4 × 1 × 1 mm³, respectively. In case of Pr-substitution, the typical sizes of platy and bulky crystals of Y_0·8Pr_0·2Ba₂Cu₃O_7−δ and Y_0·6Pr_0·4Ba₂Cu₃O_7−δ materials are ≈ 2 × 3 × 0·1 mm³ and 5 × 1 × 1 mm³ and ≈ 1 × 1·5 × 0·1 mm³ and 7 × 0·2 × 0·1 mm³, respectively. The morphology and growth habit of the as-grown single crystals and the critical transition temperature (T _c) of the oxygenated crystals were found to depend on the Pr-content. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Hybrid open-frameworks: hydrothermal synthesis,structure determinations and magnetic properties of MIL-29, two copper diphosphonates (CuII(H2O))2{O3P–X–PO3} with X=C2H4, CH2–C6H4–CH2

Two copper diphosphonates were hydrothermally synthesized using ethylenediphosphonic and p-xylenediphosphonic acids, synthesized according the Arbuzov method. The structure of (Cu^II(H₂O))₂{O₃P–CH₂–CH₂–PO₃} already described from powder data, was solved from single crystal data. Its symmetry is monoclinic (space group P2₁/c (no. 14)) with lattice parameters a=8.0670(4) Å, b=7.5889(4) Å, c=7.3997(4) Å, β=116.281(2)°, V=406.18(4) ų, Z=2. The structure of (Cu^II(H₂O))₂{O₃P–CH₂–(C₆H₄)–CH₂–PO₃} was solved by powder X-ray diffraction (space group P2₁/c (no. 14)) with lattice parameters a=10.812(1) Å, b=7.577(1) Å, c=7.412(1) Å, β=92.34(1)°, V=606.7(2) ų, Z=2. Both structures are built up from identical inorganic layers covalently linked by the organic chains which act as pillars. The inorganic layers contain dimers of edge-sharing CuO₄(H₂O) square pyramids linked by the PO₃C tetrahedra. Both compounds are antiferromagnetic at 4(1) K.     

Electrical properties of LiNi3/5Fe1/5Cu1/5VO4 ceramics

In this work, LiNi_3/5Fe_1/5Cu_1/5VO₄ ceramic was prepared by solution-based chemical route whose X-ray diffraction results reveal an orthorhombic unit cell structure with lattice parameters of a = 9.6329 (23) Å, b = 6.8527 (23) Å, c = 5.4667 (23) Å. Complex electrical impedance and electrical conductivity were studied using complex impedance spectroscopy technique. The analysis of complex electrical impedance shows: (i) grain interior, grain boundary and electrode-material interface contributions to electrical response and (ii) the presence of temperature dependent electrical relaxation phenomena in the material. A detailed study of electrical conductivity indicates that electrical conduction in the material is a thermally activated process.     

Mutual phase stabilization of aluminium phosphate and titania in AlPO4-TiO2 binary system

Crystallization behaviour of amorphous aluminium phosphate (AlPO₄) and titania (TiO₂) in a mixed system of the two (5:1) has been reviewed in the light of our recent results. The polymorphous aluminium phosphate in such a binary system grows exclusively in a single phase over a temperature range 500–1150°C. The phase is reported to have a tridymite-like structure belonging to orthorhombic system with cell parametersa=9·638±0·0019,b=8·664±0·0017 andc=18·280±0036Å. Titania in the system preferentially retains its anatase phase morphology up to a temperature (950°C) well beyond its normal anatase → rutile transformation temperature showing a phenomenon of stabilization of this phase in such mixture. An interfacial reaction mechanism that can explain the observed phenomenon of mutual phase stabilization has been discussed and implications of this result towards the use of such technique for stabilization of various polymorphous compounds in a single phase has been pointed out.     

Growth and applications of superconducting and nonsuperconducting oxide epitaxial films

Growth and characterization of high-temperature-superconducting YBa₂Cu₃O₇ and several metallic-oxide thin films by pulsed laser deposition is described here. An overview of substrates employed for epitaxial growth of perovskite-related oxides is presented. Ag-doped YBa₂Cu₃O₇ films grown on bare sapphire are shown to giveT _c=90 K, critical current >10⁶ A/cm² at 77 K and surface resistance =450μΩ. Application of epitaxial metallic LaNiO₃ thin films as an electrode for ferroelectric oxide and as a normal metal layer barrier in the superconductor-normal metal-superconductor (SNS) Josephson junction is presented. Observation of giant magnetoresistance (GMR) in the metallic La_0·6Pb_0·4MnO₃ thin films up to 50% is highlighted.     

Compounds WAl<Subscript>4</Subscript>, WAl<Subscript>3</Subscript>, W<Subscript>3</Subscript>Al<Subscript>7</Subscript>, and WAl<Subscript>2</Subscript> and Al-W-N combustion products

X-ray diffraction data are presented for combustion products in the Al-W-N system. New, nonequilibrium intermetallic compounds have been identified, their diffraction patterns have been indexed, and their unit-cell parameters have been determined. The phases α-and β-WAl₄ are shown to exist in three isomorphous forms, differing in unit-cell centering. The phases α′-, α″-, and α?-WAl₄ are monoclinic, with a ₀ = 5.272 Å, b ₀ = 17.770 Å, c ₀ = 5.218 Å, β = 100.10°; point groups C12/c1, A12/n1, I12/a1, respectively. The phases β′-, β″-, and β?-WAl₄ are monoclinic, with a ₀ = 5.465 Å, b ₀ = 12.814 Å, c ₀ = 5.428 Å, β = 105.92°; point groups A112/m, B112/m, I112/m, respectively. The compounds WAl₂ and W₃Al₇, identified each in two isomorphous forms, differ in cell metrics (doubling) but possess the same point group: P222. WAl ₂ ^′ : orthorhombic, a ₀ = 5.793 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. WAl ₂ ^″ : orthorhombic, a ₀ = 11.586 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. W₃Al ₇ ^′ : orthorhombic, Pmm2, a ₀ = 6.225 Å, b ₀ = 4.806 Å, c ₀ = 4.437 Å. W₃Al ₇ ^″ : orthorhombic, Pmm2, a ₀ = 12.500 Å, b ₀ = 4.806 Å, c ₀ = 8.874 Å. The new phase WAl₃: triclinic, P1, a ₀ = 8.642 Å, b ₀ = 10.872 Å, c ₀ = 5.478 Å, α = 104.02°, β = 64.90°, γ = 107.15°.     

Thermal conductivity of Ni, Co, and Fe-doped La5Ca9Cu24O41 thin films measured by the 3ω method

The effect of Ni-, Co- and Fe-substitution for Cu on the thermal conductivity of La₅Ca₉Cu₂₄O₄₁ thin films is investigated. Highly b-axis oriented polycrystalline films were grown onto MgO (100) substrates using the pulsed laser deposition technique. Thermal conductivity measurements were made between 90 K and 300 K using the dynamic 3ω method. The room-temperature thermal conductivity of 1% Ni:La₅Ca₉Cu₂₄O₄₁ and 1% Co:La₅Ca₉Cu₂₄O₄₁ is approximately 15% smaller than the thermal conductivity of pristine La₅Ca₉Cu₂₄O₄₁ thin films of similar thickness. On the other hand, the thermal conductivity of 1% Fe:La₅Ca₉Cu₂₄O₄₁ is approximately the same as the thermal conductivity of pristine La₅Ca₉Cu₂₄O₄₁ thin films of similar thickness. These results enable us to indirectly conclude that Ni²⁺ and Co²⁺ ions substitute Cu²⁺ ions in the crystal structure while Fe ions are not incorporated in the lattice.     

Structure and Superconductivity of the Y4Ba8Cu12O27 Nanosuperconductor

A nanosuperconductor was prepared and characterized in the present work. The nanosuperconductor consisted of nanocrystals with superconducting transition temperature, T _c, of 88 K. Dimension of the nanocrystals is dozens nm in diameter and hundreds nm in length. The compound of the nanocrystals has a formula of Y₄Ba₈Cu₁₂O₂₇. The structure of the nanocrystals was found to be an orthorhombic perovskite, and the lattice constants were determined as a = 7.634 Å, b = 7.758 Å, and c = 11.654 Å, respectively.     

Increased Solubility of Mg at Ca Site in Cu<Subscript>0.5</Subscript>Tl<Subscript>0.5</Subscript>Ba<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductor

The Cu_0.5Tl_0.5Ba₂Ca_2?y Mg _y Cu₃O_10?δ (y=0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0) superconductor has been synthesized at the atmospheric pressure by the solid-state reaction method. The zero resistivity critical temperature is found to increase to 98 K with Mg concentration of y=0.6, but saturates to 97 K with further enhancement of Mg to y=0.8, 1.0, and 1.5. The Mg doped material grows in tetragonal structure and follows P4/mmm symmetry with a &; c-axes lengths of 3.894 Å &; 15.091 Å for y=1.5. The axes lengths were decreased with the increase of Mg content in the unit cell, which shows that anisotropy of the material decreases. The critical current density and the quantity of diamagnetism in the samples with Mg contents are higher than in the samples without Mg. In order to realize the effects of decreased axes lengths on the phonon modes of Cu_0.5Tl_0.5Ba₂Ca_2?y Mg _y Cu₃O_10?δ , we have carried out FTIR absorption measurements.     

Investigations of Bi substitution in NdBa2Cu3Oy

Attempts to substitute Bi for Nd in orthorhombic NdBa₂Cu₃O _y , prepared in air or oxygen at about 950°C led instead to formation of Ba₂NdBiO₆, a new cubic compound witha=0.8703 nm. The possibility was then explored of preparing superconducting (Nd_1–x Bi _x )Ba₂Cu₃O _y , by first forming the tetragonal phase at 880–950°C in nitrogen or argon followed by reheating in oxygen or air at 250–500°C in order to insert the additional oxygen required to yield the orthorhombic form while avoiding oxidation of Bi³⁺ to Bi⁵⁺. X-ray diffraction studies, electrical conductivity measurements, and thermogravimetric analysis of products indicate that Bi does not enter the NdBa₂Cu₃O _y , lattice in either the tetragonal or the orthorhombic phase. Ba₂NdBiO₆ clearly forms on reheating in oxygen or air even at low temperatures, and evidence is presented that a poorly crystallized oxygen-deficient form of this compound is already present prior to the reheating.