Orthorhombic to tetragonal phase transition in high TC superconductor YBa2Cu3O7-y and NdBa2Cu3O7-z under high pressure

Abstract

X-ray powder diffraction measurements for YBa₂Cu₃O_7-y and NdBa₂Cu₃O_7-y were made at the intense synchrotron radiation source under high pressure up to 5 GPa. These samples were wrapped tightly in platinum foil to avoid deoxidizing atmosphere. The orthorhombic to tetragonal transition temperature increases with pressure in both samples. These results are discussed on the basis of the disordering of the oxygen atoms on the chain sites.     

Interface and grain boundary structures in YBa2Cu3O7-x and YBa2Cu4O8 materials

Y₂BaCuO₅YBa₂Cu₃O_7-x (Y211/Y123) interfaces in melt-processed YBa₂Cu₃O_7-x were studied by high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Yttrium enrichment and barium depletion were observed locally at the Y211/Y123 interfaces where Y123 (001) facets were present. This effect may be interpreted as the result of lattice substitution of Ba by Y near these interfaces. Cation nonstoichiometry was found near Y211/Y123 interfaces where liquid phases (Cu-Ba-O) were present. This chemical disorder introduces numerous point defects in the Y123, and these defects may act as additional pinning sites alongwith stacking faults. A comparison of grain boundary (GB) chemical composition in polycrystalline YBa₂Cu₃O_7-x and YBa₂Cu₄O₈(Y124), studied using nanoprobe parallel-detection electron energy-loss spectroscopy (EELS), is presented. The studies of Y124 show that stoichiometric grain boundaries can also form weak links between superconducting grains. It is suggested that weak-link behavior is determined largely by misorientation at grain boundaries.     

Structure of grain boundaries in YBa2Cu3O7 thin-film step-edge junctions

The atomic structure of 90° [100] (or [010]) tilt grain boundaries in YBa₂Cu₃O₇ thin-film step-edge junctions and, for comparison, in the interface between a-axis and c-axis oriented YBa₂Cu₃O₇ grains is investigated by means of high-resolution transmission electron microscopy. For (100)(001)-type boundaries two different structures are found. In the first a (001) CuO₂ plane of one grain faces a (100) Y–Ba–O plane of the other grain, in the second a (001) BaO plane faces a (100) Cu–O plane. In the former structure an incomplete unit cell of YBa₂Cu₃O₇ terminates at the boundary and a smaller strain in the adjacent CuO₂ planes is detected in comparison with the latter. It is found that a combination of a partial dislocation with a 124 stacking fault is a way to accommodate the lattice mismatch between c and 3a of YBa₂Cu₃O₇ in the boundary. For a symmetric (103)(103)-type boundary a displacement of the Cu-atoms of the CuO₂ planes is found near the boundary plane. From this a redistribution of the oxygen atoms around the Y-atoms located right in the boundary plane is inferred. The possible effect of the boundary structure on the superconducting properties of YBa₂Cu₃O₇ films is discussed.     

Specific contact resistivity of silver contacts on highT cYBa2Cu3O7 superconductors

Specific contact resistances of vacuum-evaporated silver contacts on new oxide superconductors, YBa₂Cu₃O₇ are measured using a modified Kelvin test structure. Contact resistivity increases continuously as the temperature decreases down to zero resistance point. AtT _c, its value is ∼0.15 ohm cm² which is high for technological applications. The nature of contacts appears different from that of semiconductor metal structures.     

Kinetics of oxygen desorption in Y Ba2Cu3O7-δ

In this paper the kinetics of oxygen desorption from Y Ba₂Cu₃O_7-δ has been investigated and measurements carried out in the temperature range 350–750 C. It is shown that the oxygen desorption is diffusion-controlled (and not surface-controlled), and characteristic of diffusion from small grains in a loosely coupled porous matrix. At every temperature a number of time constants with well-defined ratios appear and have been identified and measured. These time constants can be related to the average bulk diffusion constant which shows an Arrhenius behaviour with deviations around 750°C which are ascribed to the well-known orthorhombic to tetragonal transition. In this paper we establish the basic underlying physical mechanism of oxygen desorption in Y Ba₂Cu₃O_7-δ.     

Relation between highT c superconductivity and structure: A study of Ba3+x Cu6O14+δ and YBa2Cu3O7−δ systems

Superconductivity is found in tetragonal La_3−x Ba_3+x Cu₆O_14+δ and La, Ba)_6−x Sr _x Cu₆O_14+δ even though they do not possess Cu-O chains or the K₂NiF₄ structure. Resistivity measurements confirm the occurrence of a transformation from chain-superconductivity to sheet-superconductivity in YBa₂Cu₃O_7−δ as δ is varied in the range 0.0–0.5. Contribution No. 481 from the Solid State and Structural Chemistry Unit     

Ac-Josephson effect in YBa2Cu3O7/YBa2Cu3O7 point contact junctions

The ac-Josephson effect could be demonstrated up to dc voltages of 20 mV on the I–V characteristic of adjustable YBa₂Cu₃O₇/YBa₂Cu₃O₇ point contacts at 4.2 K. A detailed analysis of the rf power dependence of the size of microwave-induced constant voltage steps is given for microwave frequencies of 70 and 90 GHz. The results prove that electron pairing is responsible for the superconductivity in YBa₂Cu₃O₇.     

An “EPR” study of YBa2Cu3O7 and related high-temperature superconductors

Besides ag ≈ 2 signal which disappears on cooling to the superconductingT _c , YBa₂Cu₃O₇ and related oxides show a near-zero-field signal in the superconducting state with certain unusual features attributable to a “superconducting glassy state”. Contribution No. 443 from Solid State and Structural Chemistry Unit.     

Effect of nominal copper concentration in YBa2Cu3±x O7−δ on the superconducting transition

Variation of the copper concentration in the nominal composition YBa₂Cu_3±x O_7−δ markedly influences the superconducting transition. The transition occurs at the highest temperature with lower width, whenx = +0·25. Communication No. 82 from Materials Research Laboratory.     

Specific heat anomaly in the 90 K superconductor HoBa2Cu3O7−y

We report here the results of our heat capacityC _p measurements on a monophasic material HoBa₂Cu₃O_7−y . ΔC _p/T_c, the jump inC _p at the superconducting transition temperature (=91 K) of the material is measured to be 31 mJ/mol-K².     

Vortex-creep activation energy in YBa2Cu3O7/PrBa2Cu3O7 superlattices

YBa₂Cu₃O₇/PrBa₂Cu₃O₇ (YBCO/PBCO) superlattices with a different ratio of the superconducting and insulating layer thicknesses were prepared by high pressure dc sputtering. The vortex-creep activation energy U₀ was determined by analyzing the in-plane resistive transition of 200 μm wide bridges with the external magnetic field B oriented along the c axis. It was found that U₀ is proportional to the thickness of the YBCO layers, and does only weakly depend on the PBCO layer thickness, when the latter exceeds two unit cells. We observed a change in the variation of U₀ with the current I in the specimen: U₀ exhibits a plateau in the low-I region, then decreases significantly with increasing I. This behaviour is explained in terms of a crossover plastic vortex creep – elastic (collective) creep induced by the transport current.     

On the preparation and the characterization of superconducting thin films of YBa2Cu3O7−x

The results of our initial efforts to deposit thin films of YBa₂Cu₃O_7−x system on sapphire substrate are described. The deposited films are shiny black in appearance and are of quite uniform chemical composition. The annealed films exhibit zero resistance superconducting transition temperatureT _c(R=0) ranging between 23 K and 30 K.     

Studies of synthesizing behaviors and superconductivity of sol-gel YBa2Cu3O7-x samples in flowing oxygen atmosphere

Systematic studies of synthesizing behaviors of sol-gel YBa₂Cu₃O_7−x samples in flowing oxygen atmosphere and their superconductivity have been performed. A set of high temperature ρ-T curves has been obtained for the whole synthesizing process. After four rounds of synthesizing, the resistivity of the sample was around ρ=1.00×10⁻³Ω · cm at room temperature. The ρ-T curve of the fourth round shows that the orthorhombic to tetragonal phase transformation of the sample occurs around 600 °C, which is lower than that of the YBa₂Cu₃O_7−x sample prepared by conventional solid-state reaction method. Other measurements, such as X-ray diffraction, SEM measurement and low temperature R-T and M-T measurement, were also performed. And the R-T and M-T measurement results suggest that during the synthesizing process, there exist some state at which the sample has better superconductivity than the other states. Moreover, we found screw dislocations presenting on the sample broken surface from the SEM images. This will change the concept that the screw dislocations can only grow on the surface of the YBCO thin films and single crystals.      

Normal co-ordinate analysis of the zero wave-vector vibrations of DyBa2Cu3O7

Using Wilson’s F-G matrix method a normal co-ordinate analysis of the spectral frequencies and form of the zero-wave vector vibrations of the high temperature superconductor, orthorhombic DyBa₂Cu₃O₇ has been performed. The vibrational frequencies and the potential energy distribution of the 21 infrared-active and 15 Raman-active modes are presented. The potential constants employed here are presented and evaluated vibrational frequencies are compared with the available experimental values.     

Identification of the high-temperature superconducting phase in the Y-Ba-Cu-O system as the perovskite YBa2Cu3O7±δ

The oxide responsible for high-temperature superconductivity (onset ∼100 K, zero resistance above liquid N₂ temperature) is found to be YBa₂Cu₃O₇±δ. Contribution No. 432 from the Solid State and Structural Chemistry Unit     

Mössbauer study of Fe-substituted orthorhombic and tetragonal YBa2(Cu1-xFex)3O7-σ system

Mössbauer study of orthorhombic and tetragonal YBa₂(Cu_1-xFe_x)₃O_7-σ;x=0.02, 0.04, 0.08 has been done to investigate the two inequivalent Cu-sites. Fe substituting Cu having pyramidal oxygen co-ordination is in Fe³⁺ state while Fe substituting Cu having square planar co-ordination is in Fe³⁺ and Fe⁴⁺ states in dynamic equilibrium.     

Crystallographic and microstructural aspects of the superconductor YBa2Cu3O7-y —an overview

Crystal structures and structural inhomogeneities observed in YBa₂Cu₃O_7-y are reviewed. It is brought out that a proper understanding of the nature of the structural inhomogeneities is essential for exploiting the technological potential of this material. The need for an adequate characterization of specimens used for experiments is emphasized. Practical implications of the ferroelastic nature of the material are discussed.     

On the addition of silver to highT c (Y1Ba2Cu3O7−x ) superconductor

The superconducting transition temperature of the highT _c compound Y₁Ba₂Cu₃O_7−x does not show degradation consequent upon the addition of metallic silver up to a concentration of 60 wt per cent. The X-ray diffraction data confirm that the oxygen deficient distorted orthorhombic perovskite structure of the pure compound stays intact as if silver does not enter the crystal lattice. SEM studies do show the presence of silver at the grain boundaries only. Silver is found to restrict the grain growth.     

Oxygen stability in YBa2Cu3O7−x

The highT _c superconductor YBa₂Cu₃O_7?x has been found to lose appreciable amount of oxygen in a surprisingly low temperature range (～425–630 K), which drastically degrades the superconducting property. Accompanying this oxygen loss, the resistance behaviour has also been found to change from metallic to semiconducting.     

C-oriented SrBi2Nb2O9 films grown on YBa2Cu3O7−δ /SrTiO3 and NdGaO3

Epitaxial SrBi₂Nb₂O₉ films were grown by laser ablation on (001)YBa₂Cu₃O_7?δ /(100)SrTiO₃ and (001)NdGaO₃, with c axis normal to the substrate plane. The SrBi₂Nb₂O₉ films were grown in a layer-by-layer regime on NdGaO₃ substrates in 25-Å steps at a condensation temperature of 700 °C. Microinclusions of secondary phases and a-oriented grains were observed to exist on the surface of (001)SrBi₂Nb₂O₉ films grown on (001)YBa₂Cu₃O_7?δ /(100)SrTiO₃. The dielectric permittivity of the SrBi₂Nb₂O₉ films measured along the c axis is 123 (T=300 K, f=100 kHz), and tan δ≈0.04.