Analysis of the relation between structure and the superconducting transition in YBa2 Cu3 O∼7

Abstract

It is shown that in the high-T_c YBa₂ Cu₃ O_～7 superconductor the critical temperature is a function of the orthorhombic distortion (b–a)/a of the unit cell. From the extrapolation of the (b - a)/a ratio against T_c , a maximum critical temperature of 66 K for the tetragonal phase of YBa₂ Cu₃ O_～7 was predicted. From the correlation between the transition width δT_c and the orthorhombic distortion there was deduced an upper limit for T_c , in the orthorhombic phase of YBa₂ Cu₃ O_～7 of 94·5K.     

Atomic configurations of twin boundaries and twinning dislocation in superconductor Y0.6Na0.4Ba2Cu2.7Zn0.3O7− δ

Two types of twin boundaries in superconductor Y_0.6Na_0.4Ba₂Cu_2.7Zn_0.3O_{7? δ} , the cation-centered and oxygen-centered types, and the associated twinning dislocation have been studied by high-resolution electron microscopy. The structure map projected in the [001] direction was obtained from a single image by means of the image deconvolution technique. In this map, all columns of metallic atoms appear as individual black dots, and hence the two types of twin boundaries are distinguished from each other at atomic level. It is seen that the twinning dislocation occurs when the two types of twin boundaries meet each other. The structure model of the twinning dislocation together with the two types of twin boundaries has been derived straightforwardly based on the positions of black dots seen in the deconvoluted image.     

Twin formation and structural modulations in orthorhombic and tetragonal YBa2(Cu1-xCox)3O7-δ

Abstract

The microstructure of YBa₂(Cu_1?xCo_x)₃O_7?δ, prepared by the standard ceramic method, shows lamellar twin structures with decreasing spacings between twin walls with increasing Co content for x?0·02, developing into {110}-type cross-hatched ‘tweed’ modulation for x?0·02. Several wall junctions are found for x=0·02. The structural phase transition between macroscopically orthorhombic and tetragonal material occurs at x≈0·025; structural modulations (λ≈20Å) persist in the samples with high Co content (x>0·25). The modulations lead to a considerable broadening of the X-ray lines affected by orthorhombic splitting, and show maximum amplitude at the critical composition x≈0·025.     

Domains and Na substitution in the superconductor Y0.6Na0.4Ba2Cu2.7Zn0.3O7– δ

The high-temperature superconductor Y_0.6Na_0.4Ba₂Cu_2.7Zn_0.3O_{7– δ} , with a T _c of about 60?K, has been studied by electron diffraction and high-resolution electron microscopy. High densities of twin boundaries and anti-phase boundaries are observed and analysed. Darkness-level profiles at the positions of Ba and Y atoms in the [100] experimental and simulated images are used to determine the position and content of the Na atoms. It is shown that Na substitutes for Y in the crystal structure and the atomic ratio between Na and Y can be estimated quasi-quantitatively.     

Interfacial defects in YBa2Cu3O7-δ/SrTiO3(0 0 1) heterostructures studied by aberration-corrected ultrahigh-resolution electron microscopy

The microstructure of interfacial defects in YBa₂Cu₃O_7-δ/SrTiO₃(0?0?1) heterostructures has been investigated by aberration-corrected ultrahigh-resolution electron microscopy. We determine that c-axis-oriented YBa₂Cu₃O_7-δ thin films epitaxially grow on SrTiO₃(0?0?1) with two types of interface structure. The coalescence of nucleation sites with different types of interface structure leads to the formation of antiphase domain boundaries in YBa₂Cu₃O_7-δ thin films, which terminate at planar faults with different configurations near the interface. Stand-off misfit dislocations are observed and the dislocation core structure is explored. Based on the interface structure and interfacial defects, the initial growth mode of YBa₂Cu₃O_7-δ thin films on SrTiO₃(0?0?1) is discussed.     

Study of incommensurate modulation in V-doped La2Cu1- x (VO) x O4 + δ by transmission electron microscopy

Structural variations in a hole-rich grain of V-doped La₂Cu_{1? x} (VO) _x O_{4+ δ} (x?=?0.045, δ?=?0.011) superconducting compound have been studied by electron diffraction and high-resolution transmission electron microscopy. Two types of one-dimensional (1-D) incommensurate modulation were observed one after the other when the specimen was exposed to electron beam irradiation. The features of these two modulated structures have been studied and are discussed.     

Pressure-Induced transitions in amorphous TlxSe100−x alloys

Abstract

The electrical resistivity of bulk semiconducting amorphous Tl_xSe_100?x alloys with 0 ≤ x ≤ 25 has been investigated up to a pressure of 14 GPa and down to liquidnitrogen temperature by use of a Bridgman anvil device. All the glasses undergo a discontinuous pressure-induced semiconducting-to-metal transition. X-ray diffraction studies on the pressure-recovered samples show that the high-pressure phase is the crystalline phase. The pressure-induced crystalline products are identified to be a mixture of Se having a hexagonal structure with a = 4·37 Å and c = 4·95 Å and TlSe having a tetragonal structure with a = 8·0 Å and c = 7·0 Å     

Synthesis and evolution of crystalline garnet phases in Y3Sc5– x Ga x O12

Mixed-metal oxides with the composition Y₃Sc _x Ga_{5? x} O₁₂ (x?=?2.0, 2.1, 2.2, 2.25, 2.3, 2.4, 2.5, and 3.0) have been prepared by an aqueous sol–gel method. The effects of scandium substitution on the garnet phase formation were studied by IR spectroscopy and X-ray powder diffraction (XRD). The XRD data indicate that single-phase Y₃Sc _x Ga_{5? x} O₁₂ ceramic samples were obtained for x?=?2.0, 2.1, 2.2, 2.25, 2.3, and 2.4. The results also show that the formation of Y₃Sc _x Ga_{5? x} O₁₂ garnets depends on the molar ratio of scandium and gallium in the investigated composition, and consequently on the mean cationic radius at the Al³⁺ sites. The variation of lattice parameters for the Y₃Sc _x Ga_{5? x} O₁₂ phases with different x is reported.     

Thermal stability and glass-forming ability of Se80− x Te20Ag x (x = 0, 3, 5, 7 and 9) chalcogenide glasses

The thermal stability and glass-forming ability (GFA) of Se_{80? x} Te₂₀Ag _x (x?=?0,?3,?5,?7 and 9) chalcogenide glasses have been investigated using differential scanning calorimetry (DSC). The DSC runs have been taken at five different heating rates (10, 20, 30, 40 and 50?K/min) under non-isothermal condition. The thermal stability and GFA are monitored through determination of the temperature difference ΔT?=?T _c???T _g, where T _c is the onset crystallization temperature, T _g is the glass transition temperature, H ^l is the stability parameter, ΔH _c is the enthalpy released during crystallization and F _i is the fragility index. The activation energy of crystallization E _c and crystallization rate factor K have also been determined as indicators of the thermal stability of the above-mentioned samples. It is found that Se₇₁Te₂₀Ag₉ is the most stable among all the samples of the series.     

Crystal structure of 1–2–3 superconductor precursor material: Vacancy distribution and lattice relaxation

Abstract

The crystal structure of tetragonal YBa₂Cu₃O_7-δ, with δ close to the critical composition for the tetragonal–orthorhombic transition, has been refined. The oxygen deficiency δ = 0·65(10) is accommodated by both the O(1) and O(2) sites which coordinate the ‘square planar’ Cu(1) site. Structural relaxation around the vacancies is indicated by large anisotropic Debye-Waller ellipsoids in the structural layer formed by these atomic sites. The Y, Ba, Cu(2) and O(3) sites are not involved in the relaxation mechanism. On comparing this structure with the δ ≈ 0 and δ ≈ 1 end-members, we find that with decreasing oxygen content of the structure the Cu(2) valence type transforms from square-pyramidal character towards square-planar character. This effect gives the main contribution to the increase of the c ₀ lattice constant with decreasing oxygen content.     

Precipitation in a Cu–4Ti–2Be alloy

It is known that the binary Cu–2?wt.% Be alloy can be strengthened by ageing its supersaturated solution at 523?K. The increase in the strength arising from ageing is because of the formation of GP zones. The same level of strength is achieved by ageing the supersaturated solid solution of a dilute binary Cu–4?wt.% Ti alloy at 723?K. In contrast to Cu–2?wt.% Be alloy, the increase in strength in this alloy occurs on account of spinodal decomposition of the as-quenched microstructure through the mechanism of clustering and ordering. In the present study, decomposition of a supersaturated solid solution of the ternary Cu–4?wt.% Ti–2?wt.% Be alloy was studied. Whereas ageing of this alloy at 523?K resulted in the formation of GP zones like in a Cu–2?wt.% Be alloy, ageing at 723?K resulted in a spinodally decomposed microstructure as in a Cu–4?wt.% Ti alloy. The decomposition of the ternary alloy at the two temperatures by two different mechanisms indicates that Be and Ti in solution in Cu act, more or less, independently of each other during ageing.     

Formation of Al3Ni2-type nanocrystalline τ 3 phases in the Al–Cu–Ni system by mechanical alloying

An elemental powder mixture of Al (70 at.%), Ni (15 at.%) and Cu (15 at.%) was milled in a high-energy ball mill for various times ranging from 10 to 100?h to form ternary intermetallic alloys. X-ray diffraction and transmission electron microscopy techniques were employed for characterization of the samples. The dissolution of the individual elements into an alloy led to the formation of a τ₃ vacancy-ordered phase after 100?h of milling. This phase was found to be quite stable against milling, and no other crystalline and amorphous phases could be detected. The powder after 100?h of milling was found to contain mostly τ₃ nanophases with partial ordering, and with crystallite sizes in the range 10–20?nm along with a lattice strain of ～0.675%. The milled powder, after annealing at 700°C for 20, 40 and 60?h, revealed the formation of a strain-free and ordered τ₃ phase with a crystallite size of 80?nm, indicating grain coarsening. It is interesting to note that the mechanical energy imparted during milling could not completely destroy the vacancy ordering in the τ₃ phase, unlike other stoichiometric Al–Cu–transition metal (TM) systems, where the disordered B2 (bcc) phase is commonly observed instead of any vacancy-ordered phases.     

HIGH-Tc superconductivity in perovskite oxides of the Y─Ba─Cu─O system

Abstract

Several biphasic compositions of the type Y_3?xBa_3+xCu₆O₁₄ show an onset of superconductivity in the 90-115K range, attaining zero resistance in the 70-85K range. The phase responsible for superconductivity in these compositions is a perovskite oxide of composition YBa₂Cu₃ O₇. This oxide annealed in oxygen shows the onset of superconductivity at 120K and zero resistance at 87K. YBa₂,Cu₃O₇ shows the highest Meissner effect of all oxide superconductors. The superconducting behaviour of the two perovskite oxides, Y_0.95Ba_1.95,Cu₃O₇ and Y_1.05Ba_1.95Cu₃O₇ show interesting features; a marked decrease in resistivity is observed from room temperature itself in the former oxide with zero resistance at 89K. Electron microscopy and infrared spectra of these oxides are briefly discussed.     

Nanoindentation-induced elastic–plastic transition and size effect in α-Al2O3(0001)

The mechanical properties of α-Al₂O₃(0001) have been investigated using the technique of nanoindentation with a Berkovich indenter. Coupled with the Hertzian contact theory, a theoretical shear strength of 28?±?2?GPa was determined from the onset of pop-in events on load–displacement curves during loading, and the intrinsic hardness 30?±?3?GPa was obtained by analysis of the so-called indentation size effect, based on the concept of geometrically necessary dislocations. The predicted values of the shear strength, hardness and elastic modulus are in good agreement with available experimental data. The importance of experimentally calibrating the area function over the contact depth range prior to nanoindentation tests is emphasized.     

Formation of ω -Al7Cu2Fe phase during laser processing of quasicrystal-forming Al–Cu–Fe alloy

The formation of an ω-Al₇Cu₂Fe phase during laser cladding of quasicrystal-forming Al₆₅Cu_23.3Fe_11.7 alloy on a pure aluminium substrate is reported. This phase is found to nucleate at the periphery of primary icosahedral-phase particles. A large number of ω-phase particles form an envelope around the icosahedral phase. On the outer side, they form an interface with an α-Al solid solution. Detailed transmission electron microscopic observations show that the ω phase exhibits an orientation relationship with the icosahedral phase. Analysis of experimental results suggests that the ω phase forms by precipitation on an icosahedral phase by heterogeneous nucleation and grows into the aluminium-rich melt until supersaturation is exhausted. The microstructural observations are explained in terms of available models of phase transformations.     

Formation of self-assembled Cu-γ-Al 2 O 3 nanocomposite

We report here the formation of a novel 'self-assembled' three-dimensional nanocomposite consisting of uniformly sized, evenly spaced spherical Cu nanoparticles embedded within a polycrystalline n -Al 2 O 3 matrix. This was discovered serendipitously during the examination of an ion-beam-milled ZrO 2 specimen in a transmission electron microscope. This nanocomposite was deposited as a coating during ion-beam milling by the co-sputtering of Cu and Al from the ion-mill hardware in the presence of O. The mechanism by which this highly organized nanocomposite forms is discussed, together with broader implications of these types of metal-ceramic nanocomposite and the possibility of using sputter deposition as a generic method for making them.     

Investigations of the optical and EPR spectra for VO2+ in NaHC2O4 · H2O single crystals

Theoretical calculations of the optical absorption and electron paramagnetic resonance (EPR) spectra of VO²⁺ in NaHC₂O₄?·?H₂O single crystals were performed using the complete diagonalization energy matrix method (CDM) and the perturbation theory method (PTM) for the 3d ¹ electronic configuration. The calculated results are in good agreement with experimental results. The negative signs of hyperfine structure constants A _∥ and A _⊥ for VO²⁺ in NaHC₂O₄·H₂O single crystals are suggested from the calculations. Comparison of the two calculation methods indicates that the PTM is a good approximation of CDM and that both theoretical methods are valid in interpretation of the optical and EPR spectra for VO²⁺ ions in NaHC₂O₄?·?H₂O crystals.     

Self-diffusion activation energies in α-Al2O3 below 1000°C – measurements and molecular dynamics calculation

Results from impedance spectroscopy measurements at temperatures between 400 and 1000°C, for single crystal and highly pure and dense polycrystalline α-Al₂O₃ samples with well-defined grain size, are compared with that from molecular dynamics calculation. Between 650 and 1000°C, the measured activation energy for conductivity is 1.5?eV for the single crystal, and increases from 1.6 to 2.4?eV as the grain size decreases from 15 to 0.5?µm. The molecular dynamics calculation leads to the conclusion that the self-diffusion activation energy is about 1.5?eV for O and 1.0?eV for Al in single crystal α-Al₂O₃. The much higher mobility of O ions makes the O ions responsible for the conductivity of the single crystal oxide. It seems that the grain boundary leads to an increase in the activation energy. However, the quantitative influence of grain boundary still needs to be explained. Between 400 and 650°C, the measured activation energy is about 1.0?eV and independent of the grain size.