Bi-O Double Layers Research Articles

Oxygen atomic displacement waves in the lattice of the Bi,Pb-2223 phase composite annealed in an atmosphere with a low oxygen (O2 + N2) content

After annealing of Bi,Pb-2223/Ag composites in a nitrogen-oxygen atmosphere with a low oxygen content in the 2223 phase, cross-polarized waves of oxygen atomic displacements have been revealed by electron diffraction in the [010]2223 direction. It has been demonstrated using the methods of X-ray photo-electron spectroscopy and nuclear microanalysis that nitrogen does not interact with the 2223 lattice and that the oxygen index (9.67 ± 0.20) becomes less than the stoichiometric index (x = 10). It has been found that the generation of atomic displacement waves is due to the deficiency of oxygen in the double Bi-O layers.

Static and Dynamic Characteristics of Thermoelectric Ceramics

Structural characteristics of the high performance cobaltite thermoelectric semiconductors have been studied intensively by means of X-ray and neutron diffraction measurements and high resolution electron microscopy (HREM). These cobaltites consists of CoO2 triangular conducting sheets and several different types of block layers, i.e., Na, Ca, Sr single layers, three or four layered rock-salt layers, where Co-O2 square lattices are situated at their middle, and Bi-O or Tl-O double layers plus alkaline oxygen layers. Cold neutron scattering technique is employed to search possible low-energy excitation modes, being unique for nearly 1D and 2D crystals, and phonon density of states, DOS, of several high performance cobaltites at temperatures in the range from 10K to the ambient. Low energy, i.e., less than 2meV, excitation modes were found in the three different thermoelectric ceramics, i.e., γ -Na0.7CoO2, [Ca2CoO3]pCoO2, and [Ca2(Cu,Co)2O4]pCoO2. Possible origin of these low energy excitations are discussed in terms of low-energy corrugation mode generated due to weak chemical bondings, for which Van-der-Waals force dominates, between the CoO2 conduction sheets. These characteristics could be the key to realize low thermal conductivity and high-ZT of these ceramics.

Morphological Effects of Grinding on Bismuth-Based Cuprate Superconductors

Morphological changes of bismuth-based cuprate superconductors, such as the 2223 phase, via a small mechanical force were studied mainly by means of high-resolution transmission electron microscopy and atomic force microscopy. Even light mechanical grinding of the sintered 2223 samples led to the formation of superfine particles that had a width of {approximately} 5 nm. Moreover, the authors found first that the superfine particles had a discrete thickness of odd numbers of a half unit length c/2 of the 2223 phase, e.g. 1.9 nm, 5.6 nm, and so on. The formation of the superfine particles was due to cleavage at Bi-O double layers that were bound weakly to each other in a unit cell.

Direct atomic scale imaging of grain boundaries and defects in Bi-2223/Ag high- Tc superconducting tapes

Grain boundaries and defects in (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+ δ (Bi-2223)/Ag composite tapes are investigated by the atomic resolution Z-contrast imaging technique in the scanning transmission electron microscope. One of the most prevalent types of boundary between superconductor grain colonies is the c-axis twist boundary that is located in the middle of the double BiO layers. Although no amorphous layers are found to exist at these boundaries, local phase variations at/near the boundaries are frequently observed. Another typical grain boundary is the small-angle asymmetrical tilt boundary (ATB), which is parallel to the basal planes of one grain. At the small-angle ATB, different types of phases with various numbers of CuO 2 layers in their unit cell are formed periodically along the boundary plane. This means that this type of the boundary also occurs in the middle of the BiO double layers. These local phase variations exist in close proximity to the Ag interface and may give rise to weak link behavior. Additionally, many dislocations and stacking faults are observed within the filaments. Although these defects may provide a benefit in the pinning of magnetic flux, they also give rise to local variations of the superconducting properties.

Why the large-angle twist-boundaries in Bi2Sr2CaCu2O8+δ can carry super-current?

Our recent investigation on superconducting properties of Bi2Sr2CaCu2O8+δ bicrystals showed that the [001] twist grain-boundaries, regardless of their misori-entation angle, can carry the same critical current as their constituent single crystals. To find the origin of the absence of the weak-link behavior of the boundaries, we studied the structure of these electromagnetically characterized bicrystals using advanced transmission electron microscopy. The robust superconductivity at these boundaries was found mainly due to the high anisotropy of the crystals and the softness of the double BiO layers at the boundaries, which result the presence of the undisturbed CuO2 layers adjacent to the boundary plane. The structural characteristics of these boundaries are identical to those found in Bi-based tapes, suggesting that the large-angle twist boundaries are not a current-carrying barrier in this important material.

Structural origin of misorientation-independent superconducting behavior at [001] twist boundaries inBi2Sr2CaCu2O8+δ

A systematic investigation of the structure and properties of [001] twist boundaries was made in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ bicrystals. Contrary to conventional wisdom, all these boundaries, regardless of their misorientation angle, carried almost the same critical current as their constituent single crystals at magnetic fields up to 9 t. The origin of this misorientation-independent superconducting behavior at twist boundaries was sought by detailed structural characterization using high-resolution and nanoprobe transmission-electron microscopy. The robust electromagnetic properties of these grain boundaries were mainly attributed to the high anisotropy of the crystals, and to the softness of the double BiO layers at the boundaries which allow the ${\mathrm{CuO}}_{2}$ layers adjacent to the boundary plane to remain undisturbed. The structural characteristics of these boundaries are identical to those found in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{10+\mathrm{\ensuremath{\delta}}}$ tapes, suggesting that the large-angle [001] twist boundaries are not a current-limiting obstacle in this important conductor.

Intercalation Route to New Hybrid Organic-Inorganic Superconductors

Recently, we have successfully synthesized new type of high- Tc superconductors of (HgX2)0.5Bi2Sr2Cam-1CumOy(X˭Br, I, and m = 1, 2) by means of intercalation technique. From the X-ray diffraction(XRD) and X-ray absorption spectroscopy(XAS) analyses, we found that the intercalated mercuric halides are stabilized as the isolated gaseous molecules in the interlayer space of superconducting lattice, leading to an increase of basal spacing. A small amount of charge transfer between guest molecule and host lattice could be observed upon intercalation with a slight depression of Tc . For the extension of metal halide intercalation into high-Tc , superconductor, we have developed new type of hybridized superconducting nanohybrid with periodic stacking of organic and superconducting layer by stepwise synthetic approach, where organic compounds are stabilized in the form of organic-inorganic complex salt in-between Bi-O double layers of Bi-based high- Tc cuprates. The synthetic strategy proposed here can be applicable to many other layered systems

Intercalation Route to New Hybrid Organic-Inorganic Superconductors

Recently, we have successfully synthesized new type of high-Tc superconductors of (HgX2)0.5Bi2Sr2Cam-1CumOy(X˭Br, I, and m = 1,2) by means of intercalation technique. From the X-ray diffraction(XRD) and X-ray absorption spectroscopy(XAS) analyses, we found that the intercalated mercuric halides are stabilized as the isolated gaseous molecules in the interlayer space of superconducting lattice, leading to an increase of basal spacing. A small amount of charge transfer between guest molecule and host lattice could be observed upon intercalation with a slight depression of Tc . For the extension of metal halide intercalation into high-T c, superconductor, we have developed new type of hybridized superconducting nanohybrid with periodic stacking of organic and superconducting layer by stepwise synthetic approach, where organic compounds are stabilized in the form of organicinorganic complex salt in-between Bi-O double layers of Bi-based high- Tc cuprates. The synthetic strategy proposed here can be applicable to many other layered systems.

TEM study of twist boundaries and colony boundaries in (Bi,Pb) 2Sr 2Ca 2Cu 3O x silver-sheathed tapes

The grain boundaries in high- j c (Bi,Pb) 2Sr 2Ca 2Cu 3O x silver-sheathed tapes have been investigated using transmission electron microscopy. The typical microstructural feature in these tapes are the colonies, which consist of grains with a common c-axis. The grains within a colony are piled up along the common c-axis and are separated by [001] twist boundaries. It was found that the rotation angle of these twist boundaries takes several definite values. Colony boundaries are formed between colonies having slightly tilted c-axes with respect to each other and are virtually free from amorphous material and intergranular phases. Two basic types of colony boundaries have been observed: the small-angle c-axis tilt boundary and the edge-on c-axis tilt boundary. The weak bonds between the BiO double layers are responsible for the mica-like crystalline structure of Bi 2Sr 2Ca n−1 Cu n O x and their importance concerning the formation of colony boundaries is discussed.

Permanent photoinduced changes in the transport properties of Bi2Sr2CaCu2O8+ delta thin films.

We have observed permanent photoinduced decreases in both normal-state conductivity and ${\mathit{T}}_{\mathit{c}}$ for ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ epitaxial thin films. These changes which are induced by illuminating the films with high-power-density He-Ne laser light are opposite to and substantially smaller than the case of persistent photoconductivity in oxygen-deficient ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$. Moreover, no relaxation is observed even at room temperature. However, a characteristic time for the changes of a few tens of hours and insensitivity to illumination with infrared light are similar to the case of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$. These similarities, as well as the substantial dependence of the ${\mathit{T}}_{\mathit{c}}$ decrease on post-annealing conditions for these films, strongly suggest that these photoinduced phenomena are caused by a displacement of oxygen atoms in the Bi-O double layers.

Crystal structure, chemical composition, and extended defects of the high-Tc (Bi,Pb)2Sr2Ca(n)-1CunO4 + 2n + delta compounds.

This paper summarizes results obtained by high-resolution transmission electron microscopy on the crystal structure and microstructure of the (Bi,Pb)2Sr2Ca(n)-1CunO4 + 2n + delta high-Tc superconducting oxides. The experimental basis for the work presented here are high-resolution structure images obtained at ultra-thin (3 nm) areas of carefully prepared transmission electron microscope (TEM) samples. The analysis was carried out on a 400 kV TEM equipped with a pole piece yielding 0.17 nm point-to-point resolution. From the images obtained the projected crystal potential of the cations can be extracted directly, as confirmed by detailed image simulation. Structural analysis of the oxygen sublattice remains an unsolved problem by high-resolution TEM (HRTEM), mainly because of the small scattering factors, and thus the contribution of the oxygen sublattice to the image contrast is small. The (BiPb)2Sr2Ca(n)-1CunO4 + 2n + delta phases are modulated structures that can be understood as an average structure plus a superimposed displacement field. The crystal structure consists of BiO double layers and perovskite-type cuboids (containing Sr, Ca, Cu, and O), which are sandwiched between the BiO double layers. The displacement field can be directly analyzed by HRTEM, and the largest displacement amplitudes of 70 pm were determined for the Bi atoms in the n = 1 compound. The chemical composition of the n = 2 and n = 3 compounds was determined by EDX in the TEM for the cation sublattice. A significant (Ca + Sr) deficiency (approximately 10%) with respect to Cu was found. The (Sr + Ca)/Cu mole fraction ratio was 1.31 for the Bi-2212 phase and 1.14 for the Bi(Pb)-2223 phase. The oxygen content cannot be determined by EDX in the TEM with the accuracy necessary for a correlation with electrical and superconducting properties. The defect structure present in these materials, that is, intergrown lamellae with different crystal structures and equal or different chemical compositions, stacking faults, and grain boundaries, is summarized. The importance of grain boundaries for understanding and improving superconducting properties is emphasized.

High-pressure systhesis of bismuth oxycarbonate Bi 2(Bi, Sr, Ca) 6Ca(Cu, C) 4C 2O Z

A new compound, Bi 2Sr 6CaCu 4C 2O Z was synthesized at ∼ 2 GPa. Lattice parameters of the new compound are a≈ b=0.54nm and c=5.92nm. A high-resolution transmission electron microscopy (HRTEM) image has revealed two CO, four CuO 2, six SrO and one Ca planes stacked between double BiO layers. Electron microprobe analyses suggest that the ideal chemical formula changes significantly by partial substitution of Sr and Cu by Ca and C atoms, respectively.

The effects of anisotropy on the vortex dynamics and transport dissipation in Bi-Sr-Ca-Cu-0 compounds

Abstract The effects of the high anisotropy of Bi2Sr2CaCu2O8 on vortex dynamics and dissipation is reviewed. This high anisotropy results from the very weak coupling of conduction-electron states across the double Bi-O layers separating the conducting Cu-O bilayers. A model, based on Josephson coupling between the Cu-O bilayers, is shown to be consistent with the irreversibility behavior and the c-axis resistivity. The implications for conductor applications are described.

Microstructure and superconducting properties of attrition-milled Bi2Sr2CaCu2Ox

The microstructure and superconducting properties of Bi2Sr2CaCu2Ox (Bi-2212) during high-energy attrition milling were investigated in detail by a combination of x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and magnetization techniques. The starting superconducting powder was milled in a standard laboratory attritor using yttria-stabilized ZrO2 balls and a stainless steel tank. After selected time increments, the milling was interrupted and a small quantity of milled powder was removed for further analysis. It was found that the deformation process rapidly refines Bi-2212 into nanometer-size crystallites, increases atomic-level strains, and changes the plate-like morphology of Bi-2212 to granular submicron clusters. At short milling times, the deformation seems localized at weakly linked Bi-O double layers, leading to twist/cleavage fractures along the {001} planes. The Bi-2212 phase decomposes into several bismuth-based oxides and an amorphous phase after excessive deformation. The superconducting transition is depressed by about 10 K in the early stages of milling and completely vanishes upon prolonged deformation. A deformation mechanism is proposed and correlated with the evolution of superconducting properties. The practical implications of these results are presented and discussed.

Role of double Bi-O layers in flux pinning properties of Bi-oxides

We study DC magnetization of Bi-2212 single crystals and Bi-2223 polycrystalline pellets before and after intercalation with metal-phthalocyanines. Disruption of the interblock coupling due to the insertion of insulating ZnPc molecules between the unit cell blocks brings about a sharp decrease of irreversibility temperature T irr (in units of T irr/ T c) in the 2212 system, while T irr in the 2223 system is only moderately affected. This finding indicates that the interblock coupling is less significant for the flux pinning properties in the 2223 as compared to the 2212 system, probably due to a larger number of Cu-O layers contained within the unit cell block. On the other hand, similar experiment with NiPc has shown that the introduction of magnetic ions between the double Bi-O layers may be an efficient way to improve the flux pinning properties.

Ｂｉ‐２２２３相の微細化と熱的挙動

Refining behavior due to a small mechanical force and re-formation process of the 2223 phase on heating have been examined for the samples with a nominal composition of Bi1.8Pb0.4Sr2Ca2Cu3.2Oz by XRD, TEM, ED, AFM and TG-DTA. The 2223 phase is brittle and easily cleaved at Bi-O double layers in its crystals by a small mechanical force, resulting in changing into the superfine powder with a thickness of 1/2c. On heating the ground powder at 500°C to 800°C in air, various phases were formed from the superfine powder.

Hole Doping Mechanism in Bismuth Cuprates

The role of oxygen for the hole doping has been investigated for single‐phased (Bi,Pb)2Sr2Ca2Cu3O.z Defects introduced by oxygen have been clarified to be interstitial atoms by pycnometric density measurements. The c axis decreases and the modulation periods along the b axis also decrease with increasing oxygen content in the crystal. It is likely that extra oxygen ions exist between the BiO double layers and those oxygen ions are responsible for the modulation. Samples with oxygen content ranging from 9.99 to 10.18 showed superconductivity. The critical temperature Tc, exhibited a maximum value (109 K) at 10.07 oxygen atoms in the formula unit, which coincided with the average Cu valence of 2.13. The data indicated that hole formation was governed by oxygenation reaction in this system.

X-Ray Diffraction Line Broadening: Modeling and Applications to High-Tc Superconductors.

A method to analyze powder-diffraction line broadening is proposed and applied to some novel high-Tc superconductors. Assuming that both size-broadened and strain-broadened profiles of the pure-specimen profile are described with a Voigt function, it is shown that the analysis of Fourier coefficients leads to the Warren-Averbach method of separation of size and strain contributions. The analysis of size coefficients shows that the “hook” effect occurs when the Cauchy content of the size-broadened profile is underestimated. The ratio of volume-weighted and surface-weighted domain sizes can change from ~1.31 for the minimum allowed Cauchy content to 2 when the size-broadened profile is given solely by a Cauchy function. If the distortion co-efficient is approximated by a harmonic term, mean-square strains decrease linearly with the increase of the averaging distance. The local strain is finite only in the case of pure-Gauss strain broadening because strains are then independent of averaging distance. Errors of root-mean-square strains as well as domain sizes were evaluated. The method was applied to two cubic structures with average volume-weighted domain sizes up to 3600 Å, as well as to tetragonal and orthorhombic (La-Sr)2CuO4, which exhibit weak line broadenings and highly overlapping reflections. Comparison with the integral-breadth methods is given. Reliability of the method is discussed in the case of a cluster of the overlapping peaks. The analysis of La2CuO4 and La1.85M0.15CuO4(M = Ca, Ba, Sr) high-Tc superconductors showed that microstrains and incoherently diffracting domain sizes are highly anisotropic. In the superconductors, stacking-fault probability increases with increasing Tc; microstrain decreases. In La2CuO4, different broadening of (h00) and (0k0) reflections is not caused by stacking faults; it might arise from lower crystallographic symmetiy. The analysis of Bi-Cu-O superconductors showed much higher strains in the [001] direction than in the basal a-b plane. This may be caused by stacking disorder along the c-axis, because of the two-dimensional weakly bonded BiO double layers. Results for the specimen containing two related high-Tc phases indicate a possible mechanism for the phase transformation by the growth of faulted regions of the major phase.

O1s core levels of Bi-Sr-Ca-Cu-O superconductors studied by X-ray photoelectron spectroscopy

The O1s core level spectra recorded for the Bi 2Sr 2Ca n−1 Cu n O 2 n+2+δ ( n = 1, 2, and 3) oxides are investigated by taking into account the layered structures, the well developed cleavage between double Bi-O layers, and the surface sensitivity of photoelectron spectroscopy. The results show that the O1s core lines with binding energies of 527.9, 528.7, and 529.4 eV can be attributed to oxygen atoms in the Cu-O 2, Bi-O, and Sr-O layers, respectively. In the Bi 2Sr 2Ca 1− x Y x Cu 2O y system, the O1s line corresponding to oxygen atoms in the Cu-O 2 planes shifts significantly to the high binding energy side with the substitution of Y for Ca. These results are discussed in terms of hole concentrations of metal-oxygen bonds.

New cuprates with (Bi, Cu)-O monolayers: (Bi,Cu)Sr 2LnCeCu 2O 9−δ

New Bi-based cuprates with the “1-2-2-2” structure have been synthesized. The tetragonal structure of (Bi,Cu)Sr 2LnCeCu 2O 9−δ (space group I4/mmm) can be derived from that of the recently discovered superconductors (Bi,Cu)Sr 2YCu 2O 7− z , in which Y separates the CuO 2 layers responsible for the occurrence of superconductivity. When Y is replaced by fluorite-type blocks, LnCeO 2 (with Ln = Y, Sm, Eu, Gd, Dy and Ho), and the residual upper half of each unit cell is shifted by [ 1 2 1 2 0], the resu (Bi,Cu)Sr 2LnCeCu 2O 9−δ are isostructural to the superconductor (Pb,Cu)(Sr,Eu) 2(Eu,Ce) 2Cu 2O 9−δ. The compounds (Bi,Cu)Sr 2YCu 2O 7− z and (Bi,Cu)Sr 2LnCeCu 2O 9−δ with (Bi,Cu)-O monolayers can be considered as sister series analogous t the Bi 2Sr 2CaCu 2O 8 and the Bi 2Sr 2(Ln,Ce) 2Cu 2O 10 compounds, which both contain double Bi-O layers. A quantitative ana of almost-single-phase material for LnHo with respect to cation contents by EDS techniques suggests an approximate stoichiometry Bi 0.3Sr 1.9Ho 1.2Ce 0.9Cu 2.2O 9−δ. When synthesized at ambient pressure, the investigated samples are not superconducting above T = 1.9 K, for any choice of Ln.