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Superconductive La2CuO4+delta with T-c similar to 40 K has been obtained by chemical oxidation using NaClO solution. The oxygen-decrease processes have been studied during the thermal treatment processes at different temperatures. The material separates into discrete phases with different transition temperatures for samples treated at 100 degrees C consistent with the oxygenation process reported previously. However, Tc decreases continuously in samples treated at 300 degrees C, similar to the superconducting properties of La2-xSrxCuO4. In addition, the coexistence of superconducting and antiferromagnetic phases cannot be observed in our samples. The interrelation between the transition temperature and the treatment conditions such as temperature and time is discussed. (c) 2005 Elsevier B.V. All rights reserved.

The correlation between the oxygen partial pressure, p(O 2 ), at the annealing process after heat treatment (post-annealing) and the J c value at 4.2 K in 10 T is investigated for Bi-2212/Ag tapes and Bi-2212 single crystal. The p(O 2 ) was controlled in the range from 0.01 to 14.5 atm, and high oxygen-pressure post-annealing was performed for the samples by using an autoclave furnace. The J c -B characteristics of Bi-2212/Ag tape were found to depend strongly on the condition of post-annealing; the optimum condition of post-annealing for obtaining a Bi-2212/Ag tape with the highest J c value is considered to be the p(O 2 ) 6 atm at 300 °C. The improvement of the J c value by post-annealing under the condition of p(O 2 ) < 6 atm is considered to be due to the decrease in the electromagnetic anisotropy that occurs as a consequence of the change in the excess oxygen content of the Bi-2212 phase. On the other hand, it is assumed that the degradation of the J c value by higher oxygen-pressure post-annealing is the result of the deterioration of the T c value. It suggests that the delicate balance between the decrease in the electromagnetic anisotropy and the deterioration of the T c by over-doping should be optimized for the improvement of the Bi-2212/Ag tapes.

We have prepared the superconducting La 2 CuO 4+ δ electrochemically oxidized at 333 K. Each sample is a single-phase superconductor; the low- T c phase ( T c =32 or 36 K, oxidation time=24 h, δ =0.068) or the high- T c phase (45 K, 72 h, 0.093). We have studied the relation between the hole concentration n H , determined by the Hall effect measurement as n H ≡1/ eR H and the valence of Cu 2+ p , determined by the iodometric titration in La 2 CuO 4+ δ . In highly oxidized La 2 CuO 4+ δ , the p /2-dependence of n H has been found to be different from that of the ordinary (La 1− x Sr x ) 2 CuO 4 . Similar phenomena were observed in (La 1− x Sr x ) 2 CuO 4+ δ ( x =0.050, 0.075, 0.100) electrochemically oxidized at 333 K. These results suggest that in La 2 CuO 4+ δ or (La 1− x Sr x ) 2 CuO 4+ δ , the intercalated excess oxygen plays a different role in conduction from Sr in the ordinary (La 1− x Sr x ) 2 CuO 4 .

The structure of a La 2 CuO 4+ y single crystal with the excess oxygen content y =0.03 has been studied by means of neutron diffraction at T =13 K. Although the extra oxygen content of y =0.03 is inside the miscibility gap of La 2 CuO 4+ y (0.01 y Cmca symmetry. It was confirmed that the extra oxygen, O4, is located close to the site (1/4, 1/4, 1/4). The structural parameters for all atoms lay between those previously determined for the oxygen-poor and oxygen-rich phases representing the edges of the miscibility gap, y =0.01 and 0.06, respectively. Our data allow us to conclude that (1) the phase-separated and non-phase-separated La 2 CuO 4+ y compounds are structurally identical, and (2) the crystal structure of La 2 CuO 4+ y changes monotonically when the extra oxygen content changes from 0 to 0.05.

Polycrystalline samples of Bi 2 Sr 2− x La x CaCu 2 O y and pure Bi2212 with different oxygen content have been prepared and studied by Raman spectroscopy. No frequency change of O Bi A g mode is observed. The O Sr A g mode exhibits remarkable downward frequency shift. The analysis indicates that the content of excess oxygen inserted into the Bi 2 O 2 layer is the important factor causing frequency shift of the O Sr A g mode.

X-ray structure analyses were performed with HgBa2CuO4+δ and Hg1−xPbxBa2CuO4+δ single crystals. The onset of the superconducting transition of the crystals ranges from 78–92 K. Hg-1201 crystallizes with space group P4/mmm and has lattice pafameters a = 3.8907(2) A and c = 9.556(1) A . The structure was refined to R = 0.023 (wR2 = 0.036) in the ideal 1201 type with an excess oxygen content of 6(2)% and an occupancy of 97% at the Hg site. Pb-doping decreases the lattice parameters to a = 3.8836(2) A and c = 9.553(1) A , and the refinement converged at R = 0.042 (wR2 = 0.088). Pb atoms are shifted by 0.5 A to (0.14, 0, 0) and the excess oxygen atom O(3) by 0.45 A to (0.38, 1 2 , 0). It has not a well defined position in the lattice, but is distributed randomly in a ring-like configuration around the ( 1 2 , 1 2 , 0) site, supporting a Pb clustering model. Rather high anisotropic temperature factors U11 and U22 of Hg and apical O(1) atoms are observed, which are also present in higher members of the Hg(Pb)-12(n − 1)n series. They are discussed in terms of corrugated OHgO bonds. Comparison with other structure refinements of 1201 shows a correlation of the transition temperature with the position of the Ba atom, which seems to be a good indicator for the hole doping evel in 1201.

A study of the magnetic properties of La 2 Cu 1 − x Zn x O 4 + δ (0 ≤ x ≤ 0.13; δ = 0.02) by means of magnetization measurements ( M vs T , H ) is reported. The Neel temperature of the 3D antiferromagnetic order (AF) is found to decrease with Zn substitution much more rapidly than in samples without excess oxygen. The effect of Zn substitution on the metamagnetic M vs. H transition is discussed, analyzing the temperature dependence of both the critical field H c and the weak ferromagnetic moment M s for different compositions x .

YBa2Cu3−xAlxO6+δ single crystals with different Al contents (x = 0.06, 0.14 and 0.19) were heat treated in different atmospheres. The oxidized as-grown crystals show a macroscopically tetragonal tweed-structure. The oxygen content δ in the Cu(1) layer of these oxidized samples is significantly lower than 1, which results from the Al coordination disturbing the ordering in the chain-oxygen sublattice. Neutron-diffraction data show that upon reduction at 1044 K the oxygen content δ within the Cu(1) layer remains at 0.25 due to pinning of excess oxygen by the aluminum at x = 0.19. Crystals annealed in reducing atmospheres at temperatures above 973 K develop orthorhombic long-range order after reoxidation at 673 K, probably due to short-range clustering of the AlOn defects. The obtained orthorhombicity of the reoxidized material decreases with decreasing temperature of the reduction treatment. No change in the structural state is obtained when the reducing treatment is performed below 950K; the reoxidized samples remain macroscopically tetragonal. Annealing the samples at 1073 K in oxygen restores the tweed structure. An evolution of the tweed structure with annealing time towards orthorhombic long-range order was not observed.

Electrochemicaly oxidated La 2 CuO 4+δ was investigated by means of SQUID-magnetometry and 139 La NQR. Several superconducting phases with different thermal stability have been discovered. It is supposed that the differences between the most stable phases are connected with the ordering of the excess oxygen.

The compositional diagrams representing the stability regions of the 2212 and 2201 phases of the bismuth system were constructed for several temperatures above 820°C. The actual densities of both phases were determined by pycnometry and compared with the theoretical densities. It was found that the maximum ion deficiency is about 2% for the 2212 phase and nearly zero for the 2201 phase. The valences of Bi and Cu were determined by chemical analysis and the excess oxygen was calculated. The maximum excess oxygen was found to be 2.5% and 3% for the 2212 and 2201 phases, respectively. From the ion deficiency and the excess oxygen, it was concluded that the excess oxygen occupies not the vacancy but the interstitial site.

The dependence of the superconducting transition temperature T c on the excess oxygen content δ has been investigated for Bi 2 >Sr 2 >CaCu 2 >O 8+δ annealed under high pressures of oxygen. With increasing δ, T c decreases for δ≲0.3 and then becomes constant for δ≳0.3. It has been found by chemical titration that this strange δ dependence of T c is related to the δ dependence of the average valency of Cu ions. That is, the average valency increases with increasing δ for δ≲0.3 and then is saturated for δ≳0.3. For δ≳0.3, the Curie-Weiss component in the DC magnetic susceptability has been found to increase with increasing δ. Therefore, the saturation is ascribed to the fact that most of the excess oxygen atoms exceeding δ≈0.3 are introduced in the form of O 2 instead of O 2- .

A model of the electronic structure is proposed for the centres produced in YBa 2 Cu 3 O 6+ x by excess oxygen in the basal plane at x ⪡1, showing them to contribute considerably to the low temperature magnetic susceptibility but not to the ESR signal.

A new method has been developed for the reversible removal of excess oxygen atoms or holes in the Tl-Ba-Ca-Cu-O type of superconductor without altering its general lattice structure. It has been found by this method that changes in T c and Raman spectrum of the superconductor can be simply correlated with the concentration of excess oxygen and that a 634 cm −1 phonon is closely associated with the superconducting holes. Replacement of 16 O by 18 O atoms at the holes causes a 3.6% decrease in the frequency of this phonon without a similar change in T c . With these new results as supporting evidence, a least vibration model of superconductivity is suggested.

The ceramic La 2 CuO 4+ δ with excess oxygen is found to have two superconducting phases, whose transition temperatures are 32 K (36 K) of the low T c phase and 45 K of the high T c phase, respectively. In this paper we report the normal state susceptibilities of the two kinds of almost single superconducting phases. The susceptibility of the high T c phase was found to be similar to that of optimally doped (LaSr) 2 CuO 4 . The susceptibility gradually decreased with decreasing temperature from 300 K. On the other hand, in the low T c phase, the first large susceptibility peak suggesting some antiferromagnetic ordering was observed at about 260 K, and then the second small peak at about 215 K. In the preliminary measurements, the susceptibility of the powdered low T c phase showed a Curie-type increase from about 215 K down to the superconducting transition temperature.

We have studied oxygen evolution, as a function of increasing temperature, of four oxygen-doped high T c cuprates. The oxygen effusion spectra of La 2 CuO 4+δ , YBa 2 Cu 3 O 6+δ , HgBa 2 CuO 4+δ , and Bi 2 Sr 2 CaCu 2 O 8+δ share unusually similar two-peak feature. Although excess oxygen ions (δ) occupy different crystallographic sites in different systems, i.e., interstitial site in La 2 CuO 4+δ and Bi 2 Sr 2 CaCu 2 O 8+δ , CuO chain in YBa 2 Cu 3 O 6+δ , and “normal” lattice site in HgBa 2 CuO 4+δ , the oxygen atoms evolve in two distinct steps upon heating. Considering the strong coupling between excess oxygen ions and doped holes, we suggest that this two-peak feature is intrinsic to the doped holes of all high T c cuprates.

The relation between the crystal structure and superconductivity has been investigated for La 2-x Bi x CuO 4+δ with excess oxygen, in which the phase separation observed in La 2 CuO 4+δ is suppressed. In the range of 0.03≲δ≲0.1 and T≲100 K, the crystal structure is orthorhombic for x = 0.05, while it is tetragonal for x =0.10. With increasing δ, T c increases and then reaches the maximum value, 20–25 K, at δ ≳ 0.05 for 0.05⩽x⩽0.10, independent of the crystal structure. The overall symmetry of the crystal structure is concluded not to be so responsible for the appearance of high-T c superconductivity.

The spectroscopic evidence of metal-insulator (M-I) transition in Bi2Sr2Ca1−xEux Cu2O8+;δ has been identified. Observed spectral changes due to Eu doping have been related with (i) incorporation of excess oxygen atoms between the BiO layers and (ii) a decrease in the hole concentration in the CuO2 planes.

The results of our electronic structure calculations for the hole carrier densities in the one, two, and three CuO 2 layer Bi- and Hg-based compounds are presented. We find that, in the absence of oxygen atoms in the plane of Hg, the hole density in the CuO 2 planes is nearly zero, and thus the presence of an excess oxygen in the Hg-planes is critical for superconductivity in these compounds. We also find Bi2201 to be highly overdoped, Bi2212 to be overdoped, and Bi2223 to have nearly optimum hole density.

Antiferromagnetic nuclear magnetic resonance (AFNMR) of the Cu(2) site and the Cu-NQR of the Cu(1) site are investigated as a function of excess oxygen, y, in Pb 2 Sr 2 ( Y 1− x Ca x ) Cu 3 O 8+ y . We confirm the re-appearance of the antiferromagnetic order of the Cu(2) moments and the change of the valence of the Cu(1) site with hole-doping introduced by extra oxygen in the superconducting Pb 2 Sr 2 ( Y 0.5 Ca 0.5 ) Cu 3 O 8+ y .

Substitution of allovalent ions in the 2212 and 2201 phases changes the excess oxygen content and gives a maximum Tc(zero) value at appropriate concentrations of allovalent cations. The excess oxygen content for the maximum Tc(zero) value becomes the boundary between the oxygen doping region and the oxygen overdoping one.

The x-ray diffraction patterns for the Bi 2 Sr 2 Ca 1−x Y x Cu 2 O 8+y samples sintered in air and Ar indicated that the reduction in the number of excess oxygen ions induced a structural change from orthorhombic to pseudotetragnal. When the oxygen contents decreased, the period of modulation in Bi 2 Sr 2 Ca 0.4 Y 0.6 Cu 2 O 8+y increased from 4.2b to 4.4b.

Single phase samples of Tl 2 Ba 2 CuO 6+δ with a T c between 80 K and less than 4 K were investigated by 205 Tl NMR experiments indicate that the oxidation state of the thallium ions is predominantly 3+. The resonance line shapes, line positions and spin-lattice relaxation times as well as their temperature dependences are comparable for all samples of Tl 2 Ba 2 CuO 6+δ , irrespective of their T c . Redistribution of holes between TlO layers and CuO 2 planes can hence be disregarded. The hole concentration in Tl 2 Ba 2 CuO 6+δ is dominated by the amount of excess oxygen in the TlO layers. The change in T c with the relative change in oxygen content suggests that the phase diagram of Tl 2 Ba 2 CuO 6+δ is comparable to that of La 2− x Sr x CuO 4 , YBa 2 Cu 3 O 6+ x , Bi 2 Sr 2 CuO 6+δ and Bi 2 Sr 2 CaCu 2 O 8+δ . This leads to the conclusion that nominal Tl 2 Ba 2 CuO 6+δ samples may be regarded as being overdoped with holes.

Solid phases and their relations in the Bi-Sr-Cu-O system were investigated at 840°C in air. Bi 2+ x Sr 2− x Cu 1+ y O Z (0.1 x y x /2), Bi 17 Sr 16 Cu 7 O Z , Bi 2 Sr 3 Cu 2 O Z , Bi 4 Sr 8 Cu 5 O Z and BiSr 3 O Z have newly been found. Structural data obtained by means of powder XRD and TEM for these are presented. The superconductor with a T c ∼ 10 K corresponds to the Bi-poorest end of the solid solutions mentioned above. Carrier holes are created by the presence of excess oxygen but these tend to be trapped locally for x > 0.13. The one-dimensional modulation mode showed an interesting composition dependence, from which a modified mechanisms of the modulation has been deduced.

A strong correlation of the superconducting transition temperature T c with the weight loss during thermal cycling and annealing of Bi 2 Sr 2 CaCu 2 O 8+ x is reported. An increase of T c (measured by DC magnetization from 68 K to a maximum of 95 K corresponding to a weight loss of 0.16% (measured by thermogravimetry) is observed. It is believed that the weight loss is due to oxygen and that excess oxygen resides in the BiO planes.