Showing papers in "Physica C-superconductivity and Its Applications in 1990"

Structural anomalies, oxygen ordering and superconductivity in oxygen deficient Ba2YCu3Ox

Abstract: We report the characterization of series of oxygen deficient Ba2YCu3Ox samples for 7 ≥ x ≥ 6 prepared by Zr gettered annealing at 440°C. Measurements include complete crystal structure analysis at 5 K by powder neutron diffraction, electron microscopy study of the oxygen ordering, and magnetic measurements of the superconducting transitions, with particular attention to the transition widths. The results show for the first time that the 90 K and 60 K plateaus in Tc as a function of oxygen stoichiometry are associated with plateaus in the effective valence of the plane coppers. We also correlate the disappearance of superconductivity for x

Pinning and creep in layered superconductors

Abstract: The thermal- and disorder-induced effects in the mixed state of a layered high- T c superconductor (in a field H ‖ c ) are studied. The flux line lattice is of a quasi-2D type at sufficiently high field H ⪆ H 0 where H 0 is proportional to the mass anisotropy m / M and estimated to be around 3 T for the Bi- and Tl-based superconductors. At H 》 H 0 the FLL melts at a temperature T m close to the temperature of 2D dislocation-mediated melting. At T > T m the system is in the normal state. At low temperatures T T g ⋍ T m / In ( H / H 0 ) the system is in the vortex glass state with zero linear resistivity: ∂ V /∂ j →0 as j →0. In the intermediate temperature range T g T T m the energy barriers for the plastic motion of vortices are finite and thermally activated flux flow (TAFF) occurs. In the vortex glass state, different regimes of flux pinning and creep are identified and the behaviour of the critical current j c as a function of temperature and magnetic field is estimated. Power-law behaviour of the effective pinning energy U ( j ) at j ⪌ j c is obtained for the case of 2D collective creep.

Time-dependent structural phenomena at room temperature in quenched YBa2Cu3O6.41: Local oxygen ordering and superconductivity

Abstract: The superconducting transition temperature, T c , of YBa 2 Cu 3 O 6.41 quenched from 500°C into liquid nitrogen increases from 0 to 20 K while annealing at room temperature during the first few days following the quench. Using neutron powder diffraction and Rietveld refinement we show that this time-dependent increase of T c is accompanied by changes in the structural properties. The a -and c -axes contract by 0.04%, while the percentage shortening of the b -axis is three times smaller. We attribute this behavior to charge transfer between the chains and planes, with the smaller contraction along the b -axis arising from oxygen ordering in the chains. Clear evidence for this charge transfer is seen in the relevant Cu-O and Cu-Cu distances. No significant changes in the average occupancies of oxygen sites are observed. We attribute the increase in T c to local ordering of oxygen atoms around the Cu(1) atoms with no change in the average site occupancies.

Synthesis and properties of YBa2Cu3O7 thin films grown in situ by 90° off-axis single magnetron sputtering

Abstract: High quality superconducting films of YBa 2 Cu 3 O 7− x were deposited in situ using single target 90° off-axis sputtering. We have investigated their superconducting DC and RF properties, their normal state properties, and their microstructures. These films are distinctly different from bulk crystals and post-deposition annealed films. Sharp superconducting transition temperatures can be reproducibly obtained by control of deposition parameters. The T c can be varied from 75 to 89 K. The optimization of properties other than T c and the control of film texture occur under conditions different from those for which the highest T c is obtained. Normal state conductivities are as high as or higher than those of single crystals. Critical current densities reach 6 × 10 7 A/cm 2 at 4.2 K. All the above properties are relatively insensitive to compositional variations. The T c 's have a much weaker dependence on the c -axis lattice parameters than do those of bulk samples. The measured low-temperature penetration depth is 1400 A and surface resistance at 4.2 K and 10 GHz is as low as 16 μΩ. Microstructural studies show sharp interfaces between films and their substrates and a variety of defect structures. Many of the properties of in situ films can be explained by clean grain boundaries and the characteristics of the surface growth occuring during in situ deposition.

Flux pinning and creep in very anistropic high temperature superconductors

Abstract: The effect of thermal fluctuations on collective flux-pinning and creeps is investigated for thin-film superconductors and layered superconductors with weak Josephson coupling between the layers in a field normal to the layers and much less than B c2 . Temperature and field dependences of the critical current j c in a two-dimensional (2D) system are obtained. The activation barriers for 2D flux-creep are shown to grow infinitely as U ( j )∝ j -μ at j ⪡ j c , which is characteristics for the vortex-glass state. At very small currents this behaviour is cut off by the plastic motion of edge-dislocation pairs which are either induced by disorder or thermally created, leading to linear current-voltage behaviour inhibiting the existence of a vortex-glass state in 2D systems. The Josephson coupling in layered superconductors changes the dimensionality of the vortex lattice. It is shown that a sufficiently large field when the lattice constant a o becomes less than the characteristic length of the interlayer coupling a 0 r 3D =( R j a 0 ) 1 2 or B > B 2D =Ф 0 / R 2 J , ( R J =Г 1 2 s is the effective Josephson length, s is the interlayer spacing and Г= m z / m is the mass anisotropy), the fluctuations of the vortex lines become of 2D nature. This means in particular that 3D-lattice melting will take place at T = T m , T m is the dislocation-mediated melting temperature of a 2D vortex-lattice. The mixed state at B > B 2D is studied and the possibility of different regimes of pinning and creep is demonstrated. The crossover from 2D to 3D pinning is found when the pinning length R c exceeds r 3D . The crossover conditions are derived and displayed in a schematic phase diagram.

Pressure-induced charge transfer and dTc/dP in YBa2Cu3O7−x

Abstract: Subtle pressure-induced structural changes in YBa2Cu3O6.93 and YBa2Cu3O6.60 have been measured by neutron powder diffraction for samples in a hydrostatic helium-gas pressure cell. Small, but significant, differences in the compression of particular Cu-O bonds (notably Cu(2)-O(4)) are observed. However, when the charges on the two copper sites are calculated, requiring overall charge conservation versus pressure, it is found that the net pressure-induced charge transfer of holes from Cu(1) to Cu(2) is essentially the same for both systems. We conclude that the much smaller value of dTc/dP for YBa2Cu3O6.93 results from the fact that, in the 90 K superconductor, the Tc has already reached its optimum value and the introduction of additional hole carriers cannot further increase Tc.

The role of surface effects in magnetization of high-Tc superconductors

Abstract: Experimental data on the field and time dependencies of the isothermal magnetization for Tl and Bi 2212 single crystal are reported. It is shown that, due to some specific features of high-Tc superconductors, Bean-Livingston's surface barrier plays an unusual and important role in the processes of their magnetization. The values of the thermodynamic critical field for both substances are estimated.

Controlled preparation of all high Tc SNS-type edge junctions and DC-SQUIDs

Abstract: High-Tc SNS-type Josephson junctions and DC SQUIDs were successfully fabricated using hetero-epitaxially grown multilayers of YBa2Cu3Ox and PrBa2Cu3Ox. These layers are c-axis oriented and hence edges of the multilayers give rise to a current flow in the ab-plane between the electrodes of a Josephson junction. The necessary structuring was done by Ar ion beam etching. The individual junctions exhibit a supercurrent up to 80 K. The IcRn-product of these junctions usually has a lower limit of 8 mV at 4.2 K. Voltage modulation of the first DC SQUIDs can be observed up to 66 K. Details on the fabrication and measurements are presented.

The relationship between bond-valence sums and Tc in cuprate superconductors

Abstract: The recently demonstrated correlation of T c with copper or oxygen bond valence sums for a wide range of superconducting cuprates is shown to contradict the behaviour observed for YBa 2 CU 3 O x as x is varied from 6 to 7. This conflict is removed by introducing two independent variables, namely, the sum of and difference of the copper and oxygen bond valence sums. The latter is a measure of the hole density on the CuO 2 plane and as this increases T c passes through a maximum T c (max). The former indicates the relative distribution of these holes on the oxygen or copper sites for a given compound. T c (max) correlates closely with the preference for hole distribution on the oxygen atoms. A general phase-diagram using these bond sum parameters is presented.

Frequency dependence of AC susceptibility in high-temperature superconductors Flux creep and critical state at grain boundaries

Abstract: The loss peak of the AC susceptibility in polycrystalline high- T c superconductors shifts slightly to higher temperatures with increasing frequency of the applied AC magnetic field It is shown that a flux creep term, added to the current density term in the critical state equation, can account for the observed frequency dependence The magnitude of the peak shift is predicted to increase with decreasing average grain size and decreasing grain boundary junction current density The model predictions are compared with the experimental data of Nikolo et al Some of the parameters used in the calculation are determined by fitting data for χ′ and χ″ over the full temperature range using a recently developed model for granular superconductors In addition, the relation between the intergranular pinning potential and the activation energy, which is extracted from log-frequency versus inverse χ″-peak temperature data, is clarified

Quaternary phase relations near YBa2Cu3O6+x at 850°C in reduced oxygen pressures

Abstract: The quatenary phase relations near YBa 2 Cu 3 O 6+ x at 850°C were determined by oxygen coulometric titration and microanalysis. The phase relations change markedly as the oxygen pressure decreases, due primarily to the formation of a liquid phase and a reduced oxide, BaCu 2 O 2 . Nine invariant reactions were discovered, including decomposition of YBa 2 Cu 3 O 6+ x into Y 2 BaCuO 5 , BaCu 2 O 2 and YBa 3 Cu 2 O 6+ y . The lower limit of oxygen partial pressure at which pure YBa 2 Cu 3 O 6+ x is stable is 4.0×10 −4 atm O 2 at 850°C; the lower limit increases in the presence of BaCuO 2 and Cu 2 O impurities. These results indicate that the temperatures used for single crystal growth of YBa 2 Cu 3 O 6+ x could be lowered by using reduced oxygen pressures or, alternatively, that constant temperature crystal growth could be induced by increasing the oxygen partial pressure.

Raman scattering from single crystals of cupric oxide

Abstract: Raman scattering experiments have been carried out on single crystals of cupric oxide (CuO) in the temperature range from 15 K to 300 K. The symmetry of the three first order Raman active phonons has been determined. A broad peak occurs at approximately 1100 cm−1 and is assigned to multiphonon scattering on the basis of the observed temperature dependence. Another peak appears in the Raman spectrum as the temperature of the sample is lowered below the Neel temperature. The frequency, intensity and linewidth of this peak have been studied as a function of temperature. The frequency of this peak cannot be reconciled with the known spin wave spectrum of CuO and thus, on the basis of the measured temperature dependence, it has been assigned to scattering from a magnetic exciton. The Raman spectrum also contains a background continuum which is weakly peaked at about 2000 cm−1 and the possible origin of this scattering is discussed. The observed optical and magnetic properties of CuO are compared, where possible, to results that have been obtained for the high-Tc cuprates.

A new ambient-pressure organic superconductor: (BEDT-TTF)2(NH4)Hg(SCN)4

Abstract: A new ambient-pressure organic superconducting charge-transfer salt, (BEDT-TTF) 2 (NH 4 )Hg(SCN) 4 , is reported. The unit cell parameters of this salt indicate that it is isostructural to (BEDT-TTF) 2 KHg(SCN) 4 , which possesses an α-type packing motif. The broad ESR line width of 56 to 84 G at 300 K as well as its temperature dependence further indicates that this salt has an α-type packing. Conductivity measurements show that (BEDT-TTF) 2 (NH 4 )Hg(SCN) 4 is metallic to low temperatures and inductive studies by RF penetration depth measurements demonstrate that it is superconducting with an onset at 1.15 K.

Two-phase structural refinement of La2CuO4.032 at 15 K

Abstract: A two-phase refinement of a crystal for the compound La2CuO4+δ (δ=0.032), based on the neutron diffraction data collected at 15 K with the D9 diffractometer at ILL, using ǂ=0.48 A , has been carried out. One phase (30%) consists of stoichiometric La2CuO4 domains, while the other (70%) of oxygen-rich La2CuO4.048 domains. The percentages of each phase, which have been refined together with the other parameters (scale factor, positional parameters, thermal factors and occupancy factors of the oxygen atoms), agree very well with the value determined from χAC measurements. The La2CuO4.048 structure is essentially the same as the average structure reported in ref. [1], the only difference being the oxygen content which is found to be δ= δ 0.70 . The extra oxygen, O(4), is found to be in between two LaO layers in a similar position as the oxygen atoms located between two Nd layers in the N2CuO4 structure. The insertion of extra oxygen causes the displacement of some of the oxygen O(1) towards the O(3) positions. Different models are proposed for the distortion induced by this insertion according to the experimental value, 3.3 [6], found for the ratio of the amount of O(3) to that of O(4). If this ratio is assumed to be 3 and the O(3) atoms are localized about the insertion, then the formation of a short O(4)-O(3) bond would occur. The models not requiring the formation of the short bond correspond to ratios of 2 or 4. In the latter case there would be four displaced O(1), but due to the rigidity of the oxygen octahedra only two O(3) would be bonded to O(4), the other two being the apically opposite oxygen atoms of the same octahedra.

Superconductivity at 108 K in YBa2Cu4O8 at pressures up to 12 GPa

Abstract: Resistive measurements in a cryogenic diamond anvil cell show that the T c of YBa 2 Cu 4 O 8 can be increased by almost 30 K by applying a pressure of 12 GPa. The pressure derivative is, however, not constant. At p ⪷ 5 GPa, d T c /d p ≊ 5 K/GPa. At higher pressures d T c /d p decreases gradually and T c saturates at ≈ 108 K. This behaviour is reproduced by a phenomenological model where T c depends only on the number of holes in the CuO 2 planes.

The electronic specific heat of YBa2(Cu1−xZnx)3O7 from 1.6 K to 300 K

Abstract: Using a differential technique we have determined between 1.6 K and 300 K and for 0⩽ x ⩽0.1 the difference in electronic specific heat coefficients γ ( x , T )− γ (0.07, T ) n of YBa 2 (Cu 1− x Zn x ) 3 O 7 and a YBa 2 (Cu 0.93 Zn 0.07 ) reference. The results are consistent with a superconducting Fermi liquid and cannot be readily described in terms of the condensation at T c of bosons preformed above T c . We deduce γ( x , T ) from these results, and find γ n = 1.6±0.2 mJ /( g - at . K 2 ) (density of states N ( E F ) = 8.8 states /( eV . fu )) independent of x . Superconducting anomalies are seen for all samples, with T c and the anomaly height decreasing and γ( x , 0) increasing with x consistent with magnetic pair breaking. The thermodynamic critical field H c ( x , T ) is determined for 0⩽ x ⩽0.07. We observe a (nearly) universal temperature dependence for the low temperature magnetic anomalies in Zn-doped and undoped samples with broad specific heat maxima at or above 5 K. For undoped YBa 2 Cu 3 O 7 we find θ (0) = 406 K and an electronic term γ (0, 0) = 0.15±0.15 mJ /( g - at . K 2 ) substantially smaller than the generally quoted value.

Neutron diffraction study of Pr valence and oxygen ordering in the Y1−xPrxBa2Cu3O7−δ system

Abstract: The crystallographic changes that occur with increasing Pr concentration in the Y−xPrxBa2Cu3O7−δ system have been studied by neutron diffraction. As x is increased from 0 to 1, the orthorhombic a, b and c lattice parameters expand monotonically, while the orthorhombic distortion, espressed as b a , decreases, reflecting increased occupation of O(5) sites at the expense of O(4) chain site occupancy. The separation of the CuO2 planes in PrBa2Cu3O7−δ indicates that the Pr ions have an intermediate valence of ∼3.3 in this compound. Evidence is presented for increased interaction between the Pr ions and oxygen ions in adjacent CuO2 planes.

Substitutions for Cu in YBa2(Cu1−xMx)3O7−δ (M = Fe, Co, Al, Cr, Ni, and Zn)

Abstract: Substitutions for copper in YBa 2 (Cu 1− x M x ) 3 O 7− δ (1:2:3) were investigated. It was found that indirect methods to determine solubility into 1:2:3 typically used in the study of intermetallic compounds (observed variation with composition of superconducting transition temperature and of lattice parameters) are not entirely reliable. Optical micrography was used to asses the morphology of the sintered polycrystalline specimens. Differential thermal analysis provides melting point data signaling the presence of impurities which melt at lower temperatures. Electron microprobe studies were undertaken to determine actual M content of the superconducting phase and to check for the presence of impurity phases. Estimates of solubility limits are made. Transmission electron microscope studies were used to determine the twin structure and investigate the occurrence of a tweed pattern. Superconducting transition temperature as a function of x was determined below the solubility limits. In several cases T c 's observed in this work differ from some previous reports. Possible sources of these discrepancies are discussed. Variation of lattice parameters as a function of x is reported. Also described are electrical resistivity and DC susceptibility measurements for selected samples.

Texture processing of extruded YBa2Cu3O6+x wires by zone melting

Abstract: Zone melting has been used to produce texture in extruded YBa 2 Cu 3 O 6+ x wires. By controlling the directional solidification of the molten zone, oriented structures are produced that show a large magnetic hysteresis at 77 K compared to unaligned extruded YBa 2 Cu 3 O 6+ x wires. A magnetization critical current density of 1.8 × 10 4 A/cm 2 in an applied field of 20 kOe has been achieved.

Direct study of magnetic flux penetration and trapping in HTSC

Abstract: A method for visualization of the magnetic flux penetration and trapping in high-temperature superconductors (HTSC) using magneto-uniaxial ferrimagnetic films is described. The difference in the character of these processes in a number of materials (HTSC single crystals, films and ceramics) is studied. Specific features of remagnetization of Tl 2 Ba 2 CaCu 2 O x and YBa 2 Cu 3 O 7−δ single crystals in the superconducting state which do not fit the simple Bean model are found. Temperature dependences of the local magnetic susceptibility are measured in different sites of the samples. Non-uniformities of their superconducting properties related to structural defects are revealed.

Synthesis and properties of the YBa2Cu4O8 superconductor

Abstract: A new simple synthesis route was used to produce the ‘124’ superconductor YBa 2 Cu 4 O 8 . The method utilizes YBa 2 Cu 3 O 7 powder (‘123’) as a precursor, which is then converted to the 124 superconductor by reaction with a stoichiometric amount of CuO through normal grinding and sintering, without the need for high oxygen pressure processing. Sintered bars of the 124 superconductor exhibited a T c of ∼ 75 K in resistivity and AC magnetic susceptibility measurements. Transport critical current density was measured to be ∼ 150 A/cm 2 at 60 K, and showed a strong field dependence. This behavior, in combination with a relatively high J c (magn.) of 4 × 10 4 A/cm 2 at 60 K and H = 0.9 T, is indicative of Josephson weak links at grain boundaries, which is similarly observed in the 123 phase. It is also noted that the intragrain J c in the twin-free 124 superconductor is about the same as that in the twinned 123 superconductor at ∼ 15 degrees below their respective T c .

Atomic short-range order in oxygen deficient YBa2Cu3O7−σ

Abstract: Low field magnetization measurements are presented for a set of oxygen deficient single crystals of YBa 2 Cu 3 O 7−σ . It is demonstrated that the superconducting transition temperature T c not only depends on the oxygen concentration but also on the degree of oxygen vacancy ordering of the samples. Various degrees of ordering were obtained by annealing crystals with reduced oxygen stoichiometries at various temperatures below 350°C and fast quenching them to cryogenic temperatures. Depending on oxygen stoichiometry and annealing temperature, changes in T c of up to 27 K are observed. The observed annealing and quenching behavior is explained in terms of the kinetics of vacancy short-range order in the Cu-O chains. One major conclusion is that the so-called “60 K plateau” in the plot of T c versus σ is completely absent when the crystals are in their most disordered state, i.e., with the smallest degree of vacancy short-range order.

Iron configuration and superconducting properties of YBa2(Cu1-xFex)3Oy prepared by various thermal treatments

Abstract: YBa 2 (Cu 1− x Fe x ) 3 O y (0 ≦ x ≦ 0.20) was prepared by three kinds of heat treatment: (a) slow cooling from 850°C in flowing O 2 gas, (b) quenching into liquid N 2 from 930°C in air followed by reoxidation in flowing O 2 gas below 400°C and (c) reducing at 800°C in flowing N 2 gas followed by reoxidation in flowing O 2 gas below 400°C. The crystal structure and superconducting properties of the samples were examined by X-ray powder diffraction, TG, electrical resistivity and 57 Fe Mossbauer effects measurements. The region of orthorhombic phase and superconducting properties strongly depend on the thermal treatments. The results were discussed from the standpoint of Fe distribution.

Oxygen Nonstoichiometry and valences of bismuth and copper in Bi2.00Sr1.88Ca1.00Cu2.14Oy

Abstract: The oxygen nonstoichiometry of Bi 2.00 Sr 1.88 Ca 1.00 Cu 2.14 O y was determined as a function of temperature and oxygen partial pressure. It was found that the shape of y −log P 0 2 curves above 800°C is quite different from that below 800°C. Above 800°C, the y −log P 0 2 relation resembled those of other kinds of high T c superconducting oxides such as Ba 2 YCu 3 O 7−δ and (La 1− x Sr x ) 2 CuO 4−δ . However, below 800°C y slightly depended on the temperature but did not depend on the oxygen partial pressure. It was found by chemical analysis on quenched samples that the copper valence was 2.07 irrespective of the annealing condition, and the bismuth valence was nearly constant when the annealing temprature was above 800°C but increased with the decrease of the annealing temperature below 800°C. The tailing of resistivity-temperature curves in the vacinity of the critical temperature seemed to be due to the existence of Bi 5+ and neutral oxygen species.

New superconducting cuprates (Pb, Cu)(Eu, Ce)2(Sr, Eu)2Cu2Oz

Abstract: New superconducting Pb-based layered copper oxides, (Pb, Cu)(Eu, Ce)2(Sr, Eu)2Cu2Oz, have been synthesized. This new superconducting phase or the “1222” phase characteristically contains a (Pb, Cu)-O monolayer and a “fluorite-type” block layer which is sandwiched between the basal planes of double Cu-O5 pyramids. Crystallographycally, this structure agrees with that recently proposed for nonsuperconducting PbPr2.4Sr1.6Cu2.6Oz by Adachi et al. A superconductivity transition with the onset temperature of 25 K is measured for the nearly single-phase sample of the nominal composition of ( Pb 1 2 Cu 1 2 ) ( Eu 3 4 Ce 1 4 )2( Sr 7 8 Eu 1 8 )2Cu2Oz. This is the first superconductor of the “1222” structure.

Calcium-substituted superconducting RBa2Cu4O8 with Tc ∼ 90 K prepared at one atmosphere

Abstract: Calcium substituted RBa 2 Cu 4 O 8 with R = Y and Er has been prepared between 805°C and 830°C under oxygen at one atmosphere. The zero-resistance T c is displaced up from 79 K (unsubstituted) to 87 K at 10% substitution. This compound has the benefit of a high- T c while circumventing the problems of oxygen-loading and associated microcracking. The substitution level of 10% lies at the solubility limit under the preparation conditions used and neither prereaction at higher temperatures, nor the use of co-substitution with La assisted in obtaining higher substitution levels.

Correlation between the magnitude of the superconducting transition temperature and the normal-state magnetic susceptibility in Bi2Sr2CaCu2O8+y and Ti2Ba2CuO6+y as a function of oxygen content

Abstract: The static magnetic susceptibility of Bi 2 Sr 2 CaCu 2 O 8+ y and Tl 2 Ba 2 CuO 6+ y is studied as a function of oxygen content y over the temperature range 2–900 K. With increasing oxygen (hole) content the superconducting transition temperature T c ( y ) passes through a maximum for the Bi-compound and falls monotonically for the Tl-system, reaching a maximal value of c ≅92 K in both systems. As the oxygen content is varied, a correlation is observed between the magnitude of T c ( y ) and the temperature dependence of the normal-state magnetic susceptibility. The fact that this same correlation can also be seen in the published data on La 2- x Sr x CuO 4 and YBa 2 Cu 2 O 6+ y implies that it may be a universal property of hole-doped oxide superconductors.

Crystal structure of (Bi, Pb)2Sr2Can−1CunO4+2n+δ high-Tc superconductors

Abstract: The crystal structure of (Bi, Pb)2Sr2Can−1CunO4+2n+δ high-temperature superconducting phases was studied by transmission electron microscopy. The displacive modulation is largest for the n=1 material and yields displacement amplitudes of 70 pm with respect to the average structure. In all phases the largest distortions are found in the Bi-O double layers. The structural distortions were determined explicitly for the cations and amplitudes for the atomic modulation functions are given for the n=1 and n=2 phases. For Bi2Sr2CaCu2O8+δ(Tc=85 K) the space group Pbnb was determined for the average structure and accounts for reflections which cannot be indexed by Bbmb. In material doped with Pb the periodicity of the modulation changes significantly, from 4.7 times the lattice parameter b for the undoped phases to 8b for the doped n=2 phase and 8.8b for the doped n=3 phase, respectively. For the doped phases systematic extinction conditions are compatible with the space group PBb1m1b1. Due to the modulation, shifts of atoms may lead to distortions of CuO polyhedra and should effect the electronic structure of the various phases.

Thallium cuprates: The critical temperature is mainly governed by the oxygen nonstoichiometry

Abstract: The superconducting properties of the thallium cuprates, and especially their critical temperatures, are strongly affected by annealings in various gaseous atmospheres (nitrogen, oxygen, argon and hydrogen). These annealing effects have been studied for Tl 2 Ba 2 CaCu 2 O 8 , TlBa 2 CaCu 2 O 7 , TlBa 2 Ca 2 Cu 3 O 9 and Tl 2 Ba 2 Ca 2 Cu 3 O 10 as well as for the substituted oxides TlBa 2 Ca 1− x Ln x Cu 2 O 7 and Tl 2 Ba 2 Ca 1− x Ln x Cu 2 O 8 (Ln=Y, Nd). It is demonstrated that a drastic increase of T c can be obtained, due to a very small oxygen deficiency. The most spectacular effect is obtained with a gas mixture H 2 /Ar, which can be performed at a temperature as low as 300°C, leading for example for the 2212 phase to an increase of T c from 97 K to 118 K (to be compared to 125 K for the 2223 phase). Moreover it is shown by electron microscopy that at this annealing temperature the crystal remains unchanged. However, thermal treatments at 400°C or beyond alter the crystals in connection with a slight volatilization of thallium oxide. This dramatic effect of oxygen nonstoichiometry upon T c decreses as the number m of copper layers in the perovskite slabs of the superconductors TlBa 2 Ca m −1 Cu m O 2 m +3 and Tl 2 Ba 2 Ca m −1 Cu m O 2 m +4 increses. These results show that the important factor which governs T c in the thallium cuprates is the hole number per mole of copper in the structure and that this factor seems to be predominant with respect to the number m of copper layers.