Thallium Cuprate Superconductors Research Articles

Stabilization of new superconducting thallium cuprates and oxycarbonates by molybdenum

The stabilization of thallium cuprates and oxycarbonates by molybdenum has been studied. Two cuprates have been stabilized: Tl 0.8Mo 0.2Sr 2CuO 5−δ with the “1201” structure which does not superconduct and the superconductor Tl 0.8Mo 0.2Sr 2CaCu 2O 7−δ with the “1212” structure whose T c increases from 40 K, when as-synthesized, to 86 K after annealing in a reducing atmosphere. A new superconducting oxycarbonate Tl 0.8Mo 0.2Sr 4Cu 2(CO 3)O 7−δ has been synthesized, whose structure corresponds to an intergrowth of the “1201” structure of Tl 0.8Mo 0.2Sr 2CuO 5−δ with the Sr 2CuO 2CO 3 structure. This superconductor also exhibits a T c ranging from 30 K, when as-synthesized, to 80 K when annealed in a reducing atmosphere. Substitution of La for Sr in the “1201” phase allows superconductivity to be induced up to 40 K, whereas substitution of Ba for Sr in the oxycarbonate does not significantly influence the superconducting properties.

Systematic thermopower measurements of the thallium cuprates Tl(Ba,Sr) 2Ca m−1 Cu mO 2 m+3−δ and Tl 2Ba 2Ca m−1 Cu mO 2 m+4+δ

The thermoelectric power (TEP) S versus temperature has been systematically investigated for several series of the superconducting cuprates Tl(Ba,Sr) 2Ca m−1 Cu m O 2 m+3−δ ( m = 2, 3) and Tl 2Ba 2Ca m−1 Cu m O 2 m+4+δ ( m = 1, 2, 3). The consideration of the S( T c) curves allows two important points to be found evidence for. The first one deals with the fact that all these superconducting thallium cuprates are systematically overdoped whatever T c, and whatever the number of Cu or Tl layers; no underdoped superconducting cuprate could be obtained. The second point shows that there exist two classes of Tl cuprates: the weakly overdoped cuprates that exhibit a T c max ≥ 100 K (all the triple copper layer cuprates and the 2212 cuprates) and those which can be heavily doped that exhibit a T c max ≤ 90 K (the 2201 and the 1212 cuprates). The different behavior of thallium cuprates compared to YBa 2Cu 3O 7−δ and to bismuth cuprates is discussed.

Determination of oxygen content and cation valences in superconducting Tl-based cuprates by a wet chemical method.

A wet chemical route based on constant-potential electrolysis and different reactivities of ${\mathrm{Cu}}^{3+}$ and ${\mathrm{Tl}}^{3+}$ with water is proposed to distinguish the average valences of Cu and Tl in superconducting thallium cuprates. The validity of the method is demonstrated using ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$, ${\mathrm{Tl}}_{2}$${\mathrm{O}}_{3}$, and ${\mathrm{Tl}}_{2}$O as reference materials for ${\mathrm{Cu}}^{3+}$, ${\mathrm{Tl}}^{3+}$, and ${\mathrm{Tl}}^{+}$, respectively. Furthermore, the oxygen contents and the cation valences have been determined for various Tl-22(n-1)n phases (where n=1--4). With increasing n value the Cu valence is found to decrease, while the valence of thallium remains rather constant at ca. +3, regardless of the value of n. Based on these results, the possible carrier doping mechanisms in the Tl-22(n-1)n phases are discussed.

Superconducting thallium cuprates obtained by substitution of copper for thallium in the double-thallium layer cuprate (T12212)

A new series of superconducting thallium cuprates of nominal composition, (Tl 2− x Cu x )Ba 2CaCu 2O 8 (0<x<1.0), where copper seems to substitute for thallium in Tl2212, is reported. These materials, which are synthesized at 920K-1120K in sealed silver tubes, exhibit T c s in the range 110-99K. The phases are metastable, decomposing at higher temperatures (≈1150K) to a mixture of thallium cuprates, CuO and BaCuO 2. Significantly, x=1 member decomposes at 1150K to mixture of Tl2223, CuO and BaCuO 2. Chemical titrations involving oxidation of bromide ions reveal that the copper substituting for thallium in (Tl 2− x Cu x )Ba 2CaCu 2O 8 most likely occurs in the III oxidation state for x≤0.25 and in a mixed state (II, III) state for x>0.25.

Mechanisms for hole doping in thallium cuprate superconductors having single copper-oxygen sheets

This article describes some recent studies on the effect of chemical substitution on the superconducting properties of thallium cuprates containing single Cu-O sheets interleaved with single and double Tl-O layers. These studies strongly suggest that, for Tl 2Ba 2CuO 6, Tl 6s-block bands lie below the Fermi level, so that they remove electrons from the Cu 3d x 2− y 2 bands, i.e., the Tl-O layer creates holes in the CuO 2 layers. This is in contrast to TlBa 2CuO 5-type phases, where the metal chemistry forces the oxidation of CuO 2 layers. However, studies on TlSr 2− x La x CuO 5 and TlBa 1− x Sr x LaCuO 5 systems indicate that for Tl-O single rock-salt layer phases with a short in-plane Cu-O distance, such as TlSrLaCuO 5, the CuO 2 layer x 2− y 2 band is raised high enough in energy for the Tl-O layer to create holes in the CuO 2 layer

Flux pinning and critical currents in superconducting thallium cuprates

The irreversibility lines H irr( T) for the series of superconducting compounds Tl m A 2Ca n−1 Cu n O 2+ m+2 n , where A is Ba or Sr, are measured between 0 and 1 T for m = 1 and 2 and n = 1, 2, 3 and 4 and show a marked steepening with decreasing anisotropy, i.e. as m is lowered from 2 to 1. In addition, for m = 2 material H irr( T) is observed to steepen with decreasing number of CuO 2 layers. The hole dependence of the irreversibility line is also investigated in Tl 2Ba 2CuO 6+δ by changing the oxygen stoichiometry δ, H irr( T) progressively steepens with increasing hole concentration on the overdoped side of T c(max). These results are consistent with a model of flux motion arising from a cross-over from 3D vortex lines to 2D pancake vortices which are decoupled in the c-direction, and provide guidance for materials selection and optimum doping. The manufacture of superconducting, silver-clad tapes using Tl 0.5Pb 0.5Sr 2Ca 0.8Y 0.2Cu 2O 7 is described. Zero field transport critical currents in excess of 10 4 A cm −2 are obtained but the material is untextured and weak-linked, as shown by the strong field dependence of J c.

A novel method for the determination of the oxygen content of superconducting thallium cuprates

A novel wet chemical method based on the redox chemistry of the Cu(I)/Cu(II) couple and potentiostatic electrolyses is proposed for the determination of high metal valences in thallium cuprates. The sum of the extra oxidation states above +II for copper and +I for thallium, and thus the oxygen content of the sample, is obtained by reducing both Cu(III) and Tl(III) with CuCl. The excess of Cu(I) is electrolysed as a final analysis step. The quantitativity of the proposed method was demonstrated using YBa2Cu3Oz and Tl2O3 as reference materials for Cu(III) and Tl(III) respectively. Additionally, the oxygen content was determined in a single-phase superconducting sample with a nominal composition of Tl2Ba2CaCu2Oz.

On the determination of hole concentration and oxygen stoichiometry in thallium cuprate superconductors

The utility and limitation of the wet chemical methods for the determination of hole concentration and oxygen stoichiometry in thallium cuprate high T c compounds have been evaluated.

The Systematics of Irreversibility in Superconducting Thallium Cuprates

ABSTRACTThe irreversibility lines, Hirr(T), for the series of superconducting compoundsTlmA2CunO2+m+2n, where A is Ba or Sr, are measured between 0 and 1 tesla for m = 1,2 and n = 1,2,3 and 4 and show a marked steepening with decreasing anisotropy i.e. as m is lowered from 2 to 1. In addition, for m=2 material Hlrr(T) is observed to steepen with decreasing number of CuO2 layers. The hole dependence of the irreversibility line is also investigated in TI2Ba2CuO6+6 by changing the oxygen stoichiometry, 6. Hirr(T) progressively steepens with increasing hole concentration on the overdoped sida of Tc(max). These results are consistent with a model of flux motion arising from a crossover from 3-D vortex lines to 2-D pancake vortices which are decoupled in the c-direction, and provide guidance for materials selection and optimum doping.

Determination of hole concentration in superconducting thallium cuprates

A chemical method for the determination of hole concentration in superconducting thallium cuprates is described. The method is based on selective oxidation of bromide ion by holes on CuO 2 sheets. Application of this method to several thallium cuprates has provided some insight into the origin of holes in these materials.

Properties of the chemically characterized thallium cuprate superconductors

Thallium cuprates have been chemically characterized by adopting simple wet-chemical procedures for the determination of Tl and oxygen contents. The oxidation of the CuO 2 sheets in the Tl 2− y Ba 2Ca n−1 Cu n O 2 n+4− x family is primarily due to (i) an overlap of the Tl-6s band with the conduction bond of the CuO 2 sheets for smaller y≈0.0 and (ii) Tl vacancies for larger y≈0.5. In the intermediate region 0< y<0.5, the Tl-6s overlap decreases sensitively with increasing y. The unusual situation of a partially filled Tl-6s band causes an extrusion of Tl from the lattice as Tl 2O 3 upon exposure to air above 70°C. This remarkable observation will have important consequences for processing. In contrast, the Tl 1− y Ba 2Ca n−1 Cu n O 2 n+3− x family does not have a partially filled Tl-6s band is stable to oxidation without any Tl extrusion. Thallium cuprates have an oxygen deficiency in the Tl 2O 2 layers unlike the analogous bismuth cuprates, which have excess oxygen in the Bi 2O 2+ x layer. The origin of the orthorhombicity, which appears after low-temperature Tl extrusion in the Tl 2− y Ba 2CuO 6− x system, appears to be due to an ordered extrusion of “TlO” units that may be stabilized by a cooperative tilting of CuO 6 octahedra.

Chemical methods to identify the origin of oxidation in the thallium cuprate superconductors

Creation of mixed valency in the CuO 2 sheets of the copper oxide superconductors is a necessary condition to suppress long-range antiferromagnetic order and promote superconductivity. The oxidation/reduction of the (CuO 2) 2− sheets is generally achieved by suitable ionic substitutions and/or oxygen intercalation/disintercalation. The mechanism for such an oxidation in the thallium cuprate superconductors has, however, not yet been established. We have adopted a simple wet chemical method to obtain the Tl content and a modified iodometric procedure to determine the total oxygen content. Our wet chemical analyses combined with thermogravimetric analysis demonstrate for the first time that oxidation of the CuO 2 sheets in Tl 2Ba 2Ca n−1 Cu n O 2 n+4 is primarily due to an overlap of the Tl-6 s band with the σ∗ x 2−y 2 and/or π∗ bands of the CuO 2 sheets. Neither Tl vacancies nor excess oxygen in the Tl 2O 2 blocks is required to render the thallium cuprates superconductive.

VALENCE STATES OF BISMUTH AND THALLIUM IN SUPERCONDUCTING CUPRATES: A Bi–L3 AND Tl–L3 X-RAY ABSORPTION SPECTROSCOPIC STUDY

Based on a Bi – L 3 edge X-ray absorption spectroscopic study of superconducting bismuth cuprates and model Bi compounds, it is shown that Bi is essentially in the 3 + state in the cuprates. Tl – L 3 absorption edge studies a number of thallium cuprate superconductors along with those of model compounds show some evidence for the occupancy of the Tl -6s band, but Tl 3+ is the prepondarant species. Cu K-edge studies of the thallium cuprates show that Cu is essentially in the 2+ state.

Systematics in the thermopower behaviour of several series of bismuth and thallium cuprate superconductors:

The thermopower, $\alpha$, of many members of the $Bi_2Ca_{1-x}Y_xSr_2Cu_2O_{8+\delta}$, $Bi_{2-y}Pb_yCaSr_2Cu_2O_{8+\delta}$, $Tl_yCa_{1-x}Y_xBa_2Cu_2O_{6+\delta}$ $TlCa_{1-x}Ln_xSr_2Cu_2O_{6+\delta}$ and $TlSr_{2-x}La_xCuO_{5+\delta}$ systems is positive down to $T_c$, but is negative in some of them. The slope of the $\alpha-T$ plots above $T_c$ is generally negative independent of the sign of $\alpha$. The value of $\alpha$ extrapolated to 0 K, $\alpha(0)$, from the high-tem perature regime is finite and generally, positive. Compared to the behaviour of $YBa_2Cu_3O_7$, the data on the $Bi$ and $Tl$ cuprate systems examined here seem to suggest that electrons may be involved in the conduction process as well, at least in systems where both the $\alpha$ and the $\alpha-T$ slope are negative. Of the different systems examined here, $Tl_{1-x}Pb_xCaSr_2Cu_2O_{6+\delta}$ containing $Pb$ in the 4+state, deserves special mention since it shows negative $\alpha$ over the entire temperature range in addition to a negative slope suggesting that this is likely to be a genuine high-$T_c$ electron-superconductor. The thermopower of cuprates has been discussed in some detail in terms of entropic and quasiparticle contributions.

Novel series of thallium cuprate superconductors

The TlCa1−x Lnx Sr2 Cu2 O7+δ(Ln = rare earth) system shows electron-superconductivity when x= 0.25 and hole-superconductivity when x> 0.25. The Tc is generally in the 60–90K in both these regimes. TlSr2−x Lax CuOy (1021) also show electron or hole superconductivity (Tc∼40K) depending on x.

ChemInform Abstract: Structural Aspects of High‐Temperature Cuprate Superconductors

AbstractReview: (55 refs.

Structural aspects of high-temperature cuprate superconductors

High-temperature superconductivity constitutes the most sensational discovery of recent times. Since these new superconductors are complex metal oxides, chemistry has had a big role to play in the investigations. For the first time, stoichiometry, structure, bonding, and such chemical factors have formed central themes in superconductivity, an area traditionally dominated by physicists. These oxide superconductors have given a big boost to solid-state chemistry. In this Account, data is presented on some of the significant structure-property relations in the various cuprate superconductors, distilled from the recent literature and substantially based on our own contributions to this area. We shall first present the essential features of the La{sub 2-x} (Ca, Sr, Ba){sub x} CuO{sub 4} and LnBa{sub 2}Cu{sub 3}O{sub 7} systems and then discuss the bismuth and thallium cuprate superconductors.

Certain Novel Aspects of Thallium and Bismuth Cuprate Superconductors

AbstractNovel superconducting thallium cuprates of the type T1Ca1‐X LnX Sr2 Cu2O6+δ (Ln = Y or rare earth), T1Srn+1‐x Lnx Cun OY and Tl1‐x PbX Srn+1Cun08+δ are described. These cuprates as well as Bi2Ca1‐x Lnx Sr2Cu2O8+δ and TICa1‐xYxBa2 Cu2 O6+δ . show maximum T around a specific composition or oxygen content. They also show interesting changes in the sign and magnitude of the thermopower with the composition. Specially noteworthy is the negative slope of the thermopower‐temperature plots. The thermopower behaviour in these two‐band systems can be understood in terms of entropie and quasiparticle contributions. It appears that Tl1‐x Pbx CaSr2Cu2O6+δ is a genuine high T electron superconductor.

SUPERCONDUCTORS: Path to higher temperatures in doubt

Superconductivity researchers have arrived at a peculiar juncture in their efforts to raise the transition temperature (T c ) above the current record, 125 K (-148 °C). In recent weeks, widely separated groups investigating new thallium cuprate superconductors have reported conflicting results that may either signal an end to rising T c s in this chemical family or point to even higher temperatures. One widely pursued goal—synthesis of a thallium cuprate containing four consecutive copper-oxygen layers—has now been achieved by at least three laboratories. Two of them have published their results. In the Aug. 11 issue of Nature [ 334 , 510 (1988)], H. Ihara and co-workers at the Electrotechnical Laboratory, Tsukuba, Japan, announced the preparation of a sample of TlBa 2 Ca 3 Cu 4 O11 that becomes superconducting at 122 K. Last week, a Boston group led by Bill C. Giessen and Robert S. Markiewicz at Northeastern University made a similar announcement in the pages of Science [ 241 , 1198 (1988)]. Their sample...