The 90 K superconductor, YBa 2 Cu 3 O δ (YBCO), has proved to be highly adaptable chemically as it can accommodate a wide variety of cationic and anionic substitutions. Indeed, the majority of the chemical elements, excluding noble gases and actinides, have been reported to substitute to some extent into the YBCO structure. This review covers the literature on such substitutions and their effect on the properties of YBCO. Reported solubility limits are given, together with crystal symmetry and trends in unit cell parameters with dopant concentration. The dopant site is considered; this is additionally complex in the case of copper substitution because of the two distinct copper sites in the crystal structure. The effect of the dopant on the critical temperature, T c , is reviewed; the literature is often contradictory due to the dual effects of variable oxygen content and the nature of the dopant. Preparation methods appear to have an effect on solubility limits, crystal symmetry and T c , Also, the methods used to determine solubility limits are often imprecise which can lead to contradictions. The magnetic properties of doped materials are reviewed; for some dopants, particularly the magnetic lanthanides, antiferromagnetism and superconductivity co-exist. The related RBa 2 Cu 3 O δ phases (R = lanthanide), their structure, properties and behaviour on doping are reviewed in a similar way. For the larger rare earths, the related systems R 1+ x Ba 2- x CU 3 O δ , are reviewed; as x increases, the transition temperature decreases and compositions R 1.5 Ba 1.5 Cu 3 O δ are semiconducting. The upper and lower solubility limit changes with R, and for R = Dy, the upper limit is x = 0 composition, LaBa 2 Cu 3 O δ , cannot be prepared in air since substitution of La onto the Ba site occurs, forming the Ba-deficient solid solutions. The discovery of superconductivity above liquid nitrogen temperatures in YBa 2 Cu 3 O 7 , has generated a vast quantity of research; to date, > 20 000 papers have appeared on the 123 materials (source: BIDS). This review, therefore, cannot be comprehensive, but attempts to highlight important substitutions, their effect on the properties of YBCO and any contradictions in the literature.

Crystal chemical substitutions and doping of YBa2CU3Ox and related superconductors

Publisher Summary This chapter presents the phase diagrams and thermodynamic data for selected cuprates of alkaline-earth (A) and rare-earth (R) elements except Sc, along with the binary to quintenary oxides, and relevant solid-solution series with other elements. Short information on crystal structure is listed for each phase giving it its identity, as well as the concise chemical information which defines its character. The chemistry of phases is illustrated in mostly subsolidus isothermal phase diagrams, drawn after original sources, and referring to diverse chemical conditions defined by partial pressures of oxygen in particular. Reactivity towards CO2 and formation of oxide carbonates are treated systematically as well, having been one of the major experimental obstacles in pursuit of data relevant to pure oxides. The chapter also discusses the essential thermodynamic data for evaluations of phase equilibria appear in the form of entropies and enthalpies at standard and phase-transition temperatures together with heat-capacity functions. Other possibilities are temperature polynomials for standard Gibbs energies of formation or for partial pressures of gaseous components.

Chapter 190 Phase diagrams and thermodynamic properties

A superconducting Josephson junction, circuit and method of manufacture is provided for maintaining optimal chemical and structural compositions at the interface region between an insulating barrier (12) and a pair of superconductor electrodes (14) and (16). Each superconductor electrode of the Josephson junction comprises a chemical selected from the group consisting of Ba1-xKxBiO3 and Ba1-xRbxBiO3. In addition, the insulating barrier (12), formed between the electrode pair (14) and (16), is made of a chemical compound including an alkali element. The alkali element within the barrier region serves to eliminate or reduce ion migration from the BKBO or BRBO superconducting electrodes. The alkali element in the barrier (12) as well as the superconductor electrodes (14) and (16) also provides good structural matching at the barrier-electrode interface regions. As a result of the chemical and structural matching between the superconductor electrodes and the barrier, an SIS Josephson tunnel junction and integrated circuit is formed having high quality histeretic properties.

Alkali barrier superconductor Josephson junction and circuit

Oxide materials of (L x A 1-x ) i MO y , (L x A 1-x ) i M 1-z Cu z O y and (L x A 1-x ) i MO j- δ G k , wherein L is Sc, Y, lanthanides, etc.; A is Ba, Sr. Ca, etc.; M is V, Nb, Ta, T, Zr or Hf; 0<x<1; 0<z<1; i=1, 3/2 or 2; 0<y≦4; Gis F, Cl or N; δ is oxygen defect, and having a perovskite-like crystal structure, show superconductivity at a temperature higher than the liquid nitrogen temperature.

Superconducting materials including La-Sr-Nb-O, Y-Ba-Nb-O, La-Sr-Nb-Cu-O, and Y-Ba-Nb-Cu-O

We present X-ray and magnetization results for the compound [Y0.75Ca0.25]SrBaCu3−x(PO4)xOy with 0.0⩽x⩽0.30 synthesized at normal pressure and temperature. X-ray diffraction patterns show that for x= 0.0 the compound crystallizes into an orthorhombic phase. For increasing x a tetragonal phase is obtained. The Rietveld method was used to refine lattice constants. The magnetization measurements show that the samples were all superconductors over a range of temperatures from 10 to 100 K. The diamagnetic transition temperature Tc decreases with increasing (PO4)3− concentration (Tc=55 K for x=0.0 and Tc=35 K for x=0.30).

Synthesis, structure and superconductivity of [Y0.75Ca0.25]SrBaCu3−x(PO4)xOy with 0.0⩽x⩽0.30

Oxy-anion (PO 4 3− ) substituted phases of the composition REBaSrCu 2.8 (PO 4 ) 0.2 O y have been synthesized. Single phase formation is achieved only for the larger rare earths (La → Ho and Y) and all the phases crystallize in the tetragonal 123 structure. IR studies confirm the presence of a phosphate group in the structure. The total oxygen content (including that associated with the oxy-anion PO 4 3− ) has been found to be in the range 7.01 to 7.10 per formula unit. Except for RE = La, Nd, Yb and Lu, all the phases exhibit superconductivity and the T c,zero values vary from 18 to 38 K. The T c,zero increases with decreasing size of RE 3+ . ErBaSrCu 3 O 7 is multiphasic and single phase formation is achieved for the first time by phosphate substitution at the copper site. Single phase formation is encountered for x = 0.10 and 0.15 with T c,zero values of 54 and 37 K respectively.

Phase formation and superconductivity in REBaSrCu3−x(PO4)xOy (0 ≤ x ≤ 0.3)

Phases of the type LnA 2 Nb 3 O y , Ln=La, Y; A=Sr,Ba with Nb in +2 and/or +4 oxidation state have been synthesized and studied in order to explore the possibility of high T c superconductivity. Preliminary tests showed no superconductivity above 77K.

Search for superconductivity in mixed niobium oxides

The present study is aimed at carrying out studies on SmBa2(Cu3−xMx)O7 and DyBa2(Cu3−xMx)O7 (M=Fe, Ni, Zn) in order to determine the solubility limit and the Tc suppression rate. Interestingly, the solubility limit is higher for M = Zn in both the Sm and Dy — 123 systems in contrast to the Y — 123 system. Resistivity (ϱ-T) and susceptibility (χ-T) studies indicate that the initial rate of Tc suppression (upto ∼3 atom% of the dopant concentration) in both the systems lies in between that of Nd-123 and Y-123 systems.

U.V. Varadaraju

Papers

Phase formation and superconductivity in REBaSrCu3−x(PO4)xOy (0 ≤ x ≤ 0.3)

Search for superconductivity in mixed niobium oxides

Effect of 3d metal ion (Fe, Ni, Zn) substitution in REBa2(Cu3−xMx)O7 (RE=Sm, Dy)