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

Bulk Y1Ba2Cu3Oy superconductors with KClO3 or K2CO3 additives were synthesized by a solid state reaction method. The influence of additives on the AC susceptibility, morphology and microstructure of samples with nominal compositions Y1Ba2−xKxCu3Oy, Y1Ba2Cu3.5−xKxOy, Y1−0.2xBa2−0.2xKxCu3Oy, (x=0.2) were investigated. Experimental results showed a promoting effect of the additives on the formation of 123 phase in air atmosphere. Only traces of potassium were observed in all samples after the final backing, but chlorine was remained in the material. The superconducting transition temperature for all KClO3-doped samples was found to be higher than that for K2CO3-doped and undoped Y1Ba2Cu3Oy (YBCO). KClO3-doped Y1Ba1.8K0.2Cu3Oy with Tconset at 105 K was obtained. The comparison of characteristics for samples with various potassium compound in the initial batch demonstrated that each additive had a specific effect on χ(T), ΔTc and microstructure of YBCO high-temperature superconductors (HTSC). The results were discussed in terms of different interaction in which KClO3 and K2CO3 participate up to 900°C.

A study of the effect of KClO3 addition on the AC susceptibility and microstructure of high-temperature (Tconset at 105 K) YBCO ceramic superconductors

We report here SQUID (magnetization) measurements, along with supporting specific heat, Raman, SEM (scanning electron microscope), EDX (energy dispersive X-ray) and XRD (X-ray diffraction) measurements, on Cu-doped and undoped double perovskite $\mathrm{Sr}_{2}^{2+}\mathrm{Y}^{3+}\mathrm{Ru}^{5+}\mathrm{O}_{6}^{2-}$
 (abbreviated as SrY2116) system grown as single crystal using high-temperature solution growth technique. These measurements show the undoped system to be a nonmetallic (insulating) spin glass (SG) and the ∼5–30% Cu-doped (i.e. Cu-concentration/(Cu + Ru-concentration) ∼5–30%) system to be a spin glass superconductor (SGSC) with T
 c (critical temperature) ∼28–31 K and superconducting volume fraction, f
 sc∼2.2–9%. To mention, similar measurements done on undoped and Cu-doped BaY2116 and BaPr2116 systems show for them the same (SG, SGSC) behaviors. However they show a decrease in T
 c and f
 sc when diamagnetic Y3+ ions are replaced by Pr3+ spins, presumably due to enhanced internal pair breaking, and also decreased Cu–O–Cu overlap, owing to Pr3+ presence; these phenomena are known to exist in the Pr123 compound, PrBa2Cu3O7−δ
 (δ∼0), due to ∼10% of Pr3+ ions having tendency to occupy Ba2+ sites. Measurements done on undoped and Cu-doped SrHo2116 show similar SG and SGSC properties. Further, the undoped and Cu-doped SrY2116 crystals grown by hydrothermal growth technique (i.e., grown using lower temperature and high pressure) show same behaviors. From these investigations it can be said that the undoped Ru-double perovskites (A2BB′O6, B′=Ru) are SG systems and that Cu-doped Ru-double perovskites (A2BB′O
 δ
 , δ∼6, B′=Ru1−x
 Cu
 x
 , 0<x≲0.3) are SG superconductors (SGSCs). Results are discussed.

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Magnetic and Superconducting Properties of Doped and Undoped Double Perovskite Sr2YRuO6

By increasing the K concentration x, the authors observe first a small increase of Tc(x) for x 0.4 and a structural change for x=0.06. Through AC susceptibility, measurements of chi ', chi ", they observe in zero static field a broadening of chi ', as a function of x related to modifications of the intergranular situation by K+-doping. For finite static fields H, a larger broadening of chi ' occurs, increasing also with x. High temperature measurements of the irreversibility lines, related to chi ", indicate that the inter- and intragrain pinning is reduced by x; this phenomenon may be explained by the atomic disorder induced by x and the thermal fluctuations, two effects which may tend to melt the vortex lattice. But at 4.2 K, the low field hysteresis, negligible for low x values, becomes larger for x>0.4, indicating a higher pinning.

Diamagnetism and irreversibility lines in EuBa2-xKxCu3O7 compounds

Ab-initio calculations under GGA approximation have been employed to find out the effect Ba substitution by K and Ca on the structural and electronic properties twined and untwined YBCO system. In this regard, the twin boundary energy, γ, and impact of the substitution on the boundary's charge distribution have been of special consideration. Our results show that despite the structural changes the presence of K (Ca) modifies substantially density of levels at the Fermi level, which could be responsible for empirical reports of decreasing the critical temperature (T c ) by increasing the K(Ca) content. Although, the K doping reduces the γ value in YBa 2 – x K x Cu 3 O 7 system, after calcium doping it remains more or less unchanged. In addition, reduction of the carrier density occurs at twin boundary in CuO 2 layer for the substituted system with respect to the untwined YBCO system. Our results would be noticeable in conjunction with the experimentally reported twinned and alkali substituted superconductive properties of the YBCO samples.

Effects of K and Ca doping on twin boundary energy of cupperate superconductors

We have studied the crystal structure and magnetic susceptibility of high T c single phase RBa 2−x K x Cu 3 O 7−δ , where x=0 to 1 and R=Y, Eu. All the YBa 2−x K x Cu 3 O 7−δ samples have the same orthorhombic crystal structure and are superconducting with almost the same T c . In EuBa 2−x K x Cu 3 O 7 the crystal structure changes from orthorhombic to tetragonal and a progressive decrease in T c is observed when x is increased. EuBaKCu 3 O 7−δ is not superconducting. The decrease in the cation charge reduces both the oxygen concentration and the average valency of the copper.

M. Kowitt

Papers

The effect of substituting potassium for barium on the superconductivity of RBa2Cu3O7 (R=Y and Eu)