The magnetic properties of superconducting alloys. part i

The phase diagram for the LaBCCO solid solutions La 1.5- x Ba 1.5+ x-y Ca y Cu 3 O z has been determined in air at 950°C. Solid solution form over the complete composition range 0⩽ x ⩽0.5, 0⩽ y ⩽0.5 apart from composition at high x and low y , centred on the La-123 composition LaBa 2 Cu 3 O z . The LaBCCO solid solutions are tetragonal at all oxygen contents and are structurally distinct from tetragonal/orthorhombic La-123, which can be prepared as a stoichiometric phase by reaction in Ar. T c values of the LaBCCO solid solutions increase linearly with x at constant y and with y at constant x for samples prepared under the same conditions. Maximum T c is ∼78 K for x = y =0.5 for samples that were oxygen annealed at 400°C. The melting behaviour of the LaBCCO solid solutions has been determined. Melting temperatures increase with both decreasing x and y ; the La-336 composition has the highest melting temperature (∼1180°C) and melts incongruently to La 2 CuO 4 + liquid.

Superconducting La1.5−xBa1.5+x−yCayCu3Oz solid solutions I. Phase diagram, cation stoichiometry and Tc data

The high Tc superconductor, forming the focus of the present review, presents an interesting situation where, despite its oxygen content close to 7.0, the system prefers to crystallize in tetragonal structure, as distinct from exhaustively studied orthorhombic RE:123 (REBa2Cu3O7, RE = rare earth) compounds. Its optimum Tc is decided by the cation intermixing at various sites, unlike that in case of normal RE:123, where all cationic sites are fully occupied and fixed, leaving the anionic sites free for oxygen filling. In this, we review the rich crystal chemistry of this compound and discuss it in conjunction with its unusual superconducting properties. The article is divided in five parts: (1) first we discuss variously reported interesting finer structural details and superconductivity of pristine LaBaCaCu3O7-δ (La:1113) superconductor. The La, Ba and Ca sites are found to be intermixed with respect to their nominal sites of the standard RE:123 structure. (2) We examine the structural aspects and superconductivity of La1-xRExBaCaCu3O7 (RE = Nd, Dy, Sm and Pr). Unlike RE:123 and 124 compounds, the RE:1113 compounds form only with light rare earths such as La, Pr and Nd. Pr:1113 compound is found to be an insulator with Pr moments ordering magnetically at around 8 K, unlike at 17 K for Pr:123. (3) Studies of Pr substitution reveal that relative Tc depression due to Pr in the (La,Nd)1-xPrxBaCaCu3O7 system is less in comparison to that found for La or Nd site Pr substituted (La,Nd)1-xPrxBa2Cu3O7. (4) The results of superconductivity and structural details are reviewed for LaBaCaCu3-xMxO7 (M = Fe, Co, Ni, Ga and Zn) and finally (5) we summarize our conclusions. The results in terms of Tc depression due to these impurities in the parent La:1113 system are seen as similar to those observed for RE:124 (REBa2Cu4O8) and Nd:214 (Nd2-xCexCuO4), but are different from RE:123 and La:214. The results of structural details, superconductivity and magnetic ordering presented in the four parts for La1-xRExBaCaCu3-yMyO7 (RE = Nd, Dy, Sm, Pr with 1.0 ≥ x ≥ 0.0, and M = Fe, Co, Ni, Ga and Zn with 0.24 ≥ y ≥ 0.0 were obtained from X-ray and neutron diffraction and using AC and DC magnetic susceptibility, electrical resitivity and heat capacity data as a function of temperature. This review is mainly a consolidated work of the authors, including their collaboration from various laboratories and the reported data of other workers, all being duly acknowledged in the text.

PHYSICAL CHARACTERIZATION OF PURE AND SUBSTITUTED TETRAGONAL LaBaCaCu3O7-δ SUPERCONDUCTOR

We have investigated the influence of incorporation of Ca and Y ions on the structural and superconducting properties of La 3.5− x − y Y y Ca 2 x Ba 3.5− x Cu 7 O z (0.0≤ x = y ≤0.7) system by Rietveld refinement using the neutron diffraction data on five samples viz. x = y =0.0 (#1), 0.20 (#2), 0.40 (#3), 0.50 (#4) and 0.70 (#5) along with X-ray diffraction (XRD), resistivity, AC susceptibility and oxygen-content measurements. Samples with 0.20≤ x = y ≤0.70 are superconducting and T c R =0 is between 56 and 79 K. Neutron data confirms (i) the occurrence of a single phase tetragonal structure with space group P 4/ mmm for all the five samples, (ii) Ca and Y ions substitute onto the La sites, with concomitant displacement of La on to Ba sites, and (iii) increasing x = y from 0.0 to 0.50 decreases La–O(3) bond length from 2.516 to 2.446 A and increases Cu(2)–O(2) bond length from 2.200 to 2.292 A with corresponding increase in T c from 0 to 79 K due to increasing occupancy of Y/Ca at La site. A correlation exists between the Cu(2)–O(2) and La–O(3) bond lengths and the measured T c values of five samples. Rietveld analysis of XRD data for x = y =0.50 (#4) sample clearly shows the occupancy of Ca ions in the La sites and to small extent 0.03 in Ba sites. Such cation disordering between Ca and La,Ba may give different oxygen contents obtained from neutron data and iodometry.

Structural studies of the superconducting La3.5−x−yYyCa2xBa3.5−xCu7Oz system

The structure of superconducting LaBa 1.5 Ca 0.5 Cu 3 O δ (LaBCCO) was refined by the Rietveld method using powder neutron diffraction (ND) data. Data sets were collected at both room temperature and high temperature (HT). For the room temperature data, samples were heated isothermally over a range of temperatures to give a range of oxygen contents and then either cooled slowly or quenched. HT data were recorded in isothermal steps over the range 300–900°C in vacuum. The structure is similar to that of YBCO but with the A-cation distribution [La 0.5 Ca 0.5 ][La 0.5 Ba 1.5 ]Cu 3 O δ ; variable oxygen content is accommodated by changes in occupancy of the basal plane sites and the samples remain tetragonal under all conditions. Layers of Cu(2)O 5 pyramids are connected via apical oxygens O(3) to Cu(1) atoms. The Cu(2)–O bond distances vary smoothly with δ , have similar values for all three data sets and are insensitive to critical temperature ( T c ) variations seen between quenched and slowly cooled samples for a given δ . The Cu(1)–O(3) bond distances show a range of dependencies on δ and appear to correlate with T c , even though the superconductivity cannot reside with Cu(1) due to the absence, in this tetragonal structure, of infinite Cu(1)–O chains which could have provided a long-range conduction pathway. These results, in which the composition dependence of the Cu(1)–O(3) bond length appears to reflects that of T c , are in complete contrast to data on YBCO which show that Cu(2)–O(3) is the key structural parameter that mirrors T c .

Crystal structure-Tc correlations in LaBa1.5Ca0.5Cu3Oδ

The composition dependence of superconductivity and crystal structure in La(Ba1−xCax)2Cu3O7−y system was determined by the resistivity measurements and X-ray diffraction analysis. The superconducting transition temperature is raised with the increase of Ca content till x=0.6, at which the zero resistance temperature of the sample is 81.5 K. In the meanwhile, the crystal structure of the sample changed from tetragonal (x=0) to orthorhombic structure (x=0.2, 0.4, 0.6). With further increase of Ca content, the superconductivity decrease for the sample of x=0.8 with mixed phases including the orthorhombic oxygen-deficient perovskite-like (ODP) structure and no superconducting transition is found at 4.2 K for the sample of x=1 without the ODP structure. A possible explanation of these experimental results is given.

COMPOSITION DEPENDENCE OF SUPERCONDUCTIVITY AND CRYSTAL STRUCTURE IN La(Ba1−xCax)2Cu3O7−y SYSTEM

The electric and magnetic measurements have been carried out on a polycrystalline single phase SmBa2Cu3O7 sample. The normal state susceptibility of it consists of a temperature-dependent Curie-Weiss paramagnetic term with an effective Bohr magneton number peff=1.1 and the Curie-Weiss temperature θ=38 K and temperature-independent terms: the Van Vleck and Pauli paramagnetism and the ionic core and Landau diagmagnetism. The Pauli susceptibility derived from the temperature-independent normal-state susceptibility of YBa2Cu3O7 is so large that it is due to the strong electron-electron interactions among conduction electrons. We deduce the low critical field Hc1(1.5 K), which is 580 Oe and the critical magnetization current density which is of the order of 104 A/cm2 at 5 kOe and decreases with increasing applied field from the magnetization hysteresis loop of SmBa2Cu3O7 for fields up to 60 kOe and temperature at 1.5 K. Some magnetic and superconductive parameters such as Hc(0), kGL, χ, λ, etc. are also given by using the values of the low and upper critical fields and the energy gap within the Ginzburg-Landau theory.

W.Y. Guan

Papers

COMPOSITION DEPENDENCE OF SUPERCONDUCTIVITY AND CRYSTAL STRUCTURE IN La(Ba1−xCax)2Cu3O7−y SYSTEM

Magnetic characteristics of normal and superconductive states in smba2cu3o7