J.W. Hodby

Phase diagram studies and crystal growth of PrBa2Cu3O7−y

Abstract In this work a physico-chemical investigation of the crystal growth of PrBa 2 Cu 3 O 7- y (PrBCO) has been carried out with the aid of improved thermogravimetric analysis (ITGA) and differential thermal nalysis (DTA). The phase diagram of CuOBaCuO 2 has been established by ITGA, and the eutectic composition, found to be 0.72CuO+0.28BaO, was chosen as the flux for the crystal growth. Studies of the decomposition of (Y 1- x Pr x )BCO with various doping concentrations, and of the pseudo-binary phase diagram of PrBCO-flux, were made by DTA. This information was used to grow and separate crystals of PrBCO from the flux above the eutectic temperature, and single crystals of PrBCO up to 10×5×0.8 mm 3 were obtained by spontaneous nucleation. Polarised neutron diffraction experiments have been performed on these crystals and indicate some substitution of Pr on the Ba site in the as-grown samples.

Growth of high-quality 2212 BSCCO crystals in Pt crucibles and characterisation

Abstract We report the growth of pure, high-quality Bi2Sr2CaCu2Ox (BSCCO 2212) crystals, grown by a novel flux method in Pt crucibles. The growth process involves repeated remelting and recrystallisation of the same melt. The crystals appear to be chemically pure, contain no Pt contamination and demonstrate a very high as-grown Tc (onset), above 92 K, and the majority in the range 93–95 K with transition widths (10–90% level) between 4 and 8 K. The lattice parameters of a typical crystal are a=5.414(2) A , b=5.413 (2) A , c=30.893 (12) A . We discuss the growth process and the qualities of these crystals.

Improved thermogravimetric analysis and crystal growth of BiSrCaCuO

Superconducting crystals of Bi Sr Ca Cu O with and without Pb or Y dopant have been grown in platinum or alumina crucibles from KCl or self-flux. The improved thermogravimetric analysis (ITGA) enables the crystallization temperature to be measured and then the crystals to be grown by introducing a Pt wire as a nucleation centre or by top-seeding. At the end of crystal growth the crystals could be separated from the flux. Differential thermal analysis (DTA) is useful for getting some information to design the temperature program. The crystals of Bi Sr Ca Cu O, Bi(Pb) Sr Ca Cu O and Bi Sr Ca(Y) Cu O were identified as mainly 2212 phase with the two Cu O layer structure by X-ray diffraction and by electron probe microanalysis (EPMA). The as-grown crystals are superconducting with transition temperatures between 80 and 90 K.

The phase diagram and crystal growth of Y1xPrxBa2Cu3O7-y

Y 1 x Pr x )Ba 2 Cu 3 O 7- y is a unique superconductive material for the investigation of the interrelation between crystal structure, chemical composition and physical properties. In order to grow large crystals of high quality, an extensive study has been made of the Y 2O 3 -Pr 6 O I1 -BaO-CuO growth system. The eutectic composition of the BaO-CuO flux is confirmed by the improved thermogravimetric method (ITGA) as occurring at the molar composition of 7:18. The pseudo-binary phase diagram of (Y, Pr)BCO-(7BaO+18CuO) has been determined by means of differential thermal analysis (DTA), by X-ray diffraction and by study of crystal growth. Using the phase diagram, suitable compositions and temperature programs have been established for the growth of large crystals and for the separation of the crystals from the flux. Single crystals with a wide range of Pr doping have been obtained from a eutectic BaO-CuO flux. The dependence of the Pr concentration in the crystals on the composition of the initial melt has been determined by electron probe microanalysis and X-ray microanalysis. The superconducting transition temperature is found to decrease monotonically with increasing Pr concentration.

Improvement of crystal growth and superconductivity of YBa2Cu3O7- delta by additive

A series of experiments have been carried out on the influence of additives on the crystal growth and superconductivity of YBa2Cu3O7- delta (YBCO) crystals grown in alumina crucibles from the eutectic self-flux system of BaO-CuO. The single additives were B2O3, Bi2O3, BaF2 or BaCl2 and the double additives were BaF2-B2O3 or BaF2-Bi2O3. It was found that a small amount of BaF2 greatly improved not only the crystal growth, by increasing solute solubility and crystal yield, but also the superconductivity by reducing Al contamination and resulting in a chemical composition approaching stoichiometry. As a result, single crystals up to 20*10*0.5 or 12*10*1 mm3 in size have been obtained, and a superconducting transition temperature of 92.5 K with a transition width of 1 K in the as-grown state, or Tc=93 K with transition width <1 K after annealing in oxygen have been reached by using BaF2 as additive. A model is proposed to interpret these improvements: it incorporates F- induced monomolecular clusters in the high-temperature solution and preferential adsorption of F- on the surface of the growing crystal.

Phase diagram and crystal growth of Pb2Sr2(YxCa1-x)Cu3O8+y

Abstract Fundamental research on the crystal growth of Pb2Sr2(YxCa1-x)Cu3O8+y (PSYCCO) has been carried out by differential thermal analysis (DTA), phase diagram studies, and improved thermogravimetric analysis (ITGA). The phase diagrams of the systems PSYCCO-PbO and PSYCCO-PbO(NaCl) have been produced. These indicate that the addition of NaCl, which functions as a flux as well as separating the growth system from air, is beneficial to the crystal growth. Superconducting crystals were grown by spontaneous nucleation with and without precursor. Single crystals up to 3×2.8×2 mm3 in size have been successfully grown from PbO-NaCl flux without using a precursor. The crystals were characterized by X-ray powder analysis and by measurement of their superconducting magnetic properties in the as-grown state.

Observation of superconductivity in the organometallic intercalation compound SnSe2{Co(η-C5H5)2}0.33

Intercalation of phosphorus-doped SnSe2 with bis(cyclopentadienyl) cobalt {Co(η5-C5H5)2} produces superconducting SnSe2{Co(η5-C5H5)2}0.33 with a critical temperature of 6 K; magnetisation measurements show a hysteresis loop with a maximum at 40 G, indicating type II superconducting behaviour; the lower critical field (Bc1) is estimated at 10 G at 4.2 K and the critical current density (Ic) is ca. 500 A cm–2 at 2.5 K; the diamagnetic susceptibility corresponds to 13% of perfect diamagnetism.

Growth of doped and undoped BSCCO 2212 crystals in platinum crucibles by repeated remelting and recrystallisation

Abstract We describe the growth of Y-doped and undoped Bi 2 Sr 2 CaCu 2 O y (BSCCO 2212) crystals by a repeated melting and recrystallisation technique in platinum crucibles. The conditions for the growth of high-quality, chemically pure (undoped) and highly Y-doped crystals in platinum crucibles have been optimised. The as-grown, undoped crystals indicate a very high T c (onset) between 92–95 K and the Y-doped ones between 90 and 92 K with transition widths (10–90% level) between 4 and 10 K. It appears that, in the case of doped batches, the crystals with a high level of Y-doping are produced homogeneously and reproducibly by this method, but the low-level doping produces inhomogeneous distribution of Y within the crystal layers. The reasons are discussed.

Investigation of the crystal growth of CuO from the CuO-Bi2O3 system

The relationship between properties and structure in high-temperature superconducting copper oxides has stimulated the investigation on the properties of single crystal CuO. The phase diagram of CuO-CuBi2O4 has been established by improved thermogravimetric analysis (ITGA), differential thermal analysis (DTA) and X-ray diffraction. The crystallization temperature of CuO was determined by ITGA and the dependence of crystallization temperature on cooling rates showed about 20 °C supercooling in the crystal growth of CuO from the CuO-Bi2O3 system. Using the theory of crystal growth under stable state conditions, an initial kinetic analysis of seed crystal dissolution was also performed on the ITGA data. Using the phase diagram and the ITGA information, large crystals have been grown near the surface of the high-temperature solution or the bottom of the crucible by a combination of ITGA and the top-seeding method.

Crystal growth of CuO from BaO flux

The eutectic composition 0.72CuO-0.28 BaO is confirmed for the system CuO-BaCuO2 by improved thermogravimetric analysis (ITGA). Single crystals of CuO up to 35×6×4or20×10×3 mm3 have been obtained by spontaneous nucleation. The crystals were completely separated from flux by pouring off the molten flux above the eutectic temperature. Single crystals of BaCuO2 up to 5.5×5.5×3 mm3 were also obtained from the same batch near the eutectic composition.