Oxidation and Reduction of Bi 2 Sr 1 . 85 Cuo 6-Y Crystals

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OXIDATION AND REDUCTION OF Bi2Srl.85CuO6-y CRYSTALS EDWARD SONDER, B. C. CHAKOUMAKOS, AND B. C. SALES Solid State Division, Oak Ridge National Laboratory, Oak Ridge TN 37830-6056

ABSTRACT The effects of oxidizing and reducing atmospheres on the oxygen contents and electrical properties of Bi2SrI.85CuO 6 -y single crystals were investigated. Asgrown crystals were normally semiconducting but became metallic and superconducting after heating the crystals for several hours in oxygen at temperatures between 500 and 8000C. Previously superconducting crystals were made semiconducting by heat treatments in helium at temperatures between 700 and 7801C. Changes in the oxygen content of the crystals as well as the oxidation and reduction kinetics during the various heat treatments were monitored using standard thermogravimetric techniques. A good correlation was found between the annealing temperatures in oxygen (helium) at which there was an uptake (loss) of oxygen by the crystals, 500'C (7000 C), and the onset (loss) of metallic behavior and superconductivity. From isothermal measurents of the weight change as a function of time, it was found that the apparent activation energies for oxidation and reduction were about 0.5 eV and 2.7 eV, respectively. A partial decomposition of the crystals during the various heat treatments, however, precluded a quantitative analysis of the kinetic data. INTRODUCTION The superconductivity of the rare earth cuprates (RBa2Cu 3Oz, R = Y, Gd, Ho, etc.) is well known to depend critically on the oxygen content z with z = 7 resulting in a 90 K superconductor while z=6 produces a semiconductor. For these materials the oxidation and reduction kinetics and oxygen diffusion rates have been well documented [1]. Recently we have shown [2] that a similar superconductor-to-semiconductor transition occurs for the one layer bismuth cuprate Bi2 Srl. 85 Cu06-y as the oxygen content y is varied by appropriate heat treatments in either oxygen or helium atmospheres. The present work focuses on the correlation between changes in the electrical properties and changes in the oxygen content y of the crystals. Qualitative oxidation and reduction kinetic data are also reported. EXPERIMENTAL PROCEDURES Crystals of Bi2 Srl. 85 CuO 6 -y were grown in gold-lined A12 0 3 boats as described previously [2,31. Most of these crystals were in the form of thin (0.01 cm) cleavage flakes. For the thermogravimetric measurements single crystals with areas of about 0.3 cm 2 that had parallel and flat front and back surfaces were used. For electrical measurements, where it was necessary to use larger samples, the front and back surfaces of the crystals were not always parallel indicating that these samples were not perfect single crystals. However, the four electrical contacts were always placed on the same cleavage face of one crystal. Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society

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Electrical measurements were made between room temperature and 4.5 K using the standard four-probe technique as described p