Thermodynamic and nonstoichiometric behavior of promising Hi-Tc cuprate systems via electromotive force measurements: A
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I. INTRODUCTION
THIS article will present an overview of experimental studies that were carried out in the Chemical Technology Division at Argonne National Laboratory to investigate the nonstoichiometric and thermodynamic behavior of promising high-Tc cuprate superconductor oxide systems as a function of oxygen partial pressure, oxygen stoichiometry, and temperature via EMF measurements. The systems investigated include (1) the promising YBa2Cu3Ox (Y-123) system; (2) related rare-earth systems, NdBa2Cu3Ox (Nd-123) and GdBa2Cu3Ox (Gd-123); and (3) the bismuth perovskite systems, Bi2Sr2Ca1Cu2Ox (Bi-2212), and lead-doped Bi2Sr2Ca2Cu3Ox (Bi-2223). However, this article will mainly emphasize the results obtained with the Y-123, Nd-123, and Gd-123 systems. Comparison of our partial pressure measurements with the results of related measurements on perovskite cuprate systems can be obtained from the references cited in this article. Renewed interest in the Y-123 system and related rareearth RE-123 systems has occurred because of promising developments of coated conductors that can yield significantly higher current densities in Tesla-level magnetic fields than Bi-2212 and lead-doped Bi-2223 systems. We have reported previous oxygen partial pressure measurements on the Y-123 and Nd-123 systems.[1–4] In these studies, the oxygen content was varied in well-defined small increments by means of a coulometric titration technique, and the equilibrium partial pressure (fugacity) above the sample was established from EMF measurements.[5] This method is sensitive to detecting phase transformations, oxygen nonstoichiometry, and thermodynamic properties of Y123 and RE-123 systems, where the single phase homogeneity regions have a wide range of oxygen content in the condensed phase. The coulometric technique has also been
used in promising bismuth-cuprate perovskite systems, where the single-phase homogeneity regions have a very narrow range of oxygen content.[6,7,8] The transition temperatures have been reported to be about 90 K for Y-123, 92 K for Gd-123, and 96 K for Nd-123, where x is close to the value of 7.0 with an oxygen deficiency of x 5 ,6.8. The ionic radii of trivalent Y, Gd, and Nd are 0.99, 1.02, and 1.10 A˚, respectively,[9] and the transition temperature increases with an increase of ionic radius. Between x 5 6.5 and 7.0, the results of Veal et al.[10] for Y-123 showed two plateaus, one at 60 K and one at 90 K. It was postulated that the 60 K plateau (x 5 ,6.63 to 6.80) characterizes the ortho-II structure, and the 90 K plateau (x 5 ,6.80 to 7.0) characterizes the ortho-I structure. The lower plateau was nearly absent for the case of Nd-123.[10,11] It should be emphasized that unlike the Y-123 system, solubility of Nd in Ba and Gd in Ba has been observed for the Nd-123 and Gd-123 systems. According to Wu et al.,[12] Gd-123 (solid solution) has a solubility limit of x 5 0.2 in air, and the solubility limit can be reduced in low oxygen partial pressures resulting in precipitation of Gd2BaCuO5 and CuO as second phases. I
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