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. INTRODUCTION It is well known that the control of oxygen nonstoichiometry is crucial to high temperature superconductivity of the cuprate Y,Ba 2 Cu 3 0 J . The cuprate is thermodynamically stable over a range of oxygen content, 6 S x S 7 . M It undergoes a phase transition,4'5 crossing x — 6.5 from tetragonal (x :£ 6.5) to orthorhombic structure (x 5: 6.5). Furthermore, a superconductive transition at around 90 K has been found only in the orthorhombic cuprate.2'4'11 Tetragonal cuprate is, on the other hand, semiconductive6 or, otherwise, turns superconductive at a far lower temperature of about 60 K.7"9 This difference appears to correlate with the concentrations of Cu ions in various oxidation states ( + 1 , + 2, and +3), as illustrated by Shafer et al.,w where the superconducting transition temperature Tc rises from 0 K to ca. 90 K as the fraction of the complex [Cu-O] + of the total Cu ions increases from 0. The important question is, "How does the oxygen nonstoichiometry influence structure-sensitive properties such as electrical conductivity and thermoelectricity of this unusual material at high temperatures where sintering and annealing is usually carried out?" This paper is concerned with this question. According to the Gibbs-Duhem equation, single phase Y^ajCujOj has the degrees of freedom of 2 at the atmospheric pressure as the cationic composition is fixed at Y/Ba/Cu = 1/2/3 by compositional necessity for high Tc superconductivity.12"14 As the two independent thermodynamic variables, one may choose temperature, T, and oxygen partial pressure, POl, or its conjugate extensive parameter x via an equation of state with respect to the oxygen content, x = x(T,P02). We have, thus, measured the electrical conductivity and thermoelectric power as a function of temperature and oxygen partial pressure within the stability region1415 of the single phase Y,Ba 2 Cu 3 0 x above 400 °C. J. Mater. Res., Vol. 4, No. 1, Jan/Feb 1989

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To our knowledge, high temperature studies on the transport properties of this system are rare. Choi et al.16 quite recently found from these high temperature properties that the conduction mechanism changes from metallic to p-typz and to n-type semiconducting as temperature increases from room temperature. The first transition falls on ca. 400 °C, and the latter p-n transition temperature depends on the oxygen potential in the atmosphere. II. EXPERIMENTAL The cuprate specimen was prepared via a conventional ceramic processing technique. The starting powder, Y2O3 (99.9%), BaCO 3 (99.8%), and CuO (99.9%) were intimately mixed in the appropriate ratio, cold pressed, and calcined at 950 °C in air for a day with an intermediate regrinding in an agate mortar and pestle. The calcined sample was reground, cold pressed into a disc of 1" diameter and 1/8" thick at a pressure of 200 kg/cm 2 and subsequently cold-isostatically pressed under 600 kg/cm 2 , and finally sintered at 950 °C in air for about three days. An oversintering was intended to min