Oxygen Stoichiometry and the High T c Superconducting Oxides
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more accurate (±3%). The iodiometric method,8 in which the material is dissolved in an acidified solution of KI followed by back titration of the iodine formed, gives the average oxidation state of Cu and thereby the amount of oxygen (assuming that the valence of the other metal ions are known). In the presence of Bi the iodometry is not reliable because of the possible formation of Bil complexes. An alternative technique9 consists in the dissolution of the compound in hydrochloric acid in the presence of an excess of Fe(II). Fe(II) reduces Cu(III) to Cu(II) and the remaining Fe(II) is back titrated by Cr 2 0 7 so that the amount of Cu(III) in the compound, and the corresponding oxygen, can be determined.
La-Based Cuprates, 40 K Materials Introduction Oxide compounds have been extensively studied through the years because they exhibit a broad spectrum of electrical, magnetic, and optical properties providing both scientific and technological interest. Most oxides are insulators, but a few of them (e.g., LiTi2041 or BaPb,.xBij032 show metallic conductivity and even superconductivity at low temperatures. The discovery of superconductivity at 35 K by Bednorz and Miiller3 in the cuprate La-Ba-Cu-O system prompted the search for other high Tc compounds among this oxide family. Superconductivity above liquid nitrogen was then rapidly achieved with the Y-Ba-Cu-O system4 (Tc=90 K) and subsequently, with the Bi-Sr-Ca-Bu-O5 and Tl-Ba-Ca-Cu-O6 systems, Tc was raised to 110 K and then 125 K. A common feature of these new high Tc cuprates is that they belong to the large family of materials, termed perovskites, which have been studied over the years because of their ability to absorb or lose oxygen reversibly (i.e., for their nonstoichiometry in oxygen). It had been previously established in the field of superconductivity that Tc is extremely sensitive to compositional stoichiometry. This applies as well to these new high Tc oxides, for which it is thought that the valence (or oxidation state) of copper is one of the crucial parameters for the occurrence of superconductivity in these materials. The formal valence state of Cu can be simply modified by changing the oxygen content. In order to maintain charge neutrality per formula unit, increasing the oxygen content will increase the formal Cu valence and vice versa. MRS BULLETIN/JANUARY 1989
For brevity in this article, we discuss the changes as occurring in the formal oxidation state of the Cu, but we recognize that a hole charge state introduced, for example, by continued oxidation could be associated with either the copper (Cu2+—»Cu3+) or with the oxygen (O^-^O 1 ) leading to a net [Cu-0] +x . Indeed some recent research7 indicates that the holes are associated with the oxygen. Therefore, throughout this article, when we refer to cation valence we present the formal valence (oxygen assignment as -2) only as an expedient. La-Sr-Cu-O, Y-Ba-Cu-O, and Bi-Sr-CaCu-O are denoted respectively as the 40 K, 90 K and 85 K systems. Although all are discussed, most attention focuses on
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