Variation in T c and Carrier Concentration Caused by Change of Oxygen Content in Tl-Based Superconductors
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VARIATION IN Tc AND CARRIER CONCENTRATION CAUSED BY CHANGE OF OXYGEN CONTENT IN Ti-BASED SUPERCONDUCTORS Y. Shimakawa, Y. Kubo, T. Manako, and H. Igarashi Fundamental Research Laboratories, NEC Corporation 4-1-1 Miyazaki, Miyamae-ku, Kawasaki 213, Japan ABSTRACT T, variations observed in some Ti-based superconductors were studied. Clear correlations were found between Tc, carrier concentration, and c-axis length. In particular, for Tl2Ba 2CuO 6 , a decrease in oxygen content of about 0.1 per formula unit, which corresponded to a decrease in hole concentration of about 0.2, increased T, up to about 80K from a metallic non-superconductor, and elongated the c axis by about 0.4%. In addition, as Tc values increased systematic changes in metal-sheet separations were observed. T, variations caused by a change in oxygen content were also observed in Tl2 Ba 2CaCu 2 0 8 and T12Ba 2 Ca2Cu3O10. It was demonstrated that superconductivity appears in a certain appropriate range of carrier concentration similar to those observed in other high-Tc superconductors. INTRODUCTION Since the discovery of Tl-based superconductors [1,2], numerous investigations have been made on this system. However, from the early stage of the studies in this system, it has been quite mysterious that samples with the same crystal structure showed various Tc values. For example, Tc of 85K or 110K for T12Ba2CaCu 2O8 (2212 compound) [3,4], and Tc of 116K or 125K for Tl2 Ba 2Ca2Cu 3 Olo (2223 compound) [3,5] have been reported. In particular, in Tl 2Ba 2 Cu0 6 (2201 compound), a wide range of behaviors varying between non-superconductivity and a Tc of 85K have been reported [1,5-7], and also have been observed in our laboratory [3,81. We have succeeded in synthesizing the 2201, 2212, and 2223 samples with various Tc values by controlling the annealing condition, and revealed that variation in Tc was caused by the change in oxygen content, suggesting the relation between Tc and carrier concentration [9,10]. In the present study, we will report the variation in Tc, detailed structure, and carrier concentration caused by change of oxygen content in Tl-based superconductors, and discuss their correlations quantitatively. We will further discuss as to where the oxygen atoms, which cause the change in carrier concentration, are adsorbed and desorbed in the crystal. EXPERIMENTAL Synthesized samples were T12 Ba 2 Cu0 6 , T12Ba2CaCu 2 08, T12Ba 2 Ca 2 Cu 3 010, and TIBaLaCuO 5 . The former three have different numbers of CuO2 sheets between TI-O double layers, and the last has one CuO2 sheet (an octahedron) between TI-O mono layers [11]. They were prepared by solid state reaction. Mixtures of T12 0 3 , BaO, CaO, CuO and La20 3 powders with designed compositions were pressed into pellets. The pellets were wrapped in Au foil, and sintered at 860°C for 5-10 h in oxygen atmosphere. After grinding, they were sintered again at 880-890°C for 5-10 h in oxygen atmosphere. Structural identification was carried out by powder X-ray diffraction and all samples were confirmed to b
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