Cation Disordering in the Tl-2122 Superconductors
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CATION DISORDERING IN THE TI-2122 SUPERCONDUCTORS B. MOROSIN, D. S. GINLEY, E. L. VENTURINI, R. J. BAUGHMAN, J. E. SCHIRBER, and C. P. TIGGES. Sandia National Laboratories, Organization 1150, Albuquerque, NM 87185-5800 ABSTRACT We have examined several crystals belonging to the TI-2122 structure type (TI2CaBa 2Cu 2O5 with c=29A), and have shown that cation solid solution occurs. Such cation disorder appears to be responsible for the observed small differences in lattice parameters reported by various investigators and to contribute towards the substantial variation in the superconducting transition temperatures. INTRODUCTION We have recently examined the contributions that different cation solid solution or disorder and oxygen vacancies make on the properties of various TI-Ca-Ba-Cu-O superconductors [1]. We suggested that the observed small differences in the c-axis lattice parameters for crystals of the same structure type arise from 2such solid solution effects. The substitution of a trivalent cation, like T]3+, for divalent Ca + should affect the number of holes and, hence, the electronic and superconducting properties of the materials. Detailed structural variations were determined [I] in the Tl-I 122 and Tl-1223 (i.e., the 12 and 15A c-axis structure-types; abbreviated phase designation as well as metal atom ratios for the mixed oxide powders used to prepare the superconducting phases, or growth compositions, are given in the order TI-Ca-Ba-Cu). Here we discuss our recent results on TI-2122 crystals where substantial amounts of cation exchange are observed and found to correlate to significant changes in the transport and magnetization properties. Crystal Growth and Characterization Crystalline plates were grown by "pseudo-flux growth" in which ground and sieved mixed oxide powders were directly loaded into Pt crucibles with tight fitting Pt lids. The crucibles were placed in a vertical tube furnace under an oxygen atmosphere [2].
Rapid
heating (-5 min) to 950"C followed by a heat soak for 1 to 2 hours is achieved by moving the crucible into the preheated furnace. The temperature is then lowered over a period of 12.5 hours to 700"C and subsequently to 25"C in 5-6 hours. Only heat soak temperatures above 900"C produced the melt pockets containing well-formed platelets (1-2 mm on a side, 0.01 mm thick). The crystal production was a-function of starting compositions with a strong dependence on the Ca/Ba ratios and the amount of Cu. Occasionally much thicker plates were grown, usually from CuO-rich melts, but these usually exhibited diffraction patterns showing two or more slightly misaligned crystals even though they were of the same structure type. Formation of polycrystals was not restricted to thicker plates and was demonstrated for a few plates 0.1 mm on the side and less than 0.001 mm thick [2]. The crystal growth chemistry is quite complex, and it appears that crystal growth can occur from both TIO-rich and CuO-rich fluxes. Our growth from a TIO-rich flux tends to favor TI/Ca exchange while CuO-rich melts
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