Physical Properties of Bi 2 sr 2 cuo 6 , the Semiconducting Phase, Structurally Distinct from the N =L Bi-Cuprate Superc
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PHYSICAL PROPERTIES OF Bi2Sr2CuO 6 , THE SEMICONDUCTING PHASE, STRUCTURALLY DISTINCT FROM THE n=1 Bi-CUPRATE SUPERCONDUCTOR B. C. CHAKOUMAKOS, EDWARD SONDER, AND B. C. SALES Solid State Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6056.
ABSTRACT The stable phase that forms near 2:2:1 in the Bi-Sr-Cu oxide system is now recognized as a semiconducting phase structurally distinct from the n = 1 Bicuprate superconductor, whose actual composition (Bi2Sr2-xCuO6-y, x= 0.1-0.25, y=0-0.5) is deficient in Sr. The 2:2:1 phase forms by a slow solid-state replacement of the superconducting phase for compositions near 2:2:1. It also can be grown from melt compositions involving several phase assemblages, but single-phase products are not obtained, implying incongruent melting. Alumina and zirconia crucibles promote the growth of the superconducting phase by reacting with the melt to remove Sr, whereas Au crucibles promote the growth of the 2:2:1 phase. Owing to its fine-grained, fibrous habit, structural characterization has been difficult. Of the unit cells proposed, the C-centered monoclinic cell of Roth et al. provides the best fit to our x-ray powder and single-crystal precession data, a = 24.522(9), b = 5.426(2), c = 21.983(9) A, J3= 105.00(3)°. Perfect prismatic cleavages intersecting at -148' and -32' are indexed as (100) and (-2011 forms. Although, the superconducting n = 1 Bi-cuprate exhibits variable Sr and 0 contents, the 2:2:1 phase appears to be a single composition with no oxygen exchange observed by TGA, although it may be somewhat deficient in Cu. The 2:2:1 phase is semiconducting as determined by electrical resistivity measurements, with an apparent activation energy of 0.013 eV. For a 2:2:1 stoichiometry, the measured density of -7.2 g/cm 3 implies 16 formula units per cell. INTRODUCTION Exploration of the phase assemblages in the Bi20 3 - SrO - CuO system1 -4 has led to the discovery of several new ternary compounds in addition to the n = 1 member of the homologous family of superconducting phases (BiO)2Sr 2 Can1CunO2n+2 (n = 1, 2, 3),5,6 first encountered by Michel et al. 7 . The 2:2:1 semiconducting phase has been difficult to study because it typically forms as a fine-grained replacement of the n = 1 Bi-cuprate superconducting phase. In this paper we report the preparation of small single crystals grown from the melt and the preliminary results of a single-crystal x-ray diffraction study of Bi2Sr2CuO6 to establish the structural relationship to the n = 1 Bi-cuprate superconductor. EXPERIMENTAL Ceramic and melt processing of Bi2Sr2-xCul+zOy with 0 < x < 0.5 and 0 < z < 0.5 produces the n = 1 Bi-cuprate as a major phase when the reaction is limited to a short time. With prolonged reaction times, more nearly equilibria assemblages are obtained and they include the 2:2:1 semiconductor as a major phase. The phase assemblages involving the 2:2:1 semiconductor number at least seven (Fig. 1). The Mat. Res. Soc. Symp. Proc. Vol. 169. ! 1990 Materials Research Society
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Fig. 1.Iso
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