Studies of Oxyfluoride Analogs of Nd 1.85 Ce 0.15 CuO 4 : Preliminary Results
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STUDIES OF OXYFLUORIDE ANALOGS OF Ndl.85Ceo.1 5 CuO4: PRELIMINARY RESULTS T.A. VANDERAH* AND M.S. OSOFSKY** *Chemistry Division, Research Department, Naval Weapons Center, China Lake, CA 93555 "**Naval Research Laboratory, Washington, D.C. 30375 ABSTRACT Samarium-based oxyfluoride analogs of the apparently "n-type" 24 K superconductor, Ndl.85Ce0. 15 CuO 4 , were prepared. A series of compounds with general formula Sm2-xCexCuO4-yFy were prepared and characterized by X-ray diffraction, bromo/iodometric titration, and measurements of electrical resistivity. Homogeneous superconducting phases have not yet been obtained; preliminary results and discussion of property trends are presented. INTRODUCTION In the past year, superconducting phases with Tc values on the order of 10 to 25 K have been found in the M1.85Ce0.15CuO4.y system with M = Nd, Sm, Pr [1,2], Eu [3], and with Ce replaced by Th [3,4]. Measurements of Hall and Seebeck coefficients and chemical analysis of the formal oxidation state of copper indicate that the charge carriers in these superconductors are electrons [1,2], not holes as in the other families of cuprate superconductors. Thus, the average valence of copper in the "n-type" systems is below 2.0, which is chemically consistent with the observation that substitution of fluoride for oxide in the parent Nd 2 CuO 4 also induces superconductivity, with a Tc of 27 K exhibited by a sample of nominal composition Nd2CuO 3 .7 FO. 3 [5]. Superconducting single crystals of Ndl.85Ce0.15CuO4 have been obtained [6,7]. The crystal structure is based on that of Nd2CuO4, derived from the structure-type of the La2CuO4 family of cuprate superconductors by maintaining the cation positions but shifting the oxygens to produce square-planar four-coordination of copper and two-dimensional CuO2 sheets [1,8]. This change in structure is apparently driven by the smaller ionic radius of Nd 3 + as compared to La 3 +: the coordination of the large cation is reduced from nine to eight. A subtle superlattice modulation has been observed by electron diffraction in the "n-type" superconductors [9] but is not yet fully understood. Synthesis of the Nd2CuO 4 family of superconductors is not straightforward; the materials as prepared in air are either nonsuperconducting or have Tc values less than 10 K. Annealing in an inert gas containing a small amount of oxygen, on the order of 100 ppm [2], at temperatures from 900 to 1050'C [2-5] is necessary to induce superconductivity with zero resistance near 20 K; annealing at 875°C in dynamic vacuum (10-5 torr) has also been reportedly successful [10]. Most superconducting samples are not metallic but semiconducting above Tc, and Meissner fractions observed are less than 20%. With the aim of synthesizing a high-quality airquenchable "n-type" superconductor, a study was undertaken of the series Sm2-xCexCuO4_yFy, in which the oxidation state of copper could be held between 1.7 and 1.8, as observed in the known superconducting Nd-Ce-Cu-O and Nd-Cu-O-F phases [1-5,11], by appropriate choices of x and
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