Preparation and Characterization of La 1 Sr 2 Nb 5 O 10-x Specimens
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PREPARATION AND CHARACTERIZATION OF La Sr Nb 0 SPECIMENS 1
Donald
H.
Galvdn,
2
5
10-x
M. Avalos-Borja,
L. Cota-Araiza, J. CruzReyes, E. A. Early Instituto de Fisica-UNAM, Lab de Ensenada, Apdo. Postal 2681, Ensenada, B.C. MEXICO, * Fac. de Ciencias Quimicas UABC, Tij. B.C. MEXICO **Dept. of Physics and Institute of Pure and Applied Physical Sciences, Univ. of California-La Jolla, Ca 92093, U.S.A. ABSTRACT Recently Ogushi et al reported a La-Sr-Nb-O compound with a superconducting temperature of about 225 K. The possibility of having superconductors with such a high temperature is certainly technologically relevant. We prepared specimens with the same nominal stoichiometry and performed characterization by SEM, high resolution TEM, Scanning Auger and X-rays. INTRODUCTION Many studies have been reported on high-Tc superconducting oxides in the last three years. A pioneering discovering of the 40 K class La-Ba(Sr)-Cu-O system was followed by the attainment of superconductivty above 90 K in the Y-Ba-Cu-O system. Ogushi et al , published a series of articles claiming the possibility of a new superconductor family, La-Sr-Nb-O, with a possible T as high as 255 K. In this work, we report the production and characterization of a system made of La 2Sr NbS 01o, following the standard procedures used for the 1-2-3 superconductor. EXPERIMENTAL PROCEDURE We prepared samples of La 2Sr I Nb 0 _x, starting from La2 02, SrCO 3 and Nb220 5, 99% pure. These were mixed in the appropriate proportions to obtain the desired stoichiometry. We followed two preparation methods: First, the mixture was ground in an agate mortar and the resulting mixture was put in a ceramic crucible and sintered in air at a temperature of 1300 0 C for 14 hours, allowing it to cool inside the oven. The second method consisted of sintering two times. Once the mixture was ground together, the sample was sintered in air at a temperature of 12500 C for 16 hours. The resulting mixture was reground one more time. Pellets were formed using a pressure of 4000 pounds, resintered in oxygen at 13000 C for 16 hours, and allowed to cool inside the oven. Pellets obtained by both methods were of beige color.
Mat. Res. Soc. Symp. Proc. Vol. 209. Q1991 Materials Research Society
838
For electrical resistivity measurements, platinum elec trical leads were attached to gold film contact pads on a bar-shaped specimens using silver epoxy. A linear Research Model LR 400 ac resistance bridge operating at a frequency of 16 Hz was used for these measurements. Magnetic susceptibility was measured in an applied field of 13.4 Oe using a variable temperature vibrating sample magnetometer. The X-ray analysis was performed in a GE-XRD6 diffractometer using Cu-K radiation at a 40 KV voltage and at 20 mA current. The microstructure of the grains was studied with a JEOL 4000 microscope. The surface composition was determined by Auger Spectroscopy using a Scanning Auger Microscope PERKIN ELMER PHI-595. RESULTS AND DISCUSSION The surface morphology of the samples was studied using a JE
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