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SUPERLATHICE MODULATION IN Nd 2CuO 4 AND THE ELECTRON - DOPED SYSTEMS Nd2 .xCexCuO 4 and Nd2 CuO 4 _xFx

A. C. W. P. James, D. W. Murphy, C. H. Chen, D. J. Werder, J. Chiang*, S. M. Zahurak, R. M. Fleming, B. Batlogg and L. F. Schneemeyer AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974 *Massachusetts Institute of Technology, Cambridge, MA 02139

The discovery of superconductivity near 25K in the electron-doped systems Nd2ExCeCuO4 (x=0.15) 1,2 , Nd2ExThxCuO 4 (x='0.15) 3 , and Nd 2 CuO4 _xFx (x=0.18) 4 , all of which have the T'-Nd 2 CuO4 structure, has demonstrated that high-Tc superconductivity can arise from reduction of Cu 2+ in layered cuprates as well as from its oxidation. The published preparations of these compounds all indicate that a high temperature reduction step to incorporate the dopant ions must be followed by a lower temperature "annealing" step to make good superconductor. However, reported annealing conditions vary widely and even the best ceramic samples have rather low Meissner fractions (•20%) and a negative thermal coefficient of resistivity above Tc. This contrasts with superconducting single crystals of Nd1 .85Ce0 .15 CuO 4 , which have been made with Meissner fractions over 50% and metallic conductivity behavior in the normal state 5 6 . In order to understand the role of annealing and prepare improved ceramic samples for physical studies, we have undertaken a detailed chemical and crystallographic study of Nd2 CuOn-xFx and, to a lesser extent, Nd2-xCexCuO 4 and Nd2 CuO4 +4 . The following factors were varied in order to determine their role in the synthesis of superconducting Nd 2 CuO 4 _xFx: starting materials, reaction temperatures, cooling rates, and oxygen partial pressures (pO2's). The pO2 was varied both with gas mixtures and by reacting samples in a closed cell with an yttria - stabilized zirconia (YSZ) electrode, which could apply potentiostatically controlled pO2's down to 10-9 atm. The progress of the synthesis reaction was monitored by X-ray powder diffraction, iodometric titration and thermogravimetry (to determine average Cu valence), and measurements of superconducting properties. The following procedure was found to yield ceramic Nd 2 CuO4 _xFx with Meissner fractions up to 60%: Step 1: Pre-reaction of Nd2 0 3 , NdF 3 and CuO in the correct proportions for Nd 2 CuO 3.7 F0 .3 at 1000*C in dry air. Step 2: quench.

Reduction at 890°C and 6±lx10-5 atm 02 for 14 hours followed by rapid

Step 3: Annealing in an evacuated, sealed silica tube for 2-6 hours at 650°C. This synthesis exhibits several interesting features: in step 1, the best results were obtained with NdF3 freshly prepared by precipitation, probably because its extremely small particle size improves product homogeneity. The pre-reaction step gives a mixture of Nd 2 CuO3 .9 F0 .1 , with small quantities of NdOF and CuO. More fluorine

Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society

178

IF -4

Nd2CuO4-xFx + NdOF

,+CuO 0.

N,0

Nd2Cu04-F

CY

SUPERCONDUCTING REGION

1