Superconductivity in the (Nd,Ce,Ca) 2 Cuo 4-Z System
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SUPERCONDUCTIVITY IN THE (Nd,Ce,Ca) Takeshi Sakurai,
Toru Yamashita,
2
CuO 4 _z SYSTEM
Sumio Ikegawa and H.Yamauchi*
Superconductivity Research Laboratory, International Superconductivity Technology Center, 10-13 Shinonome 1-chome, Koto-ku 135, Tokyo, Japan * Leave of absence from Faculty of Engineering, of Windsor, Windsor, Ontario N9B3P4, Canada
the University
ASBTRACT We have controlled the carrier (electron) density in Nd 2 CuO 4 of the T' structure by introducing both Ce and Ca into the Nd-sites, and studied the superconducting transition temperature versus carrier density relationship. The phase relations in the (Nd 1 -- CexCa ? ) 2 CuO -z system were studied by means of powder X-ray cifract on and electron-probe micro-analysis techniques. For the phase diagram of the (NdI x Ce Ca ) CuO _ system, the single phase field of the T' phase was estxbiished. Samples of the (Ndo.9 _yCeo iCa ),CuO 4 _z system with compositions included in the single pha'se KieLd were synthesized. The cayrier density in these samples was varied by controlling the Ca content. The Hall effect and DC resistivity were measured to elucidate the relation between Tc and the carrier density. INTRODUCTION It has been known that (Nd 1 xCex) 2 CuO 4 _7 are electron doped superconducting cuprates[1]. Superconductivity in this system has been observed in a narrow compositional region,i.e. 0.075019. It was also anticipated that an optimum carrier 2+density might be obtained for 3+ •+ a certain ratio of Nd Ce Ca , since these ions have different valence numbers. EXPERIMENTAL PROCEDURE As the first step, the phase diagram of the (Ndl1 x Ce Ca )2CuO4_z system was studied in order to determine -hex CK solubility limit in the T'-phase. In the present work, both x and y were varied in the range of 0.0-0.15. Samples were synthsized by a solid state reaction method using high pure Nd20 3 , Ce0 2 ' CaCO3 and CuO powders as staring materials. Nd 2 0 and CeO powders were pre-fired at 1300K in air to get rid of absorbed water. Powders were weighed and mixed in a ball mill Calcination was using ZrO 2 balls and ethanol as solvent. Mat. Res. Soc. Symp. Proc. Vol. 169. c1990 Materials Research Society
162
The calcinated repeated twice at 1273K for 10h each in air. sample was pulverized, and the ground powder was3 pressed into under 50MPa bar-shaped pellets of the dimension of 3x3x20mm uniaxial pressure. The pellets were sintered at 1373K for 15h They were kept at the same temperaand fast-cooled to 1323K. ture for 15h in N2 gas flow in which the oxygen partical pressure was about 0.fPa, and quenched into liquid nitrogen in the same atmosphere. The oxygen contents in the (Nd,Ce) 2 CuO 4 z samles,i.e. 4-z, were always near 4, as determined using an inert gas fusion nonAccording to the previously dispersive IR oxygen-meter. data[l], numbers near 3.98 were obreported chemical titration . tained for 4-z in (Nd,Ce) 2 CuO 4 CexCax ) 2 CuO 4 single-phase fielB in the (Nd The T' XRD) ana diagram was determined by powder X-ray diffr~action The phase identification e
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