90 K Superconductivity in R 2 Ba 4 Cu 7 O 15 , and Rba 2 Cu 4 O 8 , R=Y, Er
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90 K SUPERCONDUCTIVITY IN R2 BaCu7 O11 AND RBasCu,Os, R=Y, Er D M POOKE, J L TALLON AND R G BUCKLEY PHYSICS AND ENGINEERING LABORATORY,
DSIR, P 0
BOX 31313,
LOWER HUTT, NEW ZEALAND
The compounds YBa2Cu,O, ("124") and Y2BaCu,O 7 1 ("247") were first observed as impurity phases in YBa 2Cu30 74 ("123") and subsequently have been produced in bulk form at high oxygen pressures [1,2]. The 124 compound has a structure similar to 123 except that the Cu0 14 chain is replaced by a Cu 202 double layer, forming ribbons extending in the b-direction. The ordered intergrowth compound 247 comprises alternating units of 123 and 124. It is now understood that 124 can be synthesized in oxygen at atmospheric pressure [3,4]. We report here that single-phase, bulk 247 can likewise be produced in 1 bar of oxygen and further, that it has a variable oxygen stoichiometry with a zero-resistance T. reaching 92 K on full oxygen loading [5]. Samples of R2BaCu 701. (R=Y, Er) were prepared by mixing stoichiometric quantities of Y20, Ba(NO3 )2 and submicron CuO powder with 0.2 mole fraction of NaNO, and pre-reacting at 700-750'C to decompose the nitrates. The material was then ground and die-pressed into pellets, and reacted/sintered in 0 flowing oxygen at 860-870 C, preferably with intermediate grinding. The alkali enhances the reaction rate and fully evaporates from the material by completion of the reaction. The pellets are then oxygen loaded by slow cooling in oxygen to 350°C; extended time near 820°C is avoided because of the possibility of conversion to 124. As shown in Fig. 1 the resultant material is very close to single phase as determined by XRD, with no indication of 123 impurity. Electrical and physical properties of the material were investigated as a function of oxygen stoichiometry by annealing in a variety of oxygen partial pressures and temperatures followed by rapid quenching into liquid N2 , as described previously [6]. Values of (1-8) were inferred from the mass changes thus obtained, and are shown in Fig. 2. Though absolute measurements of 8 were not carried out, we found slow cooling in oxygen to 350°C then annealing at 600TC in vacuo produced a mass change corresponding to a change in 5 of 0.99. This implies that the slow-cooled material was fully oxygen loaded. Further, the quenching procedure with 123 yields data consistent with the equilibrium curves of Lindemer et al [7] (shown as solid lines in Fig. 2) and, importantly, the resistive and diamagnetic transition temperatures cycle reproducibly with temperature and partial pressure. We note that, allowing for the factor of 2 difference in the definition of 6 for 123 and 247, the data for the two materials coincide. This suggests that the 123 units in 247 are the only ones to contribute to the oxygen unloading. Also shown in Fig. 2 is the resistivity ratio R(8=0)/R(5). AC susceptibility was measured at various values of oxygen stoichiometry, and these are shown in Fig. 3. The amplitude of the diamagnetic signal equals the best of our 123 samples, and remains unchanged f
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