Calcium substituted Y-Ba-Cu-O superconductors with enhanced T c
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Calcium substituted Y-Ba-Cu-O superconductors with enhanced Tc D. E. Morris, P. Narwankar, A. P. B. Sinha, K. Takano, and V. T. Shum 2-300 Lawrence Berkeley Laboratory, Berkeley CA 94920 Abstract The Tc of 124 is enhanced by Ca substitution: CaxY(l-x)Ba2Cu4Oy prepared at 200 bar at 930'*C shows Tc = 89 K for x = 0.1. The 124 phase remains stable with Ca substitution at P(0 2 ) >_ 50 bar. For P(0 2) = 2 - 16 bar, Ca substitution stabilizes a terragonal 123 phase CaxY(l-x) Ba2Cu 3 Oy, which shows a Tc (87 K for x = 0.2) comparable to that of unsubstituted orthorhombic 123. The synthesis of CaT-123 has been achieved in oxygen overpressure as small as 1 bar. 1. Introduction: Ca was substituted in Y-Ba-Cu-O superconductors prepared at elevated oxygen pressures (2 - 200 bars). Two considerations motivated this attempt at non-isovalent 3+ 2 substitution of Ca + in place of y : (1) partial substitution of Ca for Y in 124 to give CaxY(l1x)Ba2Cu4Oy would increase the hole concentration from 1 to (1+x) per formula unit if the oxygen content y remained at eight as in the parent 124 compound. By this means one could attempt to optimize the hole concentration to achieve the highest Tc. In this connection, it should be noted that compared to the chain oxygen in 123, the double chain oxygen in 124 is more tightly bound and so it is expected to maintain nearly the constant value of eight under varying oxidation conditions. (2) When a 2+ ion replaces a 3+ ion in the compound, the oxygen content would be smaller at the same formal average copper valency (FACV). The oxygen which must be added to reach eight per formula unit will be greater, and the binding energy of the added oxygen should decrease, thus one may need a higher oxygen pressure to form the 124 structure in preference to 123. In other words, on Ca substitution, one may expect to see a shift in the 123-247-124 phase stability boundaries to higher P(O2) in the P-T-Composition diagram. 2. Sample preparation: Several samples were prepared as described in [1], [2], [31 and [4]. The stafting composition was Y:Ca:Ba:Cu = (l-x) : x : 2 : 3.5 and the temperature of reaction was 930" C. After synthesis in a commercial high pressure oxygen furnace [5] at oxygen pressures of 2, 4, 8, 16, 50, 100 and 200 bar, the samples were cooled in the furnace at the rate of -5 C/m while the oxygen pressure was maintained. No additional annealing was carried out. The phases present were identified by x-ray diffraction (XRD). Some samples were multiphase: the phase fractions were estimated as described in [3] and [4]. 3. Superconducting transition temperatures: The Meissner diamagnetism (flux expulsion) of all samples was measured on a Quantum Design SQUID magnetometer by the method described in [1], [2], [31 & [4]. The Tc values are marked on Fig. I in the appropriate locations in the phase diagram. All measured samples in the 123 phase field were superconducting, with transition temperatures between 75 -86 K. For samples of Ca substituted 124 prepared at 50, 100 and 200 bar, Tc increased to a max
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