Oxygen Deficiency, Stacking Faults and Calcium Substitution in MOD YBCO Coated Conductors

The variation in transition temperature T_c with oxygen deficiency or calcium substitution in Y_1-xCa_xBa₂Cu₃O_7-δ (YBCO) is a well-known manifestation of the generic hole-doping phase diagram governing the superconducting cuprates. Less well understood is the role that microstructural defects can play in determining hole doping. We have investigated the formation of Y124-type stacking faults in metal-organic deposited (MOD) YBCO coated conductors and shown that these defects also act to reduce the hole concentration in YBCO. With low stacking-fault density, T_c can be as low as 89 K when fully oxygen loaded and can increase to a maximum of 94 K when partially unloaded. The presence of stacking faults limits the degree to which fully oxygen loaded YBCO can be overdoped. The critical current density J_c is optimized by full oxygen loading but with a moderate density of stacking faults. We have demonstrated substitution of calcium for yttrium in YBCO by modification of the MOD precursor resulting in an increase in hole concentration and consequent decrease in T_c. J_c is initially depressed by calcium substitution but partially recovers upon incorporation of a moderate density of stacking faults.