EFFECTS OF SUBSTITUTING CALCIUM FOR YTTRIUM ON THE PROPERTIES OF YBA2CU3O7-DELTA

We report measurements of the room-temperature lattice parameters, of the resistivity (rho) for temperatures up to 300 K, and of the absolute thermoelectric power up to 400 K, of single-phase sintered Y1-yCayBa2Cu3O7-delta, for y=0-0.3 and for a range of values of delta. For pure YBa2Cu3O7-delta we confirm earlier reports that an optimal delta exists, for which T(c) is maximal (T(cm)), and that the thermoelectric power changes sign in the vicinity of this concentration. Similar behavior, though with a lower T(cm) is observed in samples with y=0.1. For y=0.2 and 0.3, the maximum of T(c)(delta) flattens off into a plateau. The enhancement of the magnitude of the negative thermoelectric power by Ca doping is much less than expected. This is probably due to the fact that most of the extra charge introduced by the Ca is screened, mainly by the cointroduction of oxygen vacancies.