THERMOPOWER OF OVERDOPED BA-K-BI-O AND NI-SUBSTITUTED 214-CUPRATES

We have measured the thermoelectric power and resistivity of two diffe rent superconductor series as the doping level is increased into the o verdoped regime where T-c decreases below its maximum value: Ba1-xKxBi O3 single crystals with K content up to x=0.55, and La2-xSrxCu1-yNiyO4 -delta ceramics with Sr content up to x= 0.57. For overdoped samples i n both series, thermopowers that are small and negative are found. In overdoped Ba-K-Bi-O, the thermopower is wholly negative from T-c up to the limit of measurement near room temperature, and the temperature c oefficient of thermopower is also negative. All the thermopower versus temperature plots for Ba-K-Bi-O show a characteristic bend around 150 K, with a smaller temperature coefficient above 150 K. As Ni is subst ituted for Cu in the 214 cuprate series, there is a substantial increa se in the thermopower for Sr concentration x=0.15, but no such increas e is seen in the overdoped samples x similar to 0.5.