Electronic transport properties of PrBa2Cu3-xGaxO7-/spl delta/

We have measured the electrical resistivity, /spl rho/, and the thermopower, S, of PrBa/sub 2/Cu/sub 3-x/Ga/sub x/O/sub 7-/spl delta// in a temperature range from 300 K down to approximately 100 K. Samples with varying amounts of Ga substitution (x=0.0, 0.2, 0.4, 0.6, and 0.8) were studied, some of which were annealed in flowing O/sub 2/ for one week at 700/spl deg/C. At room temperature the resistivity of the material increases by several orders of magnitude as Ga is substituted for the Cu. The thermopower increases from 100 /spl mu/V/K for x=0.0 to approximately 300 /spl mu/V/K when x=0.2. The value does not continue to increase as more Ga is substituted. The resistivity increases as the temperature is lowered, while the thermopower exhibits a complicated temperature dependence. Annealing the material in O/sub 2/ decreases the resistivity, but does not change the room temperature value of the thermopower. Although the resistivity may follow a 3D variable range hopping expression as a function of temperature, the thermopower does not. The thermopower may show indications of magnetic transitions. The power factor (=S/sup 2///spl rho/) at room temperature is greatest for x=0.0 with a value of 10/sup -1/ /spl mu/W/(K/sup 2/-cm), it decreases over several orders of magnitude as Ga is substituted. It also decreases as the temperature is lowered. We conclude that the system PrBa/sub 2/Cu/sub 3-x/Ca/sub x/O/sub 7-/spl delta// is probably not going to be useful as a thermoelectric material, even with other substitutions such as Y for Pr.