FULLY OXYGENATED RBA(2)CU(3-X)CO(X)OY, (R=Y, EU, PR, AND 0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1) - FROM HIGH-TEMPERATURE SUPERCONDUCTORS TO HIGH-RESISTIVITY NONMETALS

Reported here are the results of measurements of the resistivity (rho) up to 300 K and of the thermoelectric power (TEP) up to 400 K of cera mic samples of the title materials. We determined also their room-temp erature lattice parameters and oxygen content as functions of Co conce ntrations. The metal-nonmetal transition in YBa2Cu3-xCoxOy and in EuBa 2Cu3-xCoxOy is marked by the onset of the deviation of the maximum abs olute TEP from a value calculated from a simple narrow-band formula. T he results indicate that the effective valency of Pr in PrBa2Cu3-xCoxO y varies from similar to 3.5 at x=0 to similar to 3 for x=0.5. For x=0 this material is close to the metal-nonmetal transition. In the nonme tallic regime the electrical transport is by activated hopping. In cer tain ranges of Co content the results are consistent with two-band hop ping conductivity with two branches, a low-temperature and a high-temp erature branch of variable-range-hopping (VRH) conductivity. In the VR H regime there is a remarkable correlation between the two resistivity parameters, rho(0) and T-0 in Mott's equation rho=rho(0) exp(T-0/T)(1 /4). The resistivity of these Co-rich compounds is very high: thus, e. g., rho(80 K) > 10(7) Omega cm in PrBa2Cu2.2Co0.8O7.15. These material s may find applications as insulating barriers in thin-film high-tempe rature superconducting devices. (C) 1996 American Institute of Physics .