SUPPRESSION OF SUPERCONDUCTIVITY BY SUBSTITUTION OF TH FOR CE IN (ND23CE1/3)2(BA2/3ND1/3)2CU3O9-D/

Single-phase compounds (space group 14/mmm) are formed in the entire c oncentration range (0.0 less-than-or-equal-to x less-than-or-equal-to 1.0) when Th is substituted in place of Ce in the Nd-223 superconducto r, i.e. [Nd2/3(Ce1-xThx)1/3]2(Ba2/3Nd1/3)2Cu3O9-d. DC and AC magnetiza tion measurements showed that the superconducting temperature T(c) dec reases from 30 K for the parent compound to 14.5 K for the compound wi th x = 0.3. The other compounds with x greater-than-or-equal-to 0.4 di d not show any sign of superconductivity down to 2 K. This is contrary to what is observed in the Nd2-xCexCuO4 compound where the complete r eplacement of Ce by Th lowers T(c) only marginally. The findings are e xplained on the basis of the difference between the size of host and t he substituent ions and on the structural characteristics of Nd-223.