Chemical control of high-Tc superconductivity of the triple-fluorite-layer copper oxide (Cu,Mo)Sr2(Ce,R)3Cu2O11+δ(R=Y,La-Yb) -: art. no. 012501

For the recently synthesized (Cu0.75Mo0.25)Sr-2(Ce0.67Y0.33)(3)Cu2O11+delta or (Cu,Mo)-1232 high-T-c superconductor, the (Ce,Y) site in the triple-fluorite-structured (Ce0.67Y0.33)-O-2-(Ce0.67Y0.33)-O-2-(Ce0.67Y0.33) block between two adjacent CuO2 planes is found flexible enough for rare earths (R) from La to Yb to fully substitute for Y. The superconductivity transition temperature, T-c, monotonically increases with decreasing size of the R constituent, from 26 K for La to 53 K for Y. At the same time the average valence of copper as evaluated from Cu L-edge x-ray absorption near-edge structure spectra is found to remain constant at 2.28 (within +/- 0.01). Since the samples (especially with large Rs) are underdoped, we believe that the smaller-for-larger R-cation substitution shifts holes from the (Cu,Mo)O1+delta charge reservoir into the CuO2 planes to increase T-c. With R=Tb and Pr, the (Cu,Mo)-1232 structure forms but the samples can not be made superconductive. The reason is found at the lower Cu valence value, i.e., 2.18 for R=Tb. Moreover, the lattice parameters, a and c, for both the R=Tb and Pr samples are significantly shorter than those expected through interpolation of data for the samples with other R constituents. Hence, we suggest that both Tb and Pr in (Cu,Mo)-1232 possess a valence state higher than III.