Chemical control of high-Tc superconductivity of the triple-fluorite-layer copper oxide (Cu,Mo)Sr2(Ce,R)3Cu2O11+δ(R=Y,La-Yb)

For the recently synthesized (Cu0.75Mo0.25)Sr2(Ce0.67Y0.33)3Cu2O11+δ or (Cu,Mo)-1232 high-Tc superconductor, the (Ce,Y) site in the triple-fluorite-structured (Ce0.67Y0.33)-O2-(Ce0.67Y0.33)-O2-(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, Tc, monotonically increases with decreasing size of the R constituent, from 26K for La to 53K 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+δ charge reservoir into the CuO2 planes to increase Tc. 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.