TC ENHANCEMENT VERSUS RARE-EARTH SIZE IN R0.9CA0.1BA2CU4O8 (R = SM, EU, GD, DY, HO, AND ER)

Six compounds of general formula R0.9Ca0.1Ba2Cu4O8 (R = Sm, Eu, Gd, Dy, Ho, and Er) have been synthesized. They crystallize in a structure isomorphous with YBa2Cu4O8. All these Ca-substituted 1:2:4 compounds are superconducting. The highest T(c) = 88 K was found for Er, the smallest ion. T(c) decreases systematically with larger rare earths to 72 K for the Sm compound. In all cases, the T(c) is increased by Ca substitution. The enhancement in T(c) (DELTA-T(c)) is the highest (10 K) for the Er compound and decreases as the rare-earth size increases; for Sm the enhancement in T(c) is only 3 K. The lattice constants a, b, and c and the unit-cell volume V increase with rare-earth size, whereas b/a decreases until the structure is nearly tetragonal for R = Sm. The systematic change in T(c) and in the T(c) enhancement by Ca substitution seemed to be a consequence of the increase in lattice constants with larger R ion. Larger Cu-O distances in the CuO2 sheets will reduce the charge transfer into the sheets. Thus when the rare-earth ion is large, and the Cu-O bonds are larger, the charge transfer is reduced. This explains why addition of holes by Ca doping at the 0.1 level will be less effective in raising T(c) for large rare earths.