Tc enhancement versus rare-earth size in R0.9Ca0.1Ba2Cu4O8 (R=Sm, Eu, Gd, Dy, Ho, and Er)

Abstract

Six compounds of general formula ${\mathit{R}}_{0.9}$ ${\mathrm{Ca}}_{0.1}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ (R=Sm, Eu, Gd, Dy, Ho, and Er) have been synthesized. They crystallize in a structure isomorphous with ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$. All these Ca-substituted 1:2:4 compounds are superconducting. The highest ${\mathit{T}}_{\mathit{c}}$=88 K was found for Er, the smallest ion. ${\mathit{T}}_{\mathit{c}}$ decreases systematically with larger rare earths to 72 K for the Sm compound. In all cases, the ${\mathit{T}}_{\mathit{c}}$ is increased by Ca substitution. The enhancement in ${\mathit{T}}_{\mathit{c}}$ (Δ${\mathit{T}}_{\mathit{c}}$) is the highest (10 K) for the Er compound and decreases as the rare-earth size increases; for Sm the enhancement in ${\mathit{T}}_{\mathit{c}}$ 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 ${\mathit{T}}_{\mathit{c}}$ and in the ${\mathit{T}}_{\mathit{c}}$ 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 ${\mathrm{CuO}}_2$ 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 ${\mathit{T}}_{\mathit{c}}$ for large rare earths.