It is shown that the superconducting-transition temperatures for all R${\mathrm{Rh}}_4$${\mathrm{B}}_4$ compounds, where R is a rare-earth (RE) element, can be combined and analyzed in a systematic way. The analysis utilizes an Abrikosov-Gorkov–type expression for pair breaking, with modifications for crystalline electric fields and changes of $T_c$ due to nonmagnetic effects, to describe $T_c$ values in concentrated systems, dilute alloys, and pseudoternary mixtures. As a result, it is seen that the exchange coupling parameter, N(0)${\mathrm{scrJ}}^2$, is larger for light RE than for heavy RE, with the increase being very large near the beginning of the rare-earth series. In view of a very similar effect in the R${\mathrm{Al}}_2$ system, it is suggested such a variation is common to RE systems.

• Received 10 January 1986

DOI:https://doi.org/10.1103/PhysRevB.34.1496