Crystal-field effects on the superconducting transition temperature of La${\mathrm{Sn}}_3$: Pr, La${\mathrm{Pb}}_3$: Pr, and La${\mathrm{Tl}}_3$: Pr

The crystal-field effects on the superconducting transition temperature of matrix $\mathrm{La}{X}_3$, ($X=\mathrm{S}\mathrm{n},\mathrm{P}\mathrm{b},\mathrm{T}\mathrm{l}$) containing praseodymium impurities are examined. Using the theory of Fulde, we have taken into account all the crystal-field levels of the impurity. Exchange integrals $\Gamma$ are evaluated from the depression of the superconducting transition temperature $T_c$ in ${\mathrm{La}}_{1-n}{\mathrm{Pr}}_n{X}_3$ solutions. The large value of $\Gamma$ found in ${\mathrm{La}}_{1-n}{\mathrm{Pr}}_n{\mathrm{Sn}}_3$ is in agreement with the observation of a Kondo effect in these compounds.