Superconductivity and Electronic Structure in the Alloy System Lead-Thallium

Abstract

The superconducting transition temperatures of Pb-Tl alloys covering the entire composition range have been determined. The $T_c\text{'}\mathrm{s}$ of the alloys are in general intermediate between those of Pb and Tl, except for samples in the concentration range 75-85 at.% Tl, for which the $T_c\text{'}\mathrm{s}$ are lower. Two other features, a continuous change of slope of $T_c$ versus composition, and a small peak structure in $T_c$, occur at about the 50 at.% composition. These features, which occur in the fcc (Pb) phase, might arise from a Brillouin-zone effect of the Merriam type. If the model for the Fermi surface in lead proposed by Anderson and Gold (based upon their de Haas-van Alphen data and using the orthogonalized plane-wave method) is employed, such an effect would be expected at the composition observed. On the other hand, an order-disorder transformation might contribute to the observed effects. A speculation regarding a connection between the electronic structure effect and a possible ordering process is made. Superconductivity behavior suggests an extension of the Pb solid solution, in a limited temperature interval, to higher Tl concentrations than those in the accepted phase diagram. The terminal solid solubility of Tl in Pb is found to be 88±2 at.% at 25 and at 295°C. The solubility of Pb in $\alpha -\mathrm{Tl}$ is 2.5±1 at.% at 25°C; in $\beta -\mathrm{Tl}$ it is 5±1 at.% at 295°C. The $T_c\text{'}\mathrm{s}$ of a few Bi-Tl samples were also measured. Structure, similar to that at ${\mathrm{Pb}}_{0.50}$ ${\mathrm{Tl}}_{0.50}$, appeared in the $T_c$ curve at ${\mathrm{Bi}}_{0.15}$ ${\mathrm{Tl}}_{0.85}$.