Superconductivity in the Alloy System Indium-Thallium

The superconducting transition temperatures of over 75 samples spanning the entire composition range of the indium-thallium alloy system have been measured. The highest $T_c$ measured was about 3.8°K in the bcc phase; the lowest was 2.5°K in the fcc phase. In all phases, $T_c$ decreases with increasing Tl content, the rate of decrease in the tetragonal (indium) phase being, however, much less steep than in the other phases. The Seraphim empirical rule, $\Delta T_c=k_{1}x+k_2\ln x$, is found not to describe our data in the dilute thallium range. An anomaly in the $T_c$-versus-composition plot at 4.4 at.% Tl is extremely similar in form to one previously identified in In-Cd as a Brillouin-zone effect. Transition temperature and isothermal magnetization data are used to locate the two-phase region separating the tetragonal and cubic phases; the boundaries of this region are: 30.1±0.4 and 31.7±0.3 at.% Tl at 3.3°K. The existence of such a region indicates that the tetragonal-cubic transformation is first-order, in agreement with the earlier room-temperature calorimetric work of Predel. The transition temperature of bcc Tl was estimated by extrapolation of the bcc alloy data to zero solute concentration; the result was 2.49°K.