Superconductivity in the Alloy System Indium-Thallium

Abstract

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\ifmmode^\circ\else\textdegree\fi{}K in the bcc phase; the lowest was 2.5\ifmmode^\circ\else\textdegree\fi{}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, $\ensuremath{\Delta}{T}_{c}={k}_{1}x+{k}_{2}\mathrm{ln}x$, 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\ifmmode\pm\else\textpm\fi{}0.4 and 31.7\ifmmode\pm\else\textpm\fi{}0.3 at.% Tl at 3.3\ifmmode^\circ\else\textdegree\fi{}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\ifmmode^\circ\else\textdegree\fi{}K.