Thermal stability of Pb-alloy Josephson junction electrode materials. VII. Concentration range of single ε-phase Pb-Bi films used in counterelectrodes

From the viewpoint of mechanical stability upon repeated thermal cycling between 298 and 4.2 K, Pb-Bi films with single ε-phase have been found to be desirable as the counterelectrode material of Pb-alloy Josephson junction devices. The Bi concentration range of the ε-phase was studied by using x-ray diffraction and electron microprobe techniques for Pb-Bi films with various Bi contents prepared by evaporation from an alloy source. The film composition was determined by controlled-potential coulometry. From the lattice constants and phase identification experiments, the Bi content range of the single ε-phase was determined to be 27.5∼31.8 wt. % (±0.3 wt. %) at room temperature. The lower limit of the Bi content of the ε-phase agrees reasonably well with those reported previously, but the upper limit of the Bi content is ∼1 wt. % higher than that of Preece and King, and ∼2 wt. % lower than that of Predel and Schwermann. Although Predel and Schwermann indicated that there would be a eutectoid at 27.5 wt. % Bi and 227 K, no evidence of phase separation of the ε-phase was obtained upon cooling as low as 4.2 K, or upon heating to 350 K. It was found by electron microprobe analysis that Bi-rich regions (β-phase) as large as 100 μm in size segregated in the Pb-Bi film surface when the Bi concentration is within the ε+β phase region.