Superconducting transition temperatures of chemically vapour-deposited tungsten-rhenium alloys

Abstract

Superconducting transition temperatures (T _c) have been measured on a series of chemically vapour-deposited (CVD) tungsten-rhenium alloys. The samples were prepared by hydrogen reduction of tungsten and rhenium hexafluoride in the composition range 3·5 to 95 at.-% Re. The maximum T _c found was 11·4 K at 42 at.-% Re which is considerably higher than the ∼5 K reported for arcmelted W-Re in this composition range. The transition temperatures of asdeposited samples were higher than those of comparable arc-melted alloys for the entire composition range regardless of crystal structure. Two reasons suggested for the enhancement of T _c are: (1) the formation of a Cr₃Si-type A15 structure, and (2) the presence of impurities trapped in the lattice. After high-temperature annealing the T _c was decreased to that of arc-melted samples. This is attributed to the decomposition of metastable phases to the equilibrium structure and/or the precipitation of gas or gaseous compounds in the form of bubbles. Long-range order parameters of samples with the A15 phase were low and showed the unusual feature of exhibiting increasing order with increasing departure from stoichiometry.