New phase formation and superconductivity in reactively diffused Nb3Sn multilayer films

Thin films of A15 Nb3Sn have been prepared by reactive diffusion of sputter-deposited Nb/Sn multilayers. The layer thicknesses were arranged in such a way as to produce Nb3Sn and desired off-stoichiometry samples. The reaction was studied by means of in situ temperature-dependent X-ray analysis in the range 50°C-950°C. It was observed that, prior to the formation of the Al5 phase, a very rapid reaction takes place between the Nb sublayers and molten Sn. In this reaction a new phase was observed which could be interpreted as a hexagonal Laves phase NbSn2. This phase transforms into orthorhombic NbSn2which then reacts with the remaining Nb to form Al5 Nb3Sn. The fact that the reaction occurs at the Nb/liquid Sn interface with the formation of the most Sn-rich phase indicates that the reaction takes place by diffusion of Nb into liquid Sn. The highest superconducting transition temperature was found to be 17.45°K. The values of the Al5 cell parameters indicated that the diffusion reaction of both on-and off - stoichiometry Nb3Sn multilayers is driven towards the Sn-rich side of Nb3Sn, with a maximum composition range of 24-28 at% Sn.