The crystal structure of (Nb0.75Cu0.25)Sn2 in the Cu-Nb-Sn system

Abstract During the processing of superconducting Nb 3 Sn wire, several intermediate intermetallic phases including a previously encountered Cu-Nb-Sn phase show up. The yet unknown crystal structure of this phase is now identified by a combination of different experimental techniques and database search to be of the hexagonal NiMg 2 type with a proposed composition of about (Nb 0.75 Cu 0.25 )Sn 2 . The structure determination started from an evaluation of the lattice parameters from EBSD Kikuchi patterns from quenched material suggesting hexagonal or orthorhombic symmetry. A database search then led to the hexagonal NiMg 2 type structure, the presence of which was confirmed by a Rietveld analysis on the basis of high energy synchrotron X-ray powder diffraction data. Assuming a partial substitution of Nb in orthorhombic NbSn 2 by Cu, the change of the valence electron concentration provokes a structural transformation from the CuMg 2 type for NbSn 2 to the NiMg 2 type for (Nb 0.75 Cu 0.25 )Sn 2 . In the previous literature the (Nb 0.75 Cu 0.25 )Sn 2 phase described here has occasionally been referred to as Nausite.