The superstructure of domain-twinned η'-Cu6Sn5

The cell of the low-temperature modification of Cu 6 Sn 5 , the η-phase, has been determined by means of electron diffraction of single domains. The basic hexagonal NiAs (B8) type cell is pentupled, reflected in the reciprocal lattice by a fivefold superstructure running along [1121]. The extensive domain-twinning macroscopically gives rise to perfect hexagonal symmetry, which explains the previously proposed hexagonal cell [a=5a NiAs , c=5c NiAs ; Bernal (1928), Nature, 122, 54]. The structure was solved by occupying every fifth trigonal bipyramidal site of the NiAs-type structure and shifting the atoms surrounding these additional Cu atoms to form Edshammar 11 polyhedra with Cu-Sn and Cu-Cu distances in the range 2.60-2.74 A