THE SUPERSTRUCTURE OF DOMAIN-TWINNED ETA'-CU6SN5

The cell of the low-temperature modification of Cu6Sn5, the eta-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 [<11(2)over bar 1>]*. 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 Angstrom. The supercell reflections from the different domains of the twinned crystals are not superimposed, which facilitated the crystal structure refinement [space group C2/c; a = 11.022 (5), b = 7.282 (4), c = 9.827 (2) Angstrom, beta = 98.84 (4)degrees and Z = 4], by means of single-crystal X-ray diffraction on a domain-twinned specimen. The Edshammar(11) polyhedra form corner-connected zigzag chains along [001]. The structure of eta'-Cu6Sn5 is hence a new superstructure-type belonging to the NiAs-Ni2In structure group. The projections of this structure along the axes of the NiAs-type base cell are void of superstructure.