Ni3Sn4 and FeAl2 as vacancy variants of the W-type ("bcc") structure

Systematization of the vast number of known crystal structures of intermetallic phases is a challenge. One previously proposed group is referred to here as vacancy variants of the W-type structure. Members of this group, may, however, not be easily recognized because of the structural irregularity introduced by the vacancies. Descriptions of the experimentally observed crystal structures of Ni3Sn4 and FeAl2 in terms of vacancy variants of the W-type structure are, respectively, derived by establishing a lattice correspondence with the W-type structure, allowing, in particular, identification of the vacant sites. In both cases only small deviatoric strains are required to obtain the experimentally encountered lattice parameters, and generally small atomic displacements occur from the ideal positions, thus demonstrating significance of the lattice correspondence. The lattice correspondences allow, for both Ni3Sn4 and FeAl2, relating reported microstructure evidence (directions/planes occurring in orientation relationships and crystal habits but also on twinning and slip) with such typical for metals and solid solutions with W-type ("bcc") structures. This demonstrates that the established lattice correspondences have a significance going beyond a descriptive one, but the underlying W-type structures reveal themselves in the materials' behavior.