A Conceptual Modelling of the Tetragonality of L10 (AuCu-I type) Intermetallics

The systematics of tetragonality in L1 (AuCu-I) type intermetallics is analysed by means of a structural mapping procedure. It is found that the Miedema parameters (metallic electronegativity and electron density at the Wigner-Seitz cell surface) order the tetragonality systematics into two broad classes, exhibited respectively by transition metal compounds and by transition – nontransition type mixed compounds. In the former category, the axial ratio ( revealed a decrease with an increase in electronegativity difference, while the reverse trend is observed for the mixed type intermetallics. The effect of ternary addition and its preferential occupation of one of the sublattices of L1 structure have been found to influence strongly the tetragonality. The observed systematics has been explained in terms of a synthesis of crystal physical and chemical viewpoints on structure and bonding in intermetallics. In particular, it is hypothesised that an antiferromagnetic like coupling between interplanar and intraplanar interactions is responsible for the relative stability of L1 over B2 (CsCl) crystal structure. It is argued that a dominantly attractive (stabilising) interlayer interaction serves to compress the lattice along the [001] axis, which is promoted by a charge transfer taking place between the constituent elements segregating to individual sublattices. On the other hand, the presence of a net repulsive interaction in the {002} planes is what is responsible for increasing the tetragonality of the lattice and pushing the structure away from a B2 packing.