The 2,240-Atom Unit Cell of PrMg1.6Zn5.4: An Intergrowth of the Laves and Heusler Structures Illustrating a Mechanism for the Stabilization of Complex Intermetallics.

The crystal structures of intermetallic phases encompass a fundamental mystery about the chemistry of metals: what drives atoms that prefer simple sphere packings as pure elements to adopt complex structures, sometimes with unit cells containing thousands of atoms? We recently developed the interface nucleus concept to interpret their most complicated arrangements in terms of chemical pressure (CP)-driven assemblies of modules of simple parent structures. The predictions of this approach earlier led us to the Pr-Mg-Zn system and the discovery of the modular structure of PrMg4Zn10 built from MgZn2- and EuMg5-type units. Herein, we present the structure of a second intergrowth compound in this system that exhibits far greater complexity: PrMg1.6Zn5.4, whose 34.50 Å cubic unit cell contains 2,240 atoms. Its structure can be simply decoded in terms of a 5 × 5 × 5 supercell of the Heusler structure (or alternatively a 10 × 10 × 10 supercell of the body-centered cubic (BCC), structure) embedded with Laves phase fragments. The nearly seamless transition between the Heusler and Laves domains observed here reflects a simple relationship between these structures, in which they can be interconverted by the exchange of a tetrahedron of atoms for a single larger atom (MgCu2 type to BCC), or vice versa (BCC to MgCu2 type). Driving forces underlying PrMg1.6Zn5.4's structure are inferred from the CP analysis of the parent structures, which highlight the overcompression of the Pr atoms in a model Heusler phase and the opportunities for relief by expanded environments available at the boundaries between the Laves and Heusler domains in PrMg1.6Zn5.4.