Structural Plasticity in the Frank‐Kasper Realm: Chemical Pressure Roles of the μ ‐ and χ‐Phase Units in the Mo−Fe−Cr System

Intermetallic phases have been known to exhibit a wide diversity since Pauling's seminal investigations into NaCd 2 in the 1920s that, along with Cd 3 Cu 4 and Mg 2 Al 3 , was shown by Samson to crystallize with a giant cubic cell containing >1000 atoms. The concept of structural plasticity – the notion that complex structures emerge from the release of internal stresses that would arise in simpler structures – has recently been used to account for one family of intermetallics, tracing the structures of Ca 2 Ag 7 , Ca 14 Cd 51 , CaPd 5 , and CaCd 6 to chemical pressure (CP) issues in the CaCu 5 type. Here, we extend the ideal of structural plasticity closer to the giant cells elucidated by Pauling and Samson through its application to a series of Mo−Fe−Cr Frank‐Kasper phases. We begin with a DFT‐CP analysis of the MgZn 2 ‐type phase MoFe 2 , which serves as a parent structure to μ‐ Mo 6 Fe 7 and χ ‐Mo 5 Cr 6 Fe 18 . The analysis reveals negative CPs around the Mo atoms arising from collisions between the Fe atoms. Tighter Mo coordination is provided in the μ ‐ or χ ‐phases by substituting some of the Friauf polyhedra of MoFe 2 with either μ ‐ and χ ‐phase units, resulting in layers or blocks of Laves‐like connectivity. Sites preferences in the μ ‐phase and the role of Cr substitution in the χ ‐phase are explained through the dual lenses of CP and electronegativity. Parallels to the features of NaCd 2 hint that such giant‐unit‐celled intermetallics can represent striking manifestations of structural plasticity.