In Search of Chemical Frustration in the Ca‐Cu‐Cd System: Chemical Pressure Relief in the Crystal Structures of Ca₅Cu₂Cd and Ca₂Cu₂Cd₉

Abstract

Abstract Structural complexity in intermetallic phases is increasingly being linked to the coexistence of or competition between mutually exclusive types of bonding or packing. We present an approach to inducing such competition through the choice of a ternary combination of elements with inherent incompatibilities in the bonding preferences of their binary subsystems, a conflict we term chemical frustration by analogy with magnetic phenomena. One system in which we perceived the potential for chemical frustration emerging is Ca‐Cu‐Cd. We describe our synthetic efforts in this system and the structures of two new compounds that we have obtained with the approximate compositions Ca 5 Cu 2 Cd and Ca 2 Cu 2 Cd 9 . Both compounds crystallize in ternary variants of binary structure types: the Cr 5 B 3 type for Ca 5 Cu 2 Cd and the Mg 2 Zn 11 type for Ca 2 Cu 2 Cd 9 . Despite their ternary compositions, the structural features of binary and even elemental phases can be discerned at the level of local coordination polyhedra. These observations can be rationalized by the position of the compounds in the phase diagram as a sort of phase‐segregation at a very small length scale, consistent with the presence of incompatible bonding modes. The impetus for the formation of ternary phases, as opposed to a multiphase sample with macroscopic separation between binary and elemental phases, is traced through μ 2 ‐Hückel chemical pressure analysis to the relief of electronic packing frustration in hypothetical binary compounds upon introducing a third element.