Electronic Packing Frustration in Complex Intermetallic Structures: The Role of Chemical Pressure in Ca2Ag7

The assignment of distinct roles to electronics and sterics has a long history in our rationalization of chemical phenomena. Exploratory synthesis in the field of intermetallic compounds challenges this dichotomy with a growing list of phases whose structural chemistry points to an interplay between atomic size effects and orbital interactions. In this paper, we begin with a simple model for how this interdependence may arise in the dense atomic packing of intermetallics: correlations between interatomic distances lead to the inability of a phase to optimize bonds without simultaneously shortening electronically under-supported contacts, a conflict we term electronic packing frustration (EPF). An anticipated consequence of this frustration is the emergence of chemical pressures (CPs) acting on the affected atoms. We develop a theoretical method based on DFT-calibrated μ2-Hückel calculations for probing these CP effects. Applying this method to the Ca2Ag7 structure, a variant of the CaCu5 type with defect planes, reveals its formation is EPF-driven. The defect planes resolve severe CPs surrounding the Ca atoms in a hypothetical CaCu5-type CaAg5 phase. CP analysis also points to a rationale for these results in terms of a CP analogue of the pressure-distance paradox and predicts that the impetus for defect plane insertion is tunable via variations in the electron count.