Intermetallic Reactivity: CaCuAl and the Role of Electronegativity in the Stabilization of Modular Structures.

The structural chemistry of intermetallic phases is generally viewed in terms of what crystal structure will be most stable for a given combination of metallic atoms. Yet, individual atoms do not always make the best reference points. As the number of elements involved in compounds increases, their structures can often appear to be assembled from structural motifs derived from simpler compounds nearby in the phase diagram rather than fundamentally new arrangements of atoms. In this Article, we explore the notion that complex multinary phases can be viewed productively in terms of motif-preserving reactions between binary compounds, as opposed to direct reactions of the component elements. We present the targeted synthesis and structure solution of CaCuAl, an intermetallic phase whose placement in the phase diagram is suggestive of a reaction between CaAl and CuAl. Single-crystal X-ray diffraction analysis reveals that this compound crystallizes in the YTaNiAl (or stuffed BaHg) structure type and is constructed from three modules: Ca@(Cu/Al) polyhedra derived from the BaAl type, Cu@Al cubes, and Al cuboctahedra. To help understand this arrangement, we identify forces driving the reactivity of one of the supposed starting materials, CaAl, through visualization of its atomic charge distribution and chemical pressure (CP) scheme, which suggest that the Al sites closest to the Ca atoms should show a high affinity for substitution by Cu atoms. Such a process on its own, however, would lead to overly long Ca-Cu distances and electron deficiency. When Cu is made available to CaAl in the Ca-Cu-Al ternary system, its incorporation in the Ca coordination environments instead nucleates domains of a fluorite-like CuAl phase, which act as nodes in the primitive cubic framework of CaAl- and fluorite-like units. The cubic holes created by this framework are occupied by Al face-centered-cubic fragments that donate electrons while also resolving negative CPs in the Ca environments. This structural chemistry illustrates how new elements added to a binary compound at sites with conflicting electronic and atomic size preferences can serve as anchor points for the growth of domains of different bonding types, a notion that can be applied as a more general design strategy for new intermetallic intergrowth structures.