Relativistic Effects Support Atomic Segregation in the Pseudobinary System Ca(Ag,Au)2

The pseudobinary system CaAg2–CaAu2 exhibits unusual atomic ordering associated with the distinct electronic and bonding characteristics of Au compared to Ag. These characteristics are strongly influenced by the substantially greater influence of relativity on Au. Although both CaAg2 and CaAu2 crystallize in the KHg2 type of structure, this type was not observed in the ternary system. Instead, two new ternary phases Ca3Ag5–xAu1+x (x = 0.78–2.48), adopting the hexagonal Fe2P-derivative Zr3Cu4Si2-type structure (hP9) and Ca3Ag1–xAu5+x (x ≈ 0.05), crystallizing in the orthorhombic Y3ZnAu5 type (oS72) were identified. Structure analyses reveal pronounced, composition-dependent Ag/Au ordering. Au–Au contacts are largely avoided in the Ag-rich intermediate, whereas an extended Au–Au bonding network develops in the Au-rich phase. Density functional theory calculations, complemented by chemical bonding and semiempirical analyses, confirm that such ordering is energetically favored over the tested alternative ordered arrangements. The observed atomic arrangements arise from the interplay of relativistically enhanced Au bonding characteristics and electron transfer from the formal cation Ca to the Ag/Au sublattice. These findings highlight the importance of relativistic effects on Au bonding, electron transfer from Ca, and geometric packing in governing Ag/Au ordering and structure selection.