Phase Stability and Competing Crystal Structures in the Formation of the Intermetallic Compounds Cu5As2 and Cu5(As,Sb)2

An experimental investigation of the Cu-As-Sb ternary system in the Cu-rich region led to the identification of a new intermetallic phase, Cu5(As,Sb)2. The compound crystallizes in the orthorhombic Mg5Ga2-type structure (oI28, Ibam), analogous to the binary parent phase Cu5As2, with lattice parameters a = 5.968–5.977(1) Å, b = 11.550–11.565(3) Å, c = 5.530–5.573(3) Å. Similar to the parent Cu5As2 phase, the ternary compound forms with slight Cu under stoichiometry and exhibits a limited compositional range, with no continuous solid solubility between the binary and ternary phases. The phase formation, compositional stability, and decomposition behavior were systematically studied using a combination of powder and single-crystal X-ray diffraction (XRD, including Rietveld refinement), metallographic analysis with optical and scanning electron microscopy with energy-dispersive X-ray spectroscopy (LOM, SEM-EDXS), electron backscatter diffraction (EBSD) and thermal analysis (DTA, DSC). The results reveal that Cu5(As,Sb)2 is a high-temperature phase forming peritectically at 650–635 °C and stable only within a limited temperature interval. No continuous solid solubility exists between the ternary compound and the parent binary phase Cu5As2. Its formation occurs in strong competition with that of two other close neighboring solid-solution compounds, [Cu3−x(As1−ySby) (Cu3P-type; hP24, P63cm) and Cu3−x(As,Sb) (Cu9TeSb2-type; cP32, Pm−3n)], reflecting a complex interplay between composition, solubility ranges and thermal history. No evidence for the existence of high-temperature (HT) and low-temperature (LT) polymorphic phases was found for either the binary compound Cu5As2 or the ternary compound Cu5(As,Sb)2. Electrical resistivity measurements on a quenched sample indicate metallic behavior. These findings provide new insight into phase stability and structure–property relationships in Cu-As-Sb alloys and contribute to the understanding of competing intermetallic phases in this system.