Crystallochemistry, thermodynamic and physical properties of the intermetallic compound Cu3−x(As,Sb)

During the investigation of the Cu-As-Sb ternary system, we identified the ternary intermetallic Cu3− x(As,Sb)compound. Its crystal structure was solved and refined by single crystal and powder X-ray diffraction. While the binary Cu3− xAs and Cu3− xSb phases crystallize in the hexagonal Cu3P-type (hP24, P63cm) and cubic anti-BiF3-type (cF16, Fm3m), respectively, Cu3− x(As,Sb) adopts the cubic Cu6AsSb prototype (cP32, Pm3n). Crystallochemical reasons lead to the exclusion that its structure could be of the UH3-type. Cu3− x(As,Sb) is isotypic with Cu12− xTeSb3; their crystal structure is a stuffed ternary derivative of the Cr3Si-type. SEM-EDX analyses reveal very large compositional ranges for this compound, mostly concerning the Sb/As ratio: 71.1− 73.9 at.% Cu, 4.0− 24.5 at.% As and 2.5− 23.5 at.% Sb, corresponding to about Cu2.5-2.8As0.23-0.98Sb0.23-0.92 (x = 0.2− 0.5).The lattice parameter and, consequently, the unit cell volume regularly expand while increasing the Sb/As compositional ratio: a ≈ 7.47 Å for Cu3− xAs0.75Sb0.25, a ≈ 7.65 Å for Cu3− xAs0.25Sb0.75 (averaged values).Cu3− x(As,Sb) forms either by a peritectic reaction for As-rich compositions, or congruently for the equiatomic Sb/As and Sb-rich compositions. The decomposition- or melting-temperature values decrease as a function of the Sb/As compositional ratio [e.g. peritectic at 710 ◦C for Cu3− xAs0.75Sb0.25 (Cu72As21Sb7), congruent melting at 690 ◦C for Cu3− xAs0.50Sb0.50 (Cu72As14Sb14), and at 675 ◦C for Cu3−xAs0.25Sb0.75 (Cu72As7Sb21)].Physical properties (electrical resistivity and magnetic susceptibility) indicate that Cu3− x(As,Sb) behaves as a good metal with electrical resistivity decreasing as the Sb/As compositional ratio increases; a peculiar anomaly in the electrical resistivity behavior (heavy-fermions like) was observed at low temperature, the origin of which needs further investigation. The compound is a standard diamagnet.