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 Cu_3−x(As,Sb) compound. Its crystal structure was solved and refined by single crystal and powder X-ray diffraction. While the binary Cu_3−xAs and Cu_3−xSb phases crystallize in the hexagonal Cu₃P-type (hP24, P6₃cm) and cubic anti-BiF₃-type (cF16, Fm$\overline{3}$m), respectively, Cu_3−x(As,Sb) adopts the cubic Cu₆AsSb prototype (cP32, Pm$\bar{3}$n). Crystallochemical reasons lead to the exclusion that its structure could be of the UH₃-type. Cu_3−x(As,Sb) is isotypic with Cu_12−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 Cu_2.5-2.8As_0.23-0.98Sb_0.23-0.92 (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 Cu_3−xAs_0.75Sb_0.25, a ≈ 7.65 Å for Cu_3−xAs_0.25Sb_0.75 (averaged values).

Cu_3−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 Cu_3−xAs_0.75Sb_0.25 (Cu₇₂As₂₁Sb₇), congruent melting at 690 °C for Cu_3−xAs_0.50Sb_0.50 (Cu₇₂As₁₄Sb₁₄), and at 675 °C for Cu_3−xAs_0.25Sb_0.75 (Cu₇₂As₇Sb₂₁)].

Physical properties (electrical resistivity and magnetic susceptibility) indicate that Cu_3−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.