Synthesis and structural characterization of orthorhombic Cu3–δ Sb (δ ≈ 0.1) and hexagonal Cu3Sb1–xInx (x ≈ 0.2) phases

Abstract

Abstract Cu3Sb is a known copper-rich phase in the Cu–Sb binary phase diagram. It is reported to be dimorphic, with a low-temperature form adopting the orthorhombic Cu3Ti structure type (space group Pmmn, No. 59). The high-temperature form crystallizes in the cubic space group F m 3 ‾ m $Fm‾{3}m$ (No. 225), and is isostructural with BiF3. Neither polymorph has been carefully characterized to date, with both structures being assigned to the respective structure type, but never refined. With this study, we provide structural evidence, based on single-crystal and powder X-ray diffraction data that the low-temperature orthorhombic phase exists with a significant amount of defects on one of the Cu-sites. As a result, its composition is not Cu3Sb, but rather Cu3–δ Sb (δ = 0.13(1)). The cubic form could not be accessed as a part of this study, but another Cu-rich phase, Cu3Sb≈0.8In≈0.2, was also identified. It adopts the hexagonal Ni3Sn structure type (space group P63/mmc, No. 194) and represents an In-substituted variant of a hitherto unknown structural modification of Cu3Sb. Whether the latter can exist as a binary phase, or what is the minimum amount of In inclusions needed to stabilize it remains to be determined. Measurements of the thermopower of Cu3–δ Sb (δ = 0.13(1)) were conducted in the range of 300–600 K and demonstrated a maximum value of ca. 50 μV/K at 600 K, indicative of a p-type transport mechanism. Electrical resistivity measurements for the same sample confirmed that it exhibits metallic-like behavior, with a room temperature value of 0.43 mΩ cm. Electronic structure calculations show the absence of a band gap. Thermal analysis was utilized to ascertain the congruent melting of both phases.