ABSTRACT: The formation and crystal structure of the binary Cu3As phase have been re-investigated.
Some physical properties were then measured on both single crystal and polycrystalline bulk. Cu3As
melts congruently at 835 C. At room temperature (RT), this compound has been found to crystallize
in the hexagonal Cu3P prototype (hP24, P63cm) with lattice parameters: a = 7.1393(1) Å and
c = 7.3113(1) Å, rather than in the anti HoH3-type (hP24, P–3c1) as indicated in literature. A small
compositional range of 74.0–75.5 at.% Cu (26.0–24.5 at.% As) was found for samples synthesized
at 300 and 400 C; a corresponding slight understoichiometry is found in one out of the four Cu
atomic sites, leading to the final refined composition Cu2.882(1)As. The present results disprove a
change in the crystal structure above RT actually reported in the phase diagram (from Gamma’ to Gamma, on heating). Instead, below RT, at T = 243 K (-30 C), a first-order structural transition to a trigonal
low-temperature superstructure, LT-Cu3-xAs (hP72, P–3c1) has been found. The LT polymorph is
metrically related to the RT one, having the c lattice parameter three times larger: a = 7.110(2) Å and
c = 21.879(4) Å. Both the high- and low-temperature polymorphs are characterized by the presence
of a tridimensional (3D) uncommon and rigid Cu sublattice of the lonsdaleite type (Cu atoms tetrahedrally
bonded), which remains almost unaffected by the structural change(s), and characteristic
layers of triangular ‘Cu3As’-units (each hosting one As atom at the center, interconnected each other
by sharing the three vertices). The first-order transition is then followed by an additional structural
change when lowering the temperature, which induces doubling of also the lattice parameter a.
Differential scanning calorimetry nicely detects the first low-temperature structural change occurring
at T = 243 K, with an associated enthalpy difference, DH(TR), of approximately 2 J/g (0.53 kJ/mol).
Low-temperature electrical resistivity shows a typical metallic behavior; clear anomalies are detected
in correspondence to the solid-state transformations. The Seebeck coefficient, measured as a function
of temperature, highlights a conduction of n-type. The temperature dependence of the magnetic
susceptibility displays an overall constant diamagnetic response
