Cu3As: Uncommon Crystallographic Features, Low-Temperature Phase Transitions, Thermodynamic and Physical Properties

Marianne Mödlinger; Alessia Provino; Pavlo Solokha; Federico Caglieris; Michele Ceccardi; Daniele Macciò; Marcella Pani; Cristina Bernini; Dario Cavallo; Andrea Ciccioli; Pietro Manfrinetti

doi:10.3390/ma16062501

Cu₃As: Uncommon Crystallographic Features, Low-Temperature Phase Transitions, Thermodynamic and Physical Properties

Materials (Basel). 2023 Mar 21;16(6):2501. doi: 10.3390/ma16062501.

Authors

Affiliations

¹ Department of Chemistry, University of Genoa, 16146 Genoa, Italy.
² Department of Physics, University of Genoa, 16146 Genoa, Italy.
³ Institute SPIN-CNR, 16152 Genoa, Italy.
⁴ Department of Chemistry, Sapienza University of Rome, 00185 Rome, Italy.

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 Cu_2.882(1)As. The present results disprove a change in the crystal structure above RT actually reported in the phase diagram (from γ' to γ on heating). Instead, below RT, at T = 243 K (-30 °C), a first-order structural transition to a trigonal low-temperature superstructure, LT-Cu_3-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, ΔH_(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.

Keywords: X-ray diffraction; copper arsenides; differential scanning calorimetry; electrical resistivity; first-order structural transition; lonsdaleite sublattice; magnetic susceptibility.

Grants and funding

101018804/European Commission