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

Area of Science:

• Solid State Chemistry
• Crystallography
• Materials Science

Background:

• The binary copper arsenide (Cu3As) phase formation and crystal structure require re-evaluation.
• Existing literature suggests conflicting crystallographic data for Cu3As, necessitating clarification.
• Understanding the structural and physical properties of Cu3As is crucial for potential applications.

Purpose of the Study:

• To re-investigate the formation and crystal structure of the binary Cu3As phase.
• To measure key physical properties of Cu3As single crystals and polycrystalline bulk.
• To clarify the low-temperature structural transitions and their impact on material properties.

Main Methods:

• Synthesis of Cu3As samples at varying temperatures (300-400 °C).
• X-ray diffraction (XRD) for crystal structure determination at room and low temperatures.
• Differential scanning calorimetry (DSC) to detect phase transitions.
• Measurements of electrical resistivity and Seebeck coefficient as a function of temperature.
• Magnetic susceptibility measurements.

Main Results:

• Cu3As crystallizes in the hexagonal Cu3P prototype (hP24, P63cm) at room temperature, not the anti-HoH3-type.
• A narrow compositional range (74.0-75.5 at.% Cu) was observed, with slight copper understoichiometry (Cu2.882(1)As).
• A first-order structural transition to a trigonal low-temperature superstructure (LT-Cu3-xAs, hP72, P-3c1) occurs at 243 K.
• Both polymorphs feature a rigid 3D lonsdaleite-type copper sublattice and triangular 'Cu3As' units.
• DSC detected the 243 K transition with an enthalpy difference of ~2 J/g.
• Electrical resistivity exhibits metallic behavior with anomalies at phase transitions; Seebeck coefficient indicates n-type conduction.
• Magnetic susceptibility shows a diamagnetic response.

Conclusions:

• The study refutes previously reported high-temperature structural changes in Cu3As.
• A new low-temperature trigonal phase (LT-Cu3-xAs) and its transition at 243 K are identified.
• The unique Cu sublattice and 'Cu3As' units are conserved across the structural transition, influencing material properties.