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

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.

Figure 1

LOM images of the Cu₃As samples synthesized at 350 °C—13 days (a) and at 500 °C—16 days (b). SEM microphotographs in Back-Scattered-Electron (BSE) mode of the same samples: 350 °C—13 days (c) and 500 °C—16 days (d).

Figure 2

SEM microphotographs in Secondary Electron (SE) mode of selected samples; (a,b) show crystals grown after thermal treatment at 300 °C—20 days, while (c,d) show crystals grown after thermal treatment at 400 °C—14 days.

Figure 3

(a) Perspective view of the rigid Cu sublattice of the lonsdaleite type formed in the RT-Cu₃As phase by the contribution of the Cu2 and Cu3 atoms only; (b) top view (along the c-axis) of the same lonsdaleite sublattice.

Figure 4

(a) Triangular ‘Cu₃As’ unit (Cu4–Cu1–Cu4) hosting one As atom (6c) and placed inside one three-rings cage of the first tridimensional Cu sublattice. One of the tetrahedral units [Cu@Cu₄] formed by all the Cu atoms pertaining to the lonsdaleite sublattice (Cu2 and Cu3 atoms) is also shown; (b) top view of one triangular ‘Cu₃As’ unit. Orange and green balls indicate Cu and As atoms, respectively.

Figure 5

(a) Three triangular units sharing one vertex. (b) Perspective view of a layer formed by the ‘Cu₃As’-triangular units. (c) Top view of the layer shown in (b). Note: for simplicity, in (b) and (c) the Cu atoms at the vertices of the ‘Cu₃As’ units (Cu4–Cu1–Cu4) are not shown. Orange and green balls indicate Cu and As atoms, respectively.

Figure 6

Perspective drawing of the RT structure of the Cu₃As compound. Each unit cell contains two layers, α and β, of vertex-sharing triangular ‘Cu₃As’ units. Orange and green balls indicate Cu and As atoms, respectively.

Figure 7

Sketch of the crystal structure of Cu₃As where the coordination polyhedra around As atoms, As@Cu₁₁, are highlighted.

Figure 8

Rietveld refinement profiles (black line) obtained for the Cu₃As samples 350 °C—13 days (a) and 400 °C—14 days (b). The observed powder patterns are highlighted in red. The lower profile (blue line) gives the difference between observed and calculated data; the Bragg angle positions are indicated by vertical bars (green).

Figure 9

Reconstructed 1kl zone intensity profiles for Cu₃As obtained at different temperatures. The green arrows indicate the lines along which the peaks associated with the formation of the LT polymorph (hP72, a′ ≈ a and c′ ≈ 3c) appear. Blue arrows indicate a set of very weak super-reflections compatible with an even larger unit cell (hP144, a′ ≈ 2a and c′ ≈ 3c).

Figure 10

(a) Perspective and (b) top view of the lonsdaleite type network, which is also preserved in the in the LT structure of Cu₃As; here, it is formed by Cu6, Cu7 and Cu8 atoms.

Figure 11

(a) Perspective view of the arrangement of the As-centered triangular units in LT—Cu₃As (only the first two of a total of six different layers are shown); (b) top view. Orange and green balls represent Cu and As atoms, respectively.

Figure 12

(a) A perspective view showing only the assembling of the six different layers of ‘Cu₃As’-units in the LT—Cu₃As structure. (b) Perspective view of the whole unit cell of the LT structure.

Figure 13

Plot of the DSC data collected on cooling (blue curve) and on heating (green curve) at 5 °C/min on a single crystal of the sample Cu₃As annealed at 300 °C—13 days. Endothermic heat flow direction is upward.

Figure 14

(a) Zero-field electrical resistivity as a function of temperature between 2 and 300 K for Cu₃As measured both on cooling and heating. (b) Electrical resistivity measured in zero and under applied magnetic field of 9 T (both on cooling) for Cu₃As. The insets in (a,b) show a magnification of the data between 225 and 255 K.

Figure 15

Seebeck coefficient as a function of temperature between 2 and 300 K for Cu₃As. The inset, which shows a magnification of the data between 225 and 255 K, highlights the discontinuity at about 242 K.

Figure 16

Magnetoresistance as a function of applied magnetic field between 0 and ± 9 T at several temperatures (10 K, 50 K, 100 K and 150 K) for Cu₃As.

Figure 17

Magnetic susceptibility (in CGS units) as a function of temperature between 100 and 300 K for both single crystal (blue line and scale on the right) and a polycrystalline sample (red line and scale on the left) of Cu₃As. Measurements were performed using an external magnetic field of 1 T oriented perpendicular to the a–b plane for the single crystal. The inset shows an overview of the data from 5 to 300 K.