Synthesis, structure, and properties of the electron-poor II-V semiconductor ZnAs

ZnAs was synthesized at 6 GPa and 1273 K utilizing multianvil high-pressure techniques and structurally characterized by single-crystal and powder X-ray diffraction (space group Pbca (No. 61), a = 5.6768(2) Å, b = 7.2796(2) Å, c = 7.5593(2) Å, Z = 8). The compound is isostructural to ZnSb (CdSb type) and displays multicenter bonded rhomboid rings Zn2As2, which are connected to each other by classical two-center, two-electron bonds. At ambient pressure ZnAs is metastable with respect to Zn3As2 and ZnAs2. When heating at a rate of 10 K/min decomposition takes place at ∼700 K. Diffuse reflectance measurements reveal a band gap of 0.9 eV. Electrical resistivity, thermopower, and thermal conductivity were measured in the temperature range of 2-400 K and compared to thermoelectric ZnSb. The room temperature values of the resistivity and thermopower are ∼1 Ω cm and +27 μV/K, respectively. These values are considerably higher and lower, respectively, compared to ZnSb. Above 150 K the thermal conductivity attains low values, around 2 W/m·K, which is similar to that of ZnSb. The heat capacity of ZnAs was measured between 2 and 300 K and partitioned into a Debye and two Einstein contributions with temperatures of θD = 234 K, θE1 = 95 K, and θE2 = 353 K. Heat capacity and thermal conductivity of ZnSb and ZnAs show very similar features, which possibly relates to their common electron-poor bonding properties.