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.