Synthesis, Structure, and Properties of the Electron-Poor II–V Semiconductor ZnAs

Abstract

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.

Origin of the highly anisotropic thermal expansion of the semiconducting ZnSb and relations with its thermoelectric applications

“…These trends are also observed in ZnAs as it shows [23] a dominant thermal expansion along a, and similar thermal variation of the Zn-As intra-ring distances and Zn-distances. The large anisotropy of the thermal expansion, and especially the large expansion along the a-direction, is linked to the strong deformation of the Zn 2 Sb 2 rings.…”

“…In a very recent study, Fischer et al [23] also found highly anisotropic thermal expansion in ZnAs with the b and c-lattice parameters decreasing with increasing temperature. Nevertheless, these observations still need to be conrmed both experimentally and theoretically.…”

“…As discussed by Haussermann's group [23,28], both Zn and Sb have peculiar ve-fold coordination with one-like and four-unlike neighbours. As discussed by Haussermann's group [23,28], both Zn and Sb have peculiar ve-fold coordination with one-like and four-unlike neighbours.…”

Review of Research on the Thermoelectric Material ZnSb

“…ZnSb, thereby, has been classified as an electron-poor framework semiconductor (EPFS) [26,31]. A quite similar interpretation for the structure has been applied to the related compounds CdSb [30,[32][33][34] and ZnAs [35].…”

“…The most common ones are cold-pressing (at room temperature with ultra-pressure 2-10GPa [83,91]), hot-pressing (>450°C with pressure of 20-300MPa [16,19,20,35]), and spark plasma sintering (SPS) (the electrical current is passed through the sample with 5min reaction time at 350-450°C [83,92]). One important difference among hot-presses is the manipulation of secondary phases; both removal and proportioning are possible, and obviously depends upon the temperature and duration, but also on details of the instrument design and the environment of the ZnSb powder.…”