GeAs: Highly Anisotropic van der Waals Thermoelectric Material

GeAs and Sn-doped GeAs were synthesized from elements. Both crystallize in a layered crystal structure in the C2/m space group (No. 12) in the GaTe structure type. The crystal structure consists of As-terminated layers separated by van der Waals gaps. 119Sn Mössbauer spectroscopy reveals that in the doped compound, Sn atoms are situated in a symmetric and homogeneous environment, most probably in the form of Sn2 dumbbells. The anisotropic crystal structure of GeAs leads to highly anisotropic transport properties. High electrical and thermal conductivities were determined along the crystallographic layers. For the perpendicular direction across the layers, a sharp drop of more than an order of magnitude was observed for the transport properties of the GeAs single crystal. As a result, an order of magnitude difference in the figure of merit, ZT, was achieved. High-temperature thermoelectric characterization of the Sn-doped compound reveals a remarkable ZT with a maximum of 0.35 at 660 K.