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 <i>C</i>2/<i>m</i> 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, <i>ZT</i>, was achieved. High-temperature thermoelectric characterization of the Sn-doped compound reveals a remarkable <i>ZT</i> with a maximum of 0.35 at 660 K