Critical mode and band-gap-controlled bipolar thermoelectric properties of SnSe

From the related article: The reliable calculation of electronic structures and understanding of electrical properties depends on an accurate model of the crystal structure. Here, we have reinvestigated the crystal structure of the high-zT thermoelectric material tin selenide, SnSe, between 4 and 1000 K using high-resolution neutron powder diffraction. Symmetry analysis reveals the presence of four active structural distortion modes, one of which is found to be active over a relatively wide range of more than ±200 K around the symmetry-breaking Pnma--Cmcm transition at 800~K. Density functional theory calculations on the basis of the experimental structure parameters show that the unusual, step-like temperature dependencies of the electrical transport properties of SnSe are caused by the onset of intrinsic bipolar conductivity, amplified and shifted to lower temperatures by a rapid reduction of the band gap between 700 and 800 K. The calculated band gap is highly sensitive to small out-of-plane Sn displacements observed in the diffraction experiments. SnSe with a sufficiently controlled acceptor concentration is predicted to produce simultaneously a large positive and a large negative Seebeck effect along different crystal directions. https://doi.org/10.1103/PhysRevMaterials.2.085405