  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.
