The curious case of the structural phase transition in SnSe insights from neutron total scattering

The thermoelectric properties of SnSe are attributed to a high-temperature phase transition, whose nature has not been resolved. Here the authors probe the transition at different length scales using neutron pair distribution function analysis and diffraction data, revealing a dynamic order-disorder nature, and shedding light on previous discrepancies. At elevated temperatures SnSe is reported to undergo a structural transition from the low symmetry orthorhombic GeS-type to a higher symmetry orthorhombic TlI-type. Although increasing symmetry should likewise increase lattice thermal conductivity, many experiments on single crystals and polycrystalline materials indicate that this is not the case. Here we present temperature dependent analysis of time-of-flight (TOF) neutron total scattering data in combination with theoretical modeling to probe the local to long-range evolution of the structure. We report that while SnSe is well characterized on average within the high symmetry space group above the transition, over length scales of a few unit cells SnSe remains better characterized in the low symmetry GeS-type space group. Our finding from robust modeling provides further insight into the curious case of a dynamic order-disorder phase transition in SnSe, a model consistent with the soft-phonon picture of the high thermoelectric power above the phase transition.

Automated summary

The authors investigated the thermoelectric properties of SnSe and found that it undergoes a dynamic order-disorder phase transition, shedding light on previous discrepancies. They used neutron pair distribution function analysis and diffraction data to probe the transition at different length scales.