Multiple emerging nano-phases are at the origin of the low lattice thermal conductivity of SnSe?

Thermoelectrics are well recognized materials for energy harvesting and many solid-state cooling devices are already present on the market. SnSe is an inexpensive binary system made by earth-abundant elements that holds the record-high performance among the semiconductor materials although the origin of its ultralow thermal conductivity is still not understood. Both X-ray and neutron diffraction experiments identified in SnSe at high temperature a transition from the α-phase (Pnma) to the β-phase (Cmcm). In this contribution using for the first time in-situ temperature-dependent X-ray Absorption Fine structure spectroscopy (T-XAFS) at Sn and Se K-edge, we showed the occurrence in the XAS data of two isosbestic points, which support the coexistence of two phases in a wide temperature range. Moreover, in the SnSe matrix Sn–O bonds are continuously formed and SnO2, SeO2 and SnSe2 nanophases are emerging at high temperature. The experiment points out how these local nanophases are formed above 600 K. These emerging nano-phases affect the mean free path of phonons reducing the lattice thermal conductivity and, together with the crystal structural phase transition, may account for the low thermal conductivity measured in SnSe single crystals. Emergence of multiple nano-phases in SnSe upon rising temperature could be at the origin of its ultralow thermal conductivity. [Display omitted] •Two isosbestic points detected in the temperature dependent XAFS spectra confirm the coexistence of two phases in SnSe.•Nanophases SnO2 anchored in bulk polycrystalline SnSe.•Emerging of multiple SnO2, SeO2, and SnSe2 nanophases during heating process.•Electronic structures and quantitative structural information were extracted.•Intrinsic low thermal conductivity could be associated to multiple nanophases detected in SnSe.