Amorphous-like thermal conductivity and high thermoelectric figure of merit in “π” SnS and SnSe

We present a detailed first-principles characterisation of the thermoelectric performance of the cubic “π” phases of SnS and SnSe. The complex structures push the “particle-like” contribution to the lattice thermal conductivity, κlatt, below the amorphous limit, resulting in an ultra-low κlatt from room temperature upwards. The cubic symmetry supports larger Seebeck coefficients than the orthorhombic phases, but higher carrier effective masses and stronger electron scattering require high doping levels to optimise the conductivity and power factors. For π SnSe, we predict a low-temperature n-type figure of merit, ZT, comparable to Bi2Te3, and a high-temperature ZT competitive with the flagship orthorhombic SnSe. These results demonstrate the exceptional promise of these systems as high-performance thermoelectrics, and highlight structural complexity as a route to optimising low-temperature ZT by minimising the κlatt.