Origins of ultralow thermal conductivity in 1-2-1-4 quaternary selenides

Engineering the thermal properties in solids is important for both fundamental physics (e.g. electric and phonon transport) and device applications (e.g. thermal insulating coating, thermoelectrics). In this paper, we report low thermal transport properties of four selenide compounds (BaAg2SnSe4, BaCu2GeSe4, BaCu2SnSe4 and SrCu2GeSe4) with experimentally-measured thermal conductivity as low as 0.31 ± 0.03 W m−1 K−1 at 673 K for BaAg2SnSe4. Density functional theory calculations predict κ < 0.3 W m−1 K−1 for BaAg2SnSe4 due to scattering from weakly-bonded Ag–Ag dimers. Defect calculations suggest that achieving high hole doping levels in these materials could be challenging due to monovalent (e.g., Ag) interstitials acting as hole killers, resulting in overall low electrical conductivity in these compounds.