One of the fundamental challenges in developing high-performance thermoelectric materials has been to achieve low lattice thermal conductivity (κL). Extrinsic approach such as formation of nanostructures in thermoelectric matrix have explored significantly for decreasing the κL in Pb and Sn chalcogenides. Formation of layered intergrowth nanostructures in SnTe matrix or in the form of 2D heterostructure nanosheets by kinetic synthesis can also lead to ultralow κL.1-2 Further, entropy driven extended solid solutions in pseudo-ternary GeTe-GeSe-GeS system exhibited low κL and high thermoelectric figure of merit (zT) of 2.1.3 Introduction of point defects, nanoprecipitates and grain boundaries scatter the phonons heavily, but scatters the electrons/holes as well, which decreases carrier mobility. Intrinsic low thermal conductivity is of practical interest due to its robustness against grain size, temperature range and other structural variations. The exploration of new materials with intrinsically low κL along with a microscopic understanding of the underlying correlations among bonding, lattice dynamics and phonon transport is fundamentally important towards designing promising thermoelectric materials. Zintl compounds, TlInTe2 and InTe [i.e. In+In3+Te2], exhibit ultralow κL due to low energy ratting modes (optical modes) of weakly bound Tl+ and In+ respectively.4, 5 These low energy optic phonon modes are strongly anharmonic, which scatter the heat-carrying acoustic phonons through phonon-phonon interactions, thereby decrease the thermal conductivity.
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