One of the fundamental challenge in developing high-performance thermoelectric materials has been to achieve low lattice thermal conductivity (κL). 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. InTe [i.e. In+In3+Te2], a mixed valent compound, exhibit an ultralow κL, which manifests an intrinsic bonding asymmetry with coexistent covalent and ionic substructures.1 The phonon dispersion of InTe exhibits, in addition to low-energy flat branches, weak instabilities associated with the rattling vibrations of In+ atoms along the columnar ionic substructure. These weakly unstable phonons originate from the 5s2 lone pairs of adjacent In+ atoms and are strongly anharmonic, which scatter the heat-carrying acoustic phonons through phonon-phonon interactions. Similarly, a Zintl compound, TlInTe2, also exhibit ultralow κL due to low energy ratting modes of weakly bound Tl.2 Soft phonon modes and optical-acoustic phonon coupling cause an ultralow lattice thermal conductivity in the room-temperature hexagonal phase of AgCuTe, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high-temperature rocksalt phase. A high thermoelectric figure of merit (zT) of 1.6 is achieved in the p-type AgCuTe at ~670 K.3 Recently, we have shown that the localized vibrations of Bi bilayer leading to ultralow lattice thermal conductivity and high thermoelectric performance in weak topological insulator n-type BiSe near room temperature.4
1. M. K. Jana, K. Pal, U. V. Waghmare, and K. Biswas, Angew. Chem Int. Ed, 2016, 55, 7792.
2. M. K. Jana, K. Pal, A. Warankar, P. Mandal, Waghmare, U. V. and Biswas, K. J. Am. Chem. Soc., 2017. 139, 4350.
3. S. Roychowdhury, M. K. Jana, J. Pan, S. N. Guin, D. Sanyal, U. V. Waghmare, and K. Biswas, Angew. Chem Int. Ed, 2018, 57, 4043.
4. M. Samanta, K. Pal. P. Pal, U. V. Waghmare, and K. Biswas, J. Am. Chem. Soc., 2018, 140, 5866