title: The Origin of Ultralow Thermal Conductivity in InTe: Lone-Pair-Induced Anharmonic Rattling.
abstract: Understanding the origin of intrinsically low thermal conductivity is fundamentally important to the development of high-performance thermoelectric materials, which can convert waste-heat into electricity. Herein, we report an ultralow lattice thermal conductivity (ca. 0.4 W m(-1)  K(-1) ) in mixed valent InTe (that is, In(+) In(3+) Te2 ), which exhibits an intrinsic bonding asymmetry with coexistent covalent and ionic substructures. The phonon dispersion of InTe exhibits, along with 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 5s(2) lone pair of the In(+) atom and are strongly anharmonic, which scatter the heat-carrying acoustic phonons through strong anharmonic phonon-phonon interactions, as evident in anomalously high mode Grüneisen parameters. A maximum thermoelectric figure of merit (z T) of about 0.9 is achieved at 600 K for the 0.3 mol % In-deficient sample, making InTe a promising material for mid-temperature thermoelectric applications.
authors.affiliation: New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore, India.
authors.forename: Manoj K
authors.initials: MK
authors.lastname: Jana
authors.affiliation: Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore, 560 064, India.
authors.forename: Koushik
authors.initials: K
authors.lastname: Pal
authors.affiliation: Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore, 560 064, India.
authors.forename: Umesh V
authors.initials: UV
authors.lastname: Waghmare
authors.affiliation: New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore, India. kanishka@jncasr.ac.in.
authors.forename: Kanishka
authors.initials: K
authors.lastname: Biswas
keywords: indium telluride; lone pairs; rattling; thermoelectrics; ultralow thermal conductivity