Evidence of Highly Anharmonic Soft Lattice Vibrations in a Zintl Rattler

Here, we present lattice dynamics associated with the local chemical bonding hierarchy in Zintl compound TlInTe2, which cause intriguing phonon excitations and strongly suppress the lattice thermal conductivity to an ultralow value (0.46–0.31 W m−1 K−1) in the 300–673 K. We established an intrinsic rattling nature in TlInTe2 by studying the local structure and phonon vibrations using synchrotron X‐ray pair distribution function (PDF) (100–503 K) and inelastic neutron scattering (INS) (5–450 K), respectively. We showed that while 1D chain of covalently bonded InTe2n-n transport heat with Debye type phonon excitation, ionically bonded Tl rattles with a frequency ca. 30 cm−1 inside distorted Thompson cage formed by InTe2n-n . This highly anharmonic Tl rattling causes strong phonon scattering and consequently phonon lifetime reduces to ultralow value of ca. 0.66(6) ps, resulting in ultralow thermal conductivity in TlInTe2. Direct evidences of bonding hierarchy and anharmonicity using the pair distribution function (PDF) and its consequent soft optical vibrations as observed from inelastic neutron scattering (INS) gives rise to low energy optical phonons. This effect scatters the heat carrying acoustic phonons and lowers the lattice thermal conductivity in Zintl type TlInTe2.