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 I n T e 2 n - 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 I n T e 2 n - 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 .