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 ${\left(I,n,{Te}_2\right)}_n^{-n}$ transport heat with Debye type phonon excitation, ionically bonded Tl rattles with a frequency ca. 30 cm^-1 inside distorted Thompson cage formed by ${\left(I,n,{Te}_2\right)}_n^{-n}$ . 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 .