Probing Tl sublattice dynamics in ultralow thermal conductivity TlInTe<inf>2</inf>

We investigated the lattice dynamics of TlInTe2 using THz time-domain spectroscopy as a function of temperature across the normal-incommensurate-commensurate transition. Our results indicate electric field screening of the LO mode on cooling and increased phonon anharmonicity in the commensurate phase. Moreover, despite the evidence of diminishing spectral weight of rattling Tl atoms, our measurements directly confirm the rigidity of the underlying InTe4 sublattice. Our data support the scenario proposed for the superionic conductors, where despite the loss of spectral weight of mobile ions, the host lattice remains intact and can transport heat even in the superionic phase.