Materials displaying ultralow lattice thermal conductivity (k_Lat) are crucial to realizing high performing thermoelectric materials to convert waste heat into electrical energy. Recently, we have discovered an ultralow lattice thermal conductivity (ca. 0.5 W/mK at 300 K) in Zintl-type TlInTe2 compound. First-principles DFT calculations in TlInTe2 reveals low-frequency optical modes arising from rattling Tl atoms and coupling to heat-carrying acoustic phonons through symmetry-allowed interactions. As such, we strongly believe that acoustic–optic anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of TlInTe2. Inelastic Neutron Scattering (INS) data as a function of temperature will enable us to extract phenomenon on an atomic scale and help us to investigate the fundamental cause for the intrinsically low k_Lat in TlInTe2.