Antibonding or Nonbonding Interaction-Driven Phonon Modes Softening and Wave-like Interband Thermal Conduction in Layered In4Te3 under the Framework of Wigner Transport Formalism

In4Te3, a p-type analogue of n-type In4Se3, exhibits anomalous thermal conductivity (kappa(L)). The value of kappa(L) is intrinsically very low, 0.7 W m(-1) K-1 at 300 K, and it is weakly temperature (T)-dependent, which deviates from the conventional kappa(L) proportional to T-1 variation. Thus, In4Te3 finds its application as a thermoelectric material in the midtemperature range. Here, employing a completely ab initio-based DFT framework, three-phonon scattering process, and Wigner transport equation, we found that In4Te3 exhibits strong anharmonicity in the acoustic region as well as the low energy optical region. The mean free path already reached the "Ioffe-Regel limit" in space even at a low T which is consistent with low kappa(L). The anharmonicity arises due to the antibonding and nonbonding overlap of In4 valence electrons with its neighbors. Such a weak bond formation capability of In4 has very low interatomic force constants, which results in overall low elastic properties, viz. sound velocity (1947.24 m s(-1)), Debye temperature (184 K), Young's modulus (51.56 GPa), etc., indicating considerable amount of lattice softening. Interestingly, those softened Einstein-like modes at similar to 0.72 THz form overlapped phonon bands and promote their wave-like tunneling. Even at low T (50 K), the line-widths of the individual modes are still larger than their interband spacing, which lowers their particle-like propagation but enhances wave-like tunneling, eventually leading to the dual particle-wave behavior of the heat carrying phonons. At high T, the intrinsic phonon-phonon interaction dominates and the calculated three-phonon scattering time lies between the "Wigner" and "Ioffe-Regel limits" in time which manifests nonstandard wave-like thermal conductivity in the system.