The Role of Off-centering Behavior and Acoustic-Optical Phonon Coupling in Heat Transport

Understanding the nature of phonon transport in solids and exploring the way to minimize the thermal conductivity are important in many fields, including the development of efficient thermoelectric materials. For a long time, the contribution of optical phonons to the lattice thermal conductivity is considered to be very small and negligible. Until recent decade, a series of studies have shown that, for some materials with special electronic configuration, optical phonons can dramatically affect the lattice thermal conductivity. Specifically, in these materials, part of their cations would be off-centering from their equilibrium positions and lead to the local distortion of the coordination structure. This off-centering behavior would introduce some low-frequency optical phonons, which can couple with the acoustic phonons and cause additional phonon scattering, thus, it is very effective to suppress the lattice thermal conductivity. This perspective clarifies the lattice dynamics of off-centering behavior and illustrates how this off-centering behavior could lead to the acoustic-optical phonon coupling. This paper also demonstrates the off-centering behavior can be introduced into a material by using the lone pair element doping and the weak sd3 orbital hybridization, and proposes a strategy to design materials with low thermal conductivity based on this.