Delocalized Bi-tetrahedral cluster induced ultralow lattice thermal conductivity in Bi3Ir3O11

Low-frequency cluster vibration plays an important role in reducing the lattice thermal conductivity of a system. Here, we show a new type of cluster vibration called delocalized cluster vibration that involves a highly symmetric local structure with lone pair electrons and marked impacts on lattice thermal transport. We take a transition-metal oxide Bi3Ir3O11 (two specific Wyckoff positions for Bi: corner-Bi1 and inner-Bi2) as an example, which exhibits counterintuitive low lattice thermal conductivity in metallic materials. Based on first-principles simulations of phonon transport in the Boltzmann transport formulation, this study shows that the four fourth neighbor Bi2 atoms form a regular Bi-tetrahedral cluster (Bi24), which collectively vibrates in certain intercorrelated patterns strongly and induces low-frequency optical phonon modes (-0.77 and-1.06 THz) along with low group velocities. The Bi24-related optical modes interrupt the acoustic vibrations and thus suppress the lattice thermal conductivity to an ultralow value (calculated value is 1.0 W/m center dot K at 300 K). Surprisingly, these Bi2-Bi2 have relatively large distances but significant interatomic force constants, indicating delocalized interactions among Bi24. This work demonstrates that the delocalized cluster vibration plays a significant role in lowering lattice thermal conductivity.

Automated summary

Low-frequency cluster vibration, specifically the delocalized cluster vibration, significantly reduces the lattice thermal conductivity by interrupting acoustic vibrations. In this study, the delocalized cluster vibration was observed in the transition-metal oxide Bi3Ir3O11, leading to a much lower thermal conductivity value compared to other metallic materials. The discovery of this phenomenon contributes to a better understanding of lattice thermal transport and opens up possibilities for designing materials with improved thermal insulation properties.