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

Abstract

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·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.