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Lattice Vibrational Hierarchy and Mean-Free-Path Filtering in Bi6Cu2Se4O6 Superlattice Thermoelectrics

Shulin Bai1,2,3,*, Haonan Shi1,4,*, Yi Wen1,*, Yixuan Hu1, Junqing Zheng1, Yongxin Qin1,5, Lizhong Su4, Shibo Liu1, Dongrui Liu1 et al.

Tian Gao1, Tao Hong1, Xiang Gao6, Fangyuan Zhu7,†, Bingchao Qin1,2,3,‡, and Li-Dong Zhao1,2,3,§

  • *These authors contributed equally to this work.
  • †Contact author: zhufy@sari.ac.cn
  • ‡Contact author: qinbingchao@buaa.edu.cn
  • §Contact author: zhaolidong@buaa.edu.cn
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Phys. Rev. X 15, 031033 – Published 31 July, 2025

DOI: https://doi.org/10.1103/44gy-zmbf

Abstract

Heteroanionic thermoelectric (TE) oxides have attracted considerable attention due to their unique phonon and carrier transports. The recent investigation of Bi6Cu2Se4O6 has opened new prospects for the development of n-type TE oxides. Herein, we first investigate the mechanism of superlattice formation and the origin of narrow band gap in Bi6Cu2Se4O6. The physical picture of lattice vibration hierarchy in Bi6Cu2Se4O6 is indexed to the lone pair rattler of Bi and liquidlike behavior of Cu since more diffusons are observed as temperature increases. Additionally, the modified band structure and crystal defects are responsible for the stable n-type transport in Bi6Cu2Se4O6 compared to the well-known p-type BiCuSeO. Combining phonon transport and carrier mean-free-path filtering, theoretical results demonstrate that n-type case exhibits two optimized carrier concentration regions with peak ZT values approaching ∼1.0 at 300 K and ∼1.4 at 900 K, respectively, while p-type case shows a higher power factor of ∼90  μW cm−1 K−2, leading to high-ranged TE performance among 300–900 K. Our results demonstrate Bi6Cu2Se4O6 is a promising TE oxide with potential applications in both refrigeration and power generation. Moreover, understanding and establishing a physical and chemical link between the superlattice structure and TE properties enables the future development of TE oxides.

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Popular Summary

Thermoelectric materials, which convert heat into electricity, are promising for energy recovery and cooling technologies. Among them, bismuth-based oxychalcogenides are known for their high performance and stability in air, making them excellent candidates for practical use. However, while efficient p-type versions of these materials are well established, their n-type counterparts—needed to complete a thermoelectric circuit—have remained elusive. In this study, we report a breakthrough with Bi6Cu2Se4O6, a compound that shows real potential as a stable and effective n-type thermoelectric oxide.

Density functional theory, orbital analysis, and molecular dynamics simulations help us uncover unique features in the material’s structure, such as lone-pair “rattling” from bismuth atoms and liquidlike vibrations of copper atoms. Experiments and dual-phonon theory suggest that many vibrational modes in the lattice behave like diffusions—random, nonwavelike carriers of heat—rather than conventional phonons. Using Boltzmann transport theory, we identify two separate ways to optimize the n-type performance of Bi6Cu2Se4O6, providing clear guidance for future materials design.

By deepening the understanding of how complex lattice dynamics affect transport properties, our work opens a new path for designing air-stable, high-performance thermoelectric oxides.

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Phys. Rev. X 15, 031033– Published 31 July, 2025
  • Received 17 January 2025
  • Accepted 27 June 2025
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