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<p>Mg<sub>3</sub>Bi<sub>2-<i>x</i></sub>Sb<sub><i>x</i></sub> compounds have emerged as promising mid-temperature thermoelectric materials, due to their excellent electrical performance and the ultralow thermal conductivity. In this study, we investigate the complex phonon interactions in Mg<sub>3</sub>Bi<sub>2-<i>x</i></sub>Sb<sub><i>x</i></sub> compounds and reveal nontrivial symmetry-protected topological crossings and avoided-crossing phenomena in the phonon dispersion, arising from the strong coupling between the acoustic and low-energy optical phonons. By combining inelastic neutron scattering measurements with first-principles simulations, we identify robust band crossing protected by crystal symmetries and mode inversion along the <i>Γ</i> <i>M</i> direction in Mg<sub>3</sub>Bi<sub>2-<i>x</i></sub>Sb<sub><i>x</i></sub> compounds, providing direct experimental evidence of topological phonons in a thermoelectric system. Furthermore, pronounced avoided crossings involving ultrasoft transverse acoustic modes are observed along the same direction, indicating significant hybridization with optical branches in the basal plane. The influence of Bi/Sb alloying on the phonon structure is also examined, revealing a broadened density of states (DOS) in the low- to mid-energy range for the Mg<sub>3</sub>BiSb alloy. These findings establish Mg<sub>3</sub>Bi<sub>2-<i>x</i></sub>Sb<i>ₓ</i> compounds as a model system for studying symmetry-protected phonon topology and strong phonon-phonon interactions, offering new insights for lattice engineering in quantum and energy materials.</p>

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Uncovering complex phonon interactions in Mg3Bi2-xSbx: topology and avoided crossings

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年4月16日

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  • Lei Chen,
  • Yuefeng Yin,
  • Ting Lu,
  • Huangshui Ma,
  • Shuxian Zhang,
  • Xinyue Zhang,
  • Zhiwei Chen,
  • Siqi Lin,
  • Siqi Huo,
  • Shengqing Xia,
  • Richard A. Mole,
  • Dehong Yu,
  • Kirrily C. Rule,
  • Weiyao Zhao,
  • Julie Karel,
  • Pingan Song,
  • John Bell,
  • Min Hong

摘要

Mg3Bi2-xSbx compounds have emerged as promising mid-temperature thermoelectric materials, due to their excellent electrical performance and the ultralow thermal conductivity. In this study, we investigate the complex phonon interactions in Mg3Bi2-xSbx compounds and reveal nontrivial symmetry-protected topological crossings and avoided-crossing phenomena in the phonon dispersion, arising from the strong coupling between the acoustic and low-energy optical phonons. By combining inelastic neutron scattering measurements with first-principles simulations, we identify robust band crossing protected by crystal symmetries and mode inversion along the Γ M direction in Mg3Bi2-xSbx compounds, providing direct experimental evidence of topological phonons in a thermoelectric system. Furthermore, pronounced avoided crossings involving ultrasoft transverse acoustic modes are observed along the same direction, indicating significant hybridization with optical branches in the basal plane. The influence of Bi/Sb alloying on the phonon structure is also examined, revealing a broadened density of states (DOS) in the low- to mid-energy range for the Mg3BiSb alloy. These findings establish Mg3Bi2-xSbₓ compounds as a model system for studying symmetry-protected phonon topology and strong phonon-phonon interactions, offering new insights for lattice engineering in quantum and energy materials.