Mode-selective anharmonicity induced by lone-pair electrons in layered oxyselenides

Materials with low thermal conductivity are essential for thermoelectric energy conversion and thermal management applications. Lone-pair electrons (LPEs), a pair of nonbonded electrons, is generally believed to increase the phonon anharmonicity, which leads to low lattice thermal conductivity. However, the specific manifestation of LPEs on the lattice dynamics and phonon transport remains elusive. Here, using Raman scattering and first-principles calculations, we unveil mode-selective anharmonicity caused by LPEs in layered oxyselenides, quantified by the linewidth of Raman-active phonon modes. Strong anharmonicity is observed not only for the phonon modes associated with the ions that contain LPEs, but also for those modes involving ions that are bonded with the LPE-containing ions. The strength of anharmonicity also depends on the vibrational directions of the phonon modes. These results establish a general guideline for identifying LPE-induced anharmonicity and provide a new perspective on optimizing thermoelectric performance.