Anharmonic Shear Vibration of Electrostatic Layers Leads to Low Thermal Conductivity in Bi2O2Se

Bi2O2Se is among the best n-type oxide thermoelectric materials with low lattice thermal conductivity and shows a robust ferroelectric state at room temperature in reduced dimensions. Herein, we have combined experimental and theoretical studies to comprehend this elusive congruence of low lattice thermal conductivity (κlat) and an incipient ferroelectric state. We have experimentally measured the κlat of Bi2O2Se in the 10-300 K temperature range, which showed that temperature-dependent κlat deviates from the usual T-1 dependence. Density functional theory-based calculation of phonon dispersion reveals that the low κlat in Bi2O2Se primarily originates from the anharmonic lattice vibration and the presence of low-frequency optical phonons, which strongly interact with the longitudinal acoustic phonons and suppress their dispersion along Γ-A and Γ-X directions in the Brillouin zone. Our study reveals that the lowest frequency optical phonons at ∼25.7 and ∼66.5 cm-1 involve a shear interlayer vibration that can promote distortion in the Bi-O bond length and displacement of Se atoms. We propose that these vibrational modes can dynamically break the centrosymmetric configuration in the reduced dimension, leading to the experimentally observed ferroelectricity in ultrathin nanosheets or flakes of Bi2O2Se as reported in the literature. We have corroborated the presence of these low-frequency phonons using low-temperature heat capacity measurement and temperature-dependent Raman spectroscopy.