Discovery and Explorationof Two Raman Modes A1g(I) and A1g(A) betweenLayers in Lamellar Bi2O2Se

Lamellar Bi2O2Se, a novel nonvan der Waals material, demonstrates exceptional electronic transport properties and complex lattice dynamics owing to its unique crystal structure. However, the understanding of its lattice dynamics─particularly phonon characteristics and lattice vibrational modes across varying temperatures─remains limited. In this study, two distinct Raman-active modes, low frequency A1g(I) and high frequency A1g(A), are identified and validated in lamellar Bi2O2Se through variable-temperature Raman spectroscopy combined with theoretical calculations. These modes correspond to antiphase and in-phase out-of-plane vibrations of Bi atoms in adjacent Bi2O2 layers, respectively. Upon cooling from 300 to 12 K, the two modes exhibit opposite trends in Raman intensity while maintaining nearly constant frequencies. Integrated analysis using variable-temperature XRD and SAED reveals that temperature-induced variations in lattice constants alter the dominant vibrational direction of interlayer Bi atoms, thereby modulating the relative intensities of the two modes. Furthermore, first-principles calculations, lattice dynamics simulations, and Raman intensity analyses corroborate the existence of these modes and confirm that their temperature-dependent behavior aligns well with experimental observations. This work provides essential insights into the lattice dynamics of nonvan der Waals layered materials and establishes a foundational framework for future studies of carrier–phonon interactions using nonlinear spectroscopy.