Phonon softening and in-plane anisotropy in KFeS₂ nanoribbons: a Raman spectroscopy investigation

In this work, a systematic Raman study of KFeS₂ nanoribbons using angle-resolved polarized, temperature-dependent (77-475 K), multi-wavelength resonance, and ultra-low frequency Raman spectroscopy was conducted. Group theory analysis reveals its polarization-dependent mode symmetries and excitation selectivity. The temperature-dependent A_g⁴ mode vibration, modeled by thermal expansion and anharmonic interactions (three-/four-phonon scattering mechanism), uncovers its lattice dynamic and phonon coupling. The abrupt phonon softening and pronounced linewidth broadening of B_g² mode was identified as a signature of phase transition, likely arising from the symmetry-breaking distortion of FeS tetrahedral framework. The resonance Raman spectra (473, 532, 632.8, and 785 nm) show enhanced electron-phonon couplings for some specific modes. What is more, ultra-low frequency measurement suggests interactions exist among the interchain lattice vibrations. These results not only establish a foundational Raman spectrum of KFeS₂, but also elucidate the temperature-dependent lattice dynamic, which uncovers the structural instability driven by anharmonic phonon interaction. The work advances understanding of low-dimensional quantum materials and provides a framework for probing coupled electronic-structural transition.