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BaGa 4 Se 7  is a promising nonlinear optical crystal at infrared frequencies and shows interesting terahertz phonon-polaritons and high nonlinear coefficients for terahertz generation. Phonons are the key players in infrared absorptions and the photon-phonon resonance phenomena at terahertz frequencies. Here, we study the phonon structures of BaGa 4 Se 7  crystal, with polarized Raman spectroscopy and theoretical calculations for phonon dispersion curves, density of states and vibration modes. An interesting phonon gap separates the modes with still or vibrating Ba atoms. We also determine the nine strongest Raman peaks’ vibration modes and Raman tensors. Our Raman mode assignments and phonon calculations show consistencies in phonon energies, phonon types, and vibration directions. Above knowledge provides a new case example for phonon gaps, offers a complete picture of the phonon structures of BaGa 4 Se 7 , and helps us understand phonon gaps, monoclinic crystals, and its phenomena at infrared and terahertz frequency ranges.

Raman spectra and phonon structures of BaGa4Se7 crystal