BaGa₄Se₇ 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 BaGa₄Se₇ 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 BaGa₄Se₇, and helps us understand phonon gaps, monoclinic crystals, and its phenomena at infrared and terahertz frequency ranges.

BaGa ₄ Se ₇是一种很有前途的在红外频率下的非线性光学晶体，显示出有趣的太赫兹声子-极化子和太赫兹产生的高非线性系数。声子是太赫兹频率下红外吸收和光子-声子共振现象的关键因素。在这里，我们研究BaGa ₄ Se ₇的声子结构晶体，具有偏振拉曼光谱和声子色散曲线，状态密度和振动模式的理论计算。一个有趣的声子间隙将具有静止或振动的Ba原子的模式分开。我们还确定了9个最强拉曼峰的振动模式和拉曼张量。我们的拉曼模式分配和声子计算显示了声子能量，声子类型和振动方向的一致性。以上知识为声子间隙提供了一个新的实例，提供了BaGa ₄ Se ₇声子结构的完整图片，并帮助我们了解了声子间隙，单斜晶体及其在红外和太赫兹频率范围内的现象。