Unusual Pressure Response of Vibrational Modes in Anisotropic TaS₃

We report on the unique vibrational properties of 2D anisotropic orthorhombic tantalum trisulfide (o-TaS₃) measured through angle-resolved Raman spectroscopy and high-pressure diamond anvil cell studies. Our broad-spectrum Raman measurements identify optical and low-frequency shear modes in pseudo-1D o-TaS₃ for the first time, and introduce their polarization resolved Raman responses to understand atomic vibrations for these modes. Results show that, unlike other anisotropic systems, only the S_∥ mode at 54 cm^-1 can be utilized to identify the crystalline orientation of TaS₃. More notably, high-pressure Raman measurements reveal previously unknown four distinct types of responses to applied pressure, including positive, negative, and nonmonotonic dω/dP behaviors which are found to be closely linked to atomic vibrations for involving these modes. Our results also reveal that the material approaches an isotropic limit under applied pressure, evidenced by a significant reduction in the degree of anisotropy. Overall, these findings significantly advance not only our understanding of their fundamental properties of pseudo-1D materials but also our interpretations of the vibrational characteristics that offer valuable insights about thermal, electrical, and optical properties of pseudo-1D material systems.