Raman signatures of inversion symmetry breaking structural transition in quasi-1D compound, (TaSe4)3I.

The breaking of inversion symmetry combined with spin-orbit coupling, can give rise to intriguing quantum phases and collective excitations. Here, we report systematic temperature dependent Raman scattering and theoretical calculations of phonon modes across the inversion symmetry-breaking structural transitions in a quasi-one-dimensional compound (TaSe4)3I. Our investigation revealed the emergence of three additional Raman-active modes in Raman spectra of the low-temperature non-centrosymmetric (NC) structure of the material. From polarization dependent Raman spectra and phonon mode symmetry analysis, we have identified the origin of these three newly appeared additional Raman-active modes. Notably, two of these modes become Raman active due to the loss of inversion symmetry, while the third mode is identified as a soft phonon mode, arising from the distinctive vibrational motion of tantalum (Ta) atoms along the -Ta-Ta- chains. Furthermore, the temperature evolution of self-energy parameters indicates significant changes in the characteristics of the Raman modes across the transition. Latent heat measurements near the phase transition using Differential Scanning Calorimetry confirm the first-order nature of the transition. Theoretical analysis, including group theory and modeling, reaffirms the displacive first-order nature of the structural transition. Our findings establish (TaSe4)3I as a model quasi-one-dimensional system with broken inversion symmetry facilitated through a displacive first-order structural transition.