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Recently, we employed electronic polarization-resolved Raman spectroscopy toreveal the strongly correlated excitonic insulator (EI) nature of Ta2NiSe5,Volkov et al. [arXiv:2007.07344], and also showed that forTa$_2$Ni(Se$_{1-x}$S$_x$)$_5$ alloys the critical excitonic fluctuationsdiminish with sulfur concentration x exposing a cooperating lattice instabilitythat takes over for large x, Volkov et al. [arXiv:2104.07032]. Here we focus onthe lattice dynamics of this EI family. We identify all Raman-active opticalphonons of fully symmetric and ac-quadrupole-like symmetries and study theirevolution with temperature and sulfur concentration. We demonstrate the changeof selection rules at temperatures below the orthorhombic-to-monoclinictransition at Tc(x) that is related to the EI phase. We find that Tc(x)decrease monotonically from 328 K for Ta2NiSe5 to 120 K for Ta2NiS5 and thatthe magnitude of lattice distortion also decreases with the sulfurconcentration x. For x < 0.7, the two lowest-frequency B2g phonon modes showstrongly asymmetric lineshapes at high temperatures due to Fano interferencewith the broad excitonic continuum present in a semimetallic state. Within theframework of extended Fano model, we develop a quantitative description of theinteracting exciton-phonon excitation lineshape, enabling us to derive theintrinsic phonon parameters and determine the exciton-phonon interactionstrength, that affects the transition temperature Tc(x). We also observesignatures of the acoustic mode scattered assisted by the structural domainwalls formed below Tc. Based on our results, we additionally present aconsistent interpretation of the origin of oscillations observed intime-resolved pump-probe experiments.

Lattice dynamics of the excitonic insulator Ta$_2$Ni(Se$_{1-x}$S$_x$)$_5$