Recently, we employed electronic polarization-resolved Raman spectroscopy to reveal the strongly correlated excitonic insulator (EI) nature of ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$ [Volkov et al., npj Quant. Mater. 6, 52 (2021)], and also showed that for ${\mathrm{Ta}}_2\mathrm{Ni}{\left({\mathrm{Se}}_{1-x}{\mathrm{S}}_x\right)}_5$ alloys the critical excitonic fluctuations diminish with sulfur concentration $x$, exposing a cooperating lattice instability that takes over for large $x$ [Volkov et al., arXiv:2104.07032]. Here we focus on the lattice dynamics of the EI family ${\mathrm{Ta}}_2\mathrm{Ni}{\left({\mathrm{Se}}_{1-x}{\mathrm{S}}_x\right)}_5$ ($x=0$,..., 1). We identify all Raman-active optical phonons of $A_g$ (fully symmetric) and $B_{2g}$ ($ac$-quadrupole-like) symmetries ($D_{2h}$ point group) and study their evolution with temperature and sulfur concentration. We demonstrate the change of selection rules at temperatures below the orthorhombic-to-monoclinic transition at $T_c\left(x\right)$ that is related to the EI phase. We find that $T_c\left(x\right)$ decrease monotonically from 328 K for ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$ to 120 K for ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{S}}_5$ and that the magnitude of lattice distortion also decreases with the sulfur concentration $x$. For $x<0.7$, the two lowest frequency $B_{2g}$  phonon modes show strongly asymmetric line shapes at high temperatures due to Fano interference with the broad excitonic continuum present in a semimetallic state. Within the framework of extended Fano model, we develop a quantitative description of the interacting exciton-phonon excitation line shape, enabling us to derive the intrinsic phonon parameters and determine the exciton-phonon interaction strength that affects the transition temperature $T_c\left(x\right)$ . While at low temperatures the intrinsic phonon parameters are in good agreement with the ab initio calculations and the anharmonic decay model, their temperature dependencies show several anomalous behaviors: (i) Frequencies of $B_{2g}$ phonons harden pronouncedly upon cooling in vicinity of $T_c\left(x\right)$ for $x<0.7$ semimetals and, in contrast, soften monotonically for ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{S}}_5$ semiconductor; (ii) the lifetime of certain phonons increases strongly below $T_c\left(x\right)$ for $x<0.7$, revealing the gap opening in the broken symmetry phase; and (iii) for most modes, the intensity shows rather strong temperature dependence that we relate to the interplay between electronic and phononic degrees of freedom. For ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$ we also observe signatures of the acoustic mode scattered assisted by the structural domain walls formed below $T_c$. Based on our results, we additionally present a consistent interpretation of the origin of oscillations observed in time-resolved pump-probe experiments.

• Received 22 April 2021
• Accepted 9 June 2021

DOI:https://doi.org/10.1103/PhysRevB.104.045102