We study the electronic phase diagram of the excitonic insulator candidates ${\mathrm{Ta}}_2\mathrm{Ni}{\left({\mathrm{Se}}_{1-x}{\mathrm{S}}_x\right)}_5$ $(x=0,\cdots ,1)$ using polarization resolved Raman spectroscopy. Critical excitonic fluctuations are observed that diminish with $x$ and ultimately shift to high energies, characteristic of a quantum phase transition. Nonetheless, a symmetry-breaking transition at finite temperatures is detected for all $x$, exposing a cooperating lattice instability that takes over for large $x$. Our study reveals a failed excitonic quantum phase transition, masked by a preemptive structural order.

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• Excitons
• Phase diagrams
• Quantum phase transitions
• Bose-Einstein condensates
• Raman spectroscopy