A high-pressure investigation of the Weyl semimetals ${\mathrm{NbIrTe}}_4$ and ${\mathrm{TaIrTe}}_4$ is presented, using infrared spectroscopy supplemented by density functional theory calculations. The experimental optical conductivity spectra as a function of pressure suggest the occurrence of a pressure-induced phase transition at a critical pressure $P_{\text{c}}=7–8\mathrm{GPa}$. This transition is most likely electronic in nature, as Raman scattering measurements provide no evidence of a significant structural phase transition. Above $P_{\text{c}}$ a significant redistribution of spectral weight occurs in the optical conductivity spectrum for both materials. A Drude-Lorentz analysis of the optical data indicates a sharp reduction in the free-carrier concentration at $P_{\text{c}}$, concomitant with the appearance of a low-energy phonon, which was initially screened by free charge carriers. A predominantly electronic origin of the phase transition is supported by the calculated electronic band structure, Fermi surface, and interband optical conductivity as a function of pressure. Our findings provide collective evidence for a pressure-induced, most likely electronic phase transition in both van der Waals materials at $P_{\text{c}}=7–8\mathrm{GPa}$, highlighting the tunability of their electronic band structure by hydrostatic pressure.

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• Electronic structure
• Optical conductivity
• Phase transitions
• Topological phases of matter
• Weyl semimetal
• Density functional theory
• Fourier transform infrared spectroscopy
• Raman spectroscopy