The nonmagnetic doublet systems $\mathrm{Pr}T{r}_2{\mathrm{Zn}}_{20}$ ($Tr=\mathrm{Rh},\mathrm{Ir}$) exhibit antiferroquadrupolar (AFQ) ordering in zero magnetic field. Under applied fields, the AFQ ordering is destroyed, and a field-induced phase (FIP) transition appears, although the mechanism remains elusive. A vibronic state below 2 K, characterized by the degeneracy of clockwise and anticlockwise “rotation,” and its breakdown at the FIP transition were proposed. We performed ultrasonic measurements of ${\mathrm{PrIr}}_2{\mathrm{Zn}}_{20}$ in magnetic fields up to 28 T at temperatures down to 0.03 K. The field ($\mathit{H}\parallel \left[110\right]$) dependence of the elastic modulus $(C_{11}-C_{12})/2$ was measured for two configurations in which the field was applied along the direction of ultrasound propagation or displacement. At 4.2 K, the difference between the two datasets is weakly dependent on the field and is reproduced by a model considering the ordinary rotational invariance effect. However, at 0.18 K, the data exhibit spontaneous separation above the FIP boundary, and this difference is not reproduced by the above model. Our experimental and theoretical results reveal an unconventional rotational effect on the elastic modulus, demonstrating that the FIP transition arises from the breakdown of the vibronic state. Above the FIP boundary, the electronic state adopts a chirality of clockwise or anticlockwise “rotation.”

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• Chirality
• Elastic modulus
• Multipole order
• Rotational states
• Vibronic effects
• Ultrasound techniques