We report low-temperature Raman signatures of the Heisenberg quantum magnet ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$, showing clear anomalies in phonon mode frequencies and linewidths below $\sim 100$ K. This crossover temperature lies in between the Jahn-Teller (JT) temperature scale ($>\text{room}$ temperature) and the temperature scale associated with the spin exchange interactions ($<12$ K). Our experimental observation is well captured by a secondary JT transition associated with a cooperative reorientation of the orbitals giving rise to anomalies in the temperature dependence of Raman frequencies and linewidths. Such orbital reorganization, in turn, affects the spin-spin exchange interactions that decide the fate of the magnet at lower temperatures and hence provide important clues to understand the energetics of the possible lower-temperature quantum paramagnetic phase.

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• Jahn-Teller effect
• Orbital order
• Perturbation theory
• Raman spectroscopy