Dynamics of the BiTeI lattice at high pressures

Raman measurements of the phonon spectrum of BiTeI at pressures of up to $20$ GPa have been performed. A decrease in the linewidth of E$^2$ vibration by almost a factor of 2 with an increase in the pressure to $3$ GPa has been detected. The frequencies of all four Raman active modes increase monotonically with the pressure. These lines are observed in spectra up to $\sim$8 GPa. Sharp change in the spectrum occurs at pressures of $8$–$9$ GPa, indicating a transition to the high-pressure phase, which holds up to $20$ GPa. This transition is reversible and hardly has any hysteresis. A sample in the high-pressure phase is single crystal.