  We have performed Raman-scattering measurements on a high-quality single
crystal of the recently discovered Fe-based superconductor
K$_{0.8}$Fe$_{1.6}$Se$_2$ ($T_c$ = 32 K). At least thirteen phonon modes were
observed in the wave number range 10$-$300 cm$^{-1}$. The spectra possess a
four-fold symmetry indicative of bulk vacancy order in the Fe-deficient planes.
We perform a vibration analysis based on first-principles calculations, which
both confirms the ordered structure and allows a complete mode assignment. We
observe an anomaly at $T_c$ in the 180 cm$^{-1}$ $A_g$ mode, which indicates a
rather specific type of electron-phonon coupling.
