We have performed polarized Raman scattering measurements on the newly discovered superconductor Ba$_{ 2}$Ti$_{ 2}$Fe$_{ 2}$As$_{ 4}$O$_{ 2}$ ($T_c = 21$ K). We observe seven out of eight Raman active modes, with frequencies in good accordance with first-principle calculations. The phonon spectra suggest neither strong electron-phonon nor spin-phonon coupling and vary only slightly with temperature, except for one E$_g$ mode associated with large displacements of As atoms near the Ti$_2$O planes. We also identify a small anomaly around 125 K in the linewidth of a A$_{ 1g}$ mode involving the same As atoms. Our results suggest that the transition at 125 K is most likely driven by electronic interactions taking place in the Ti$_2$O planes.
Categories: cond-mat.supr-con cond-mat.str-el