Phonon properties in narrow gap FeSi and FeSb2 single crystals

We review the phonon analysis of FeSi and present a new study of the Raman and infrared (IR) active phonons of the related compound FeSb2. The measurements are supplemented by a factor group analysis, enabling the full mode assignment. While in FeSi, all nine Raman active modes predicted by the theory are visible in our spectra, in FeSb2, three of the six predicted modes have been observed. The unusual temperature dependence of the Raman linewidth of FeSi has been interpreted in terms of electron-phonon interactions. In FeSb2, a Fano-like IR absorption is reported at 300K, indicating electron-phonon coupling. The asymmetry vanishes and the damping constant decreases drastically by cooling, in correspondence with the depletion of the charge carrier concentration.