Infrared Spectroscopy and DFT Calculations of Electronic and Lattice Vibrational Properties in FeGa₃

The intermetallic Fe-based semiconductors FeSb₂, FeSi and FeGa₃ distinguish themselves because of reports of Seebeck coefficients in excess of 10,000 μV/K. However, there is not yet a clear consensus on the origin of such strong thermoelectric effect in these compounds and two main scenarios are invoked: the phonon-drag effect, due to a strong coupling of lattice vibrations to the mobile charges, and electron-electron correlations, manifested in the band structure, magnetic and electrical transport properties. Here we address some of these questions in FeGa₃ by means of combined experimental measurements of infrared reflectance and Density Functional Theory (DFT) calculations. Optical band gap determined experimentally is confirmed by the theoretical values, and most noticeably, there is remarkable agreement between the observed phonon frequencies and the density functional perturbation theory (DFPT) calculations. We will discuss further the evolution of the spectral weight with temperature and the structure of the vibrational modes, as well as their potential implications on the large Seebeck coefficient in FeGa₃.