Anharmonic effect on the vibrational properties of pristine and Co-doped β-FeSi2 semiconductors

The strength of the phonon anharmonic effect of the pristine FeSi2 and Co-doped Fe0.94Co0.06Si2 is investigated by a Raman scattering study on the vibrational properties of those materials in the temperature range of 300–1523 K. All the vibrational modes exhibit significant redshifts with increasing temperature, and their spectral widths increase simultaneously. The structure transition from the semiconducting β phase to the metallic α phase is evidenced by the sudden disappearance of the vibrational modes. The extended Klemens model is applied to study the anharmonic effect on the phonon frequency shift and damping constant, and the four-phonon decaying process is expected to be the dominant one after doping the metal Co. Such an enhancement is also suggested contributing to the reduction of the thermal conductivity in Fe0.94Co0.06Si2. In addition, the vibrational properties of the mode at 250 cm−1 are more sensitive to the anharmonicity effect than that of the mode at 195 cm−1. This work provides valuable insights for understanding the high-order anharmonic effects in thermoelectric materials, especially in chemically doped materials.