Infrared response ofα- andβ-phases of LiFe5O8

Abstract

We report the infrared reflectivity of the inverse spinel LiFe${}_{5}$O${}_{8}$ in the B-site-ordered ($\ensuremath{\alpha}$) and B-site-disordered ($\ensuremath{\beta}$) phases. The infrared conductivity of the two phases, obtained through Kramers-Kronig analysis, is dominated by phonon features. The frequencies of the TO and LO phonons in the $\ensuremath{\alpha}$ phase are extracted through Lorentz-oscillator fit and they are in a good agreement with the shell-model lattice-dynamics calculations. In the disordered $\ensuremath{\beta}$ phase the number of infrared-active phonons is reduced compared to that in the ordered phase. The phonons in the $\ensuremath{\beta}$ phase are systematically broadened with respect to the corresponding phonons in the $\ensuremath{\alpha}$ phase, and their spectral shape is modeled much better by the Voigt rather than Lorentzian profile. We demonstrate that the spectral modifications in the frequency domain above 300 cm${}^{\ensuremath{-}1}$ are a result of an oscillator strength redistribution among normal modes upon the order-disorder transition.