Infrared response of α -a ndβ-phases of LiFe5O8

We report the infrared reflectivity of the inverse spinel LiFe5O8 in the B-site-ordered (α) and B-site-disordered (β) 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 α phase are extracted through Lorentzoscillatorfitandtheyareinagoodagreementwiththeshell-modellattice-dynamicscalculations.Inthedisordered β phase the number of infrared-active phonons is reduced compared to that in the ordered phase. The phonons in the β phase are systematically broadened with respect to the corresponding phonons in the α 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 −1 are a result of an oscillator strength redistribution among normal modes upon the order-disorder transition.