Revealing plasmon-phonon interaction in nanocrystalline MgFe2O4 spinels by far-infrared reflection spectroscopy

Room-temperature far-infrared reflectivity spectra of nanocrystalline, partially inverse MgFe2O4 were investigated. MgFe2O4 samples were prepared by sol-gel method and sintered at three different temperatures (400, 600 and 800 °C). Raman spectroscopy was employed to estimate the degree of inversion in the sintered samples. The degree of inversion was found to increase from 0.52 to 0.74 as the sintering temperature increased from 400 °C to 800 °C. The reflectivity spectra, besides the four infrared modes characteristic for spinels (ν1, ν2, ν3, ν4), revealed the presence of free carriers. Plasmon-longitudinal optical (LO) phonon interaction was analyzed using factorized coupled and decoupled plasmon-phonon models, combined with the Bruggeman effective medium approximation. From these models it was obtained that the ν1 and ν3 phonon modes are more strongly coupled with plasmons than the ν2 mode. A potential mechanism of plasmon-phonon interaction in inverse MgFe2O4 spinel has been discussed.