Correlated vibrations of the tetrahedral and octahedral complexes and splitting of the absorption bands in FTIR spectra of Li-Zn ferrites

We report the infrared absorption properties of Li-Zn ferrite (Li0.5-x/2ZnxFe2.5-x/2O4, x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) powder samples prepared by combustion synthesis. The FTIR spectrum for each of the sample consists of two broad absorption bands: one at ∼ 510-800 cm−1 corresponding to the vibrations of tetrahedral complexes (ν1) and the other, at ∼ 300-510 cm−1 corresponding to the vibrations of octahedral-complexes (ν2) in the spinel structure of the Li-Zn ferrites. Each of these bands is further split into four sub-bands. We have explained the observed infrared absorption bands based on the structure and the occupancy of the cations. Pronounced splitting of the ν1 and ν2 bands in pure Li0.5Fe2.5O4 were observed due to the LiO6 complexes partially replacing the FeO6 complexes at the octahedral interstitial sites. The vibrational frequencies of the LiO6 complexes are different from FeO6 complexes due to the difference in their reduced mass and valence of the cations (lower bond strength). The resolved bands smear out with increasing Zn content (x) owing to higher statistical disorder in the occupancy of the cations at the tetrahedral and octahedral sites. We observe that the vibrations of the tetrahedral and octahedral complexes are not isolated but rather correlated. Our work helps in understanding the correlation of cation occupancy with the vibrational properties of nanocrystalline Li-Zn ferrites.