Structural, elastic, optical and dielectric properties of Li0.5Fe2.5 O4 nanopowders with different particle sizes

Abstract

LiFe5O8 (Li0.5Fe2.5O4) nanopowders were synthesized by the sol-gel auto-combustion method and were annealed at different temperatures of 500, 600, 700, and 1100 °C. The spinel phase formation of these samples has been confirmed by X-ray powder diffraction (XRD) technique and Fourier transform infrared (FTIR) spectroscopy. XRD analysis revealed order to disorder phase transition of Li0.5Fe2.5O4 during the annealing temperature. This structural transition was also confirmed by differential scanning calorimetry (DSC). The scanning electronic microscopy images reveal that nanoparticles size increases from 9 nm to 500 nm with increasing annealing temperature. The influence of particle size on elastic parameters has been investigated by FTIR spectroscopy. The band gap energy estimated by UV–vis spectroscopy of LiFe5O8 nanoparticles decreased from 1.41 to 1.27 eV when the annealing temperature increases. Furthermore, impedance spectroscopy measurements have been carried out over large frequency and temperature ranges. The conductivity spectrum indicates that the tested specimens are defective. It also shows that the conductivity runs to frequency according to Jonscher’s law. Static conductivity responds to temperature in conformity with a small polaron hopping model whereas dispersive one evaluates according to correlated hopping model. The conductivity gets mixed (electronic and ionic) only when samples were annealed at 700 °C and 1100 °C. According to the Nyquist diagram, each sample is capacitive and resistive. Increasing annealing temperature led to major improvements in permittivity, and a giant low-frequency dielectric constant was observed in the sample annealed at 1100 °C. The total loss (ɛ″) is governed by the conduction mechanism and described by Giuntini's theory. The improved electrical, optical, and elastic properties indicates the potential application LiFe5O8 nanoparticles in future multifunctional devices (microelectro-mechanical systems (MEMS), optoelectronic and photovoltaic, photocatalytic activity under visible light, bolometer, low temperature co-fired ceramics (LTCC) and gas sensor applications.).