Effect of Co2+ substitution on MgAl2O4 studied by infrared reflectance spectroscopy

Infrared reflectivity spectra are measured in the frequency range between 10 and 15,500 cm−1 at room temperature for cobalt substituted nanocrystalline magnesium aluminates MgAl2−xCoxO4 (x = 0.0, 0.5, 1.0, 1.5, 2.0). Four optical phonon modes manifesting the spinel structure have been observed and analyzed using the Lorentz oscillator model. The resonant frequency, oscillator strength and the damping factor of these modes were extracted from Lorentz oscillator fit to measured spectra. The observed reflectivity spectra exhibit a typical semiconductor behavior. Born and Szigeti effective charges have been estimated and the results are discussed to understand polarization and ionicity of MgAl2−xCoxO4 system.