Time effect of annealing on phase transformations of Cu-Al-Cr nano-ferrites prepared by a co-precipitation method

Amounts of the as-prepared CuAl0.6Cr0.2Fe1.2O4 nanoparticles by the co-precipitation method have been annealed for different time t for each amount at 800 °C. The samples were characterized using XRD, IR and Mössbauer spectroscopy. The structure of the nanoparticles was transformed from cubic-to-tetragonal-to-cubic with the increase in t. This transformation was assigned to the Jahn-Teller effect (JTE) of Cu2+ ions. The transformation process from cubic to tetragonal occurred at annealing time 12 h, whereas the transformation from tetragonal-to-cubic occurred at t≥16 h. The crystallite size, lattice parameters, IR band positions and intensities, force constants, threshold frequency, Debye temperature and stiffness constant were affected by JTE and showed dependence on t. The Mössbauer spectra and parameters confirmed the transformation process by JTE and showed dependence on t. This study confirmed that the increase of annealing time can leads to moving the metallic ions between the crystal sublattices against their site preference and increasing the inversion of cation distributions.