Thermal instability of the tetragonally distorted structure of copper-iron materials

Polycrystalline CuFe2O4 samples have been successfully prepared using a simple combustion method starting from citrate-nitrate precursors. All the ferrite samples studied were prepared at the same temperatures (300, 600, 900 degrees C), but one series was slowly cooled from 900 degrees C to room temperature, and another was quenched in solid CO2. The nature of the structural transition in inverse spinels has been investigated by the synchrotron XRD technique. Slowly cooled samples are subjected to a diffusionless order-disorder transformation, due to the modified of the orientation of the Jahn-Teller distortions, and to the migration of Cu2+ ions from octahedral to tetrahedral sites This phenomenon transforms the tetragonally distorted spinel structure (F4(1)/ddm) into a cubic lattice (Fd (3) over bar m). It occurs at temperatures from similar to 360 degrees C to similar to 420 degrees C.