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

摘要

Polycrystalline CuFe_2O_4samples have been successfully prepared using a simplecombustion method starting from citrate-nitrate precursors. All the ferrite samples studiedwere prepared at the same temperatures (300, 600, 900°C), but one series was slowly cooledfrom 900°C to room temperature, and another was quenched in solid CO_2. The nature of thestructural transition in inverse spinels has been investigated by the synchrotron XRD tech-nique. 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 ofСu~(2+)ions from octahedral to tetrahedral sites This phenomenon transforms the tetragonally distorted spinel structure (F4_1/ddm)into a cubic lattice(Fd3m).It occurs at temperaturesfrom ~ 360°С to~ 420°C.