Structural phase transition in CuFe2O4 spinel

A structural transition with a reduction in symmetry of the high temperature cubic phase (sp. gr. Fd3m) to the tetragonal phase (sp. gr. I4(1)/amd) and the appearance of a ferrimagnetic structure occur in CuFe2O4 copper ferrite at T approximate to 440A degrees C. It is established by an experiment on a high-resolution neutron diffractometer that the temperature at which long-range magnetic order occurs is higher than that of tetragonal phase formation. When cooling CuFe2O4 spinel from 500A degrees C, the equilibrium coexistence of both phases is observed in a fairly wide temperature range (similar to 40A degrees C). The composition studied is a completely inverse spinel in the cubic phase, and in the tetragonal phase the inversion parameter does not exceed few percent (x = 0.06 +/- 0.04). At the same time, the phase formed upon cooling has a classical value of tetragonal distortion (gamma approximate to 1.06). The character of temperature changes in the structural parameters during the transition from cubic to tetragonal phase indicates that this transition is based on the Jahn-Teller distortion of (Cu,Fe)O-6 octahedra rather than the mutual migration of copper and iron atoms.