Neutron Diffuse Scattering Study of the High Temperature Phase of Fe ₃O ₄ - I, Determination of Atomic Displacements at the X Point in the Brillouin Zone

Diffuse scattering of neutrons from a single crystal of Fe3O4 was measured at 125 K, in the high temperature phase, in a wide range of the (001) and (0 (1) over bar 1) plane. The scattering have been interpreted as due to the lattice distortion accompanied by the charge fluctuation in the B site Fe ions of the spinel structure. Intensities at 19 X points were analysed on an assumption that the displacement of atoms are composed of phonon modes belonging to one symmetry representation of the space group of the whole spinel lattice, instead of the symmetry of some lattice points. It was concluded that the symmetry of the atomic displacement is X-3, containing 12 bases. Quantitative estimate was given for the contribution of each mode and the dynamic structure of a large polaron was determined. The structure is completely different from those proposed so far, on the Fe2+ - Fe3+ ordering model.