Topotactic crystal structure transformation from spinel ferrite to wüstite in epitaxial Fe3O4 films via Kr ion irradiation

Krypton ion irradiation was carried out on Fe3O4 epitaxial thin films grown on MgO ( 0 0 1 ) substrates. The spinel structure ( F d 3 ¯ m ) of Fe3O4 was found to be transformed into another structure as a result of the ion irradiation without any loss of the epitaxial relationship with the MgO substrate. Both total film thickness and, therefore, the composition ratio between Fe and O remained the same even after the ion irradiation process, as confirmed by both cross-sectional transmission electron microscope images and Rutherford backscattering analysis. An x-ray diffraction experiment revealed that the irradiated crystal possessed a rocksalt structure ( F m 3 ¯ m ), with the lattice constant being approximately half of that of Fe3O4. Ion irradiation, thus, gave rise to a topotactic crystal transformation to wüstite with a composition of Fe0.75O, which is a metastable phase of iron oxides.