Charge ordering in B-sites of Fe3O4: 57 Fe NMR and Mossbauer study

Fe3O4, commonly known as magnetite, is a prototypical example of a magnetic compound which has attracted the attention of the scientific community for many centuries. Its intriguing magnetic features, coupled with electric properties, stem from the peculiar charge arrangement within its crystal structure, encouraging, among others, the emergence of ferrimagnetic ordering and charge-hopping phenomenon. Despite its popularity from both the theoretical and practical perspective, its low-temperature crystal structure with oxygen positions and octahedral distortions was correctly refined only a decade ago, leaving some empty space for an experimental description of local properties, especially in the Fe(B) sites of its crystal structure, including the iron valency. Thus, in this work, we advantageously used a combination of the 57Fe nuclear magnetic resonance technique and 57Fe Mo<spacing diaeresis>ssbauer spectroscopy with previously published density functional theory calculations for independent verification of the charge arrangement in Fe3O4 using isomer shifts in some Fe(B) positions of the Fe3O4 monoclinic structure. Under certain applied conditions, the values of isomer and quadrupole shift were unambiguously determined for these Fe(B) sites, with clear assignment of their valence state. A respective physicochemical model for interpreting the Fe3O4 low-temperature 57Fe Mo<spacing diaeresis>ssbauer spectra was constructed expanding that previously proposed. The results, presented here, would stimulate a future experimental work in searching for specific conditions and measuring geometry to distinguish the Mo<spacing diaeresis>ssbauer spectral contributions from all Fe(B) positions. Discerning all 16 Fe(B) spectral components would then unambiguously provide hyperfine parameters to respective Fe(B) sites, identifying their valence state and electric field gradient tensors from the experimental viewpoint, relating it to that found from theoretical calculations.