Mixed-valent compounds of iron and other transition metals show a wide range of physical and chemical properties. Understanding the complexities of their structures and magnetism provides insights that can be used to design materials with specific desirable properties.
CaFe3O5 is a mixed-valent compound that exhibits charge ordering with the formation of ferromagnetic trimers of the form Fe3+–Fe2+–Fe3+. These dictate the low temperature magnetic structure, as shown below in the figure. This investigation used neutron powder diffraction on WISH, Monte Carlo simulation and symmetry analysis to probe the magnetic ordering in perfectly stoichiometric CaFe3O5, including the observation of an incommensurate magnetic phase forming at the onset of long range magnetic ordering.
Previous work on a sample that was slightly off-stoichiometric [Nat. Commun. 9, 2975 (2018)] had shown phase separation on cooling into a charge-averaged phase with a different magnetic structure, and the charge-ordered phase. This new study shows that the phase separation is not found in stoichiometric CaFe3O5 and, together, the works emphasise the subtleties of these mixed-valent phases, and indicate that changing the material’s exact composition and microstructure could lead to subtle control of its physical properties.
Related publication: “Single phase charge ordered stoichiometric CaFe3O5 with commensurate and incommensurate trimeron ordering.” Nat Commun 10, 5475 (2019)
Authors: SJ Cassidy (University of Oxford), F Orlandi, P Manuel (ISIS), SJ Clarke (University of Oxford)