Cation Distribution in Spinel Ferrite Nanocrystals: Characterization, Impact on their Physical Properties, and Opportunities for Synthetic Control

Metal oxide materials that adopt the spinel crystal structure, such as metal ferrites (MFe2O4), present tetrahedral (A) and octahedral B sublattice sites surrounded by oxygen anions that provide a relatively weak crystal-field splitting. The formula of a metal ferrite material is most precisely described as (M1-xFex)MxFe2-xO4, where the parentheses and square brackets denote the tetrahedral and octahedral sites, respectively, and x is the inversion parameter quantifying the distribution of M2+ and Fe3+ cations among these sites. The electronic, magnetic, and optical properties of spinel ferrites all depend on the magnitude of x, which, in turn, depends on the relative sizes of the cations, their charge, and the relative crystal-field stabilization afforded by tetrahedral or octahedral coordination. Compared to bulk spinel ferrites, the large surface-area-to-volume ratio of spinel ferrite nanocrystals provides additional structural degrees of freedom that enable access to a broader range of x values. Achieving synthetic control over the degree of inversion in addition to the size and shape is critical to tuning the properties of spinel ferrite nanocrystals. In this Forum Article, we review physical inorganic methods used to quantify x in spinel ferrite nanocrystals, describe how the electronic, magnetic, and optical properties of these nanocrystals depend on x, and discuss emerging strategies for achieving synthetic control over this parameter.