Rare-Earth Silicates as High-Temperature Surfactants for the Controlled Synthesis of ε-Fe₂O₃ Nanoparticles

The functional properties of nanocrystals can be finely tuned through controlled morphology and size. However, this can be challenging for metastable nanostructures that require harsh synthesis conditions, such as high temperatures. Here, we present a method for preparing large ε-Fe₂O₃ nanorods that are not affected by magnetic relaxation. This study presents a novel growth mechanism in which high-aspect-ratio rods evolve from spherical ε-Fe₂O₃ particles in a silica matrix containing Y³⁺. With the presence of Y³⁺, the glassy matrix undergoes a metastable binodal decomposition yielding the formation of nanodroplets of a Y-rich silicate of composition ∼Y₂Si₂O₇. This Y silicate selectively coats the ε-Fe₂O₃ planes perpendicular to the rod axis along the [100] direction but is not observed in the rod apexes. Structural optimizations and energy calculations of different crystal faces of ε-Fe₂O₃ in contact with Y₂Si₂O₇ obtained using machine-learning force fields provide an atomistic interpretation of these observations: the affinity of Y with the oxygen atoms exposed at ε-Fe₂O₃ surfaces explains the preferential capping of ε-Fe₂O₃ surfaces that present a large density of oxygen atoms and its absence in surfaces such as (100), where this density is significantly lower. The presence or absence of the silicate capping layer results in different surface energies and/or mass transfer coefficients across the interface, originating two independent Ostwald ripening processes, which drive the high aspect ratio growth. By using La³⁺ instead of Y³⁺, ε-Fe₂O₃ rods with even larger aspect ratios are obtained. Notably, this synthetic approach counteracts the progressive diminution of the average nanoparticle size observed in ε-(Fe_1-xCr_x)₂O₃ upon Cr³⁺ addition, enabling to elucidate the effect of this substitution on the intrinsic magnetic anisotropy and the anisotropy fields that determine the high-frequency ferromagnetic resonances of this phase.