Influence of Synthesis Method on The Properties of The Spinel High Entropy Oxide

Understanding and controlling sample dependence is a critical step towards any material achieving widespread use in applications. High entropy oxides (HEOs) are a class of materials with immediate potential for an array of applications but whose sample dependence has the potential to be particularly profound due to their high degree of configurational disorder. In this work, we seek to clarify the extent of sample dependence in HEOs by directly comparing the structural, magnetic, and electronic properties of an HEO with the spinel structure synthesized by five distinct methods. These methods (solid state, high pressure, hydrothermal, molten salt, and combustion) each provide distinct levels of kinetic and thermodynamic control in the growth process. The spinel HEO’s ferrimagnetic structure was determined from neutron diffraction, whilst the site distribution of the cations in each sample was calculated using x-ray absorption spectroscopy. This thorough structural and magnetic characterization allowed us to identify the likely origins of variation in the magnetic behavior of our samples. We find that while distinct differences in homogeneity, sharpness of the magnetic ordering transition, and cation distribution are apparent, the most technologically important properties, including ordering temperature (» 400 K) and saturated moment (» 1.8 µB/F.U.), are highly robust to preparation method, with the most extreme cases remaining within 10 % of one another. On the other hand, we observe a 500% variation in the sharpness of the onset of magnetic order between the most and least spatially homogenous samples. Likewise, we observe a 250% variation in coercive field, highlighting the effect of spatial homogeneity in the properties of HEOs. We conclude that this overall robustness to synthesis method make HEOs excellent candidates for a variety of industrial processes.