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Investigating the synthesis and properties of transition-metal medium & high entropy oxides Johnstone, Graham

Abstract

High entropy oxides (HEOs) are compounds defined as having a crystallographically ordered structure with substantial configurational entropy from having up to five elements occupying a site equivalent sublattice. In certain HEOs, the disorder functions as the driving force behind the stabilization of the crystal structure. Therefore, HEOs are capable of providing host to hitherto unexplored combinations of elements, particularly rare earth and 3d/4d transition metals, regardless of atomic mass, radii, charge, and magnetic character. The applications of these materials include reversible energy storage, components for electronic devices, and improved optical coatings. In this thesis I present two of my contributions to the domain of entropy materials from their exploratory synthesis to a robust characterization of their magnetic and physical properties. The first topic is focused on the magnetic properties of spinel-type HEO (MnCrFeCoNi)₃O₄ and a novel series of magnetic dilutions through the imposition of gallium onto the magnetic sublattice. Using SQUID magnetometry and neutron diffraction we confirm the presence of long-range antiferromagnetic order persistent throughout the parent and magnetically diluted samples. Moreover, magnetization as a function of field data reveals that replacing magnetic transition metals with gallium remarkably enhances the saturation and retentivity of the bulk magnetic moment. Shifting perspectives, we also explored a novel four component medium entropy oxide, (TiHfZrSn)O₂. The purpose of this investigation was to exploit the high tolerance for a wide dispersion in atomic mass among entropy materials in order to convolute the phonon scattering and recover enhanced thermal insulating properties. After fitting our measurements of heat capacity to a Debye model, we calculate the Debye temperature TD = 415.03 +/- 0.25 K. This is considerably lower than TD for the individual oxides and motivates further study of the phonon scattering with direct methods. Through further investigation into the αPbO₂ structure of (TiHfZrSn)O₂, we suspect that entropy stabilization plays a role in the structure formation for this compound. This is based on an investigation in the literature for a similar compound, (TiZr)O₄ in αPbO₂, where the entropy stabilized structure was realized almost two decades prior to the first reported entropy stabilized HEO.

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