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The prediction of new high entropy oxides (HEOs) remains a profound challenge due to their inherent chemical complexity. In this work, we combine experimental and computational methods to search for new HEOs in the tetravalent $A$O$_2$ family, using exclusively $d^0$ and $d^{10}$ cations, and to explain the observed phase stability of the $\alpha$-PbO$_2$ structure, as found for the medium entropy oxide (Ti, Zr, Hf, Sn)O$_2$. Using a pairwise approach to approximate the mixing enthalpy, we confirm that $\alpha$-PbO$_2$ is the expected lowest energy structure for this material above other candidates including rutile, baddeleyite, and fluorite structures. We also show that no other five-component compound composed of the tetravalent cations considered here is expected to form under solid state synthesis conditions, which we verify experimentally. Ultimately, we conclude that the flexible geometry of the $\alpha$-PbO$_2$ structure can be used to understand its stability among tetravalent HEOs.

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.

Working in small groups, students in ISCI 3A12 examine the science behind wine making, from the art of viticulture to its eventual consumption by the public. Students perform literature-based reviews and original research in order to understand the wine industry and its complexity; including the environmental requirements and consequences of winemaking, the short- and long-term health effects of drinking wine, and what factors contribute to the quality, aroma, and taste of wine. This research was formatted to resemble a publicly accessible scientific article, and compiled to create each Vintage of Terroir.