This past week, both Sieun and Peter gave talks at the annual American Ceramic Society Electronic materials and applications (EMA) conference on defect formation and magnetic disorder in entropy stabilized oxides.

Defect and disorder driven dielectric properties of entropy-stabilized oxides.

Abstract: Entropy-stabilized oxides (ESO) are a solid solution of five or more binary oxides in a single lattice, stabilized by the large configurational entropy from cationic disorder. Due to their tunable chemical heterogeneity and intrinsic disorder, ESO are expected to demonstrate novel functional behavior. Point defects in oxides, however, can have a strong influence on functional properties, yet an understanding of point defects in ESO is unknown. Here we present on a theoretical and experimental investigation of point defects and disorder in (MgCoNiCuZn)O-based ESO using density functional theory (DFT) and dielectric measurements. We theoretically predicted that the thermodynamic stability of vacancies in ESO strongly depends on their nearest-neighbor configuration, indicating that the types and concentrations of defects can be tuned by the composition of cations, particularly Cu. Our calculated dielectric constant varies depending on vacancy and cation composition. To experimentally characterize these materials, we have integrated single crystalline entropy-stabilized oxide thin films into vertical capacitor devices by using MgO/SrTiO₃ buffered conductive Si substrates and performed dielectric testing over a wide range of frequencies. We varied the composition of the films and observed the effect of local lattice distortion that arises from the composition of Cu on the dielectric behavior of ESO.

Magnetic Frustration Engineering Through Stereochemical Disorder in Single Crystalline Entropy-Stabilized Oxides

Abstract: A unique benefit to entropic stabilization is the increased solubility of elements, which opens a broad compositional space with subsequent local chemical and structural disorder resulting from different atomic sizes and preferred coordinations of the constituents. It is unknown, however, to what degree this structural disorder contributes to material functionalities. In the antiferromagnetic entropy-stabilized oxides studied here, we see that by tuning the concentration of local structural frustrations caused by Jahn-Teller active cations, we induce or reclaim a large degree of disorder in the magnetic lattice of the material. This effect can be utilized to tune the anisotropy and magnetic structure of the oxide to approach that of an isotropic spin glass, yet still in a single crystalline material. Our results reveal that the unique characteristics of entropy stabilized materials can be utilized to realize novel magnetism in oxide thin films.

New Publication! S. Chae, K. Mengle, J. T. Heron & E. Kioupakis Appl. Phys. Lett. 113, 212101 (2018)

Abstract: We apply hybrid density functional theory calculations to identify the formation energies and thermodynamic charge transition levels of native point defects, common impurities, and shallow dopants in BAs. We find that AsB antisites, boron-related defects such as VB, BAs, and Bi-VB complexes, and antisite pairs are the dominant intrinsic defects. Native BAs is expected to exhibit p-type conduction due to the acceptor-type characteristics of VB and BAs. Among the common impurities we explored, we found that C substitutional defects and H interstitials have relatively low formation energies and are likely to contribute free holes. Interstitial hydrogen is surprisingly also found to be stable in the neutral charge state. BeB, SiAs, and GeAs are predicted to be excellent shallow acceptors with low ionization energy (<0.03 eV) and negligible compensation by other point defects considered here. On the other hand, donors such as SeAs, TeAs SiB, and GeB have a relatively large ionization energy (∼0.15 eV) and are likely to be passivated by native defects such as BAs and VB, as well as CAs, Hi, and HB. The hole and electron doping asymmetry originates from the heavy effective mass of the conduction band due to its boron orbital character, as well as from boron-related intrinsic defects that compensate donors.

Full text available from Applied Physics Letters

Peter is off to Los Alamos National Laboratory for several months where he will be working with Aiping Chen at the Center for Integrated Nanotechnologies (CINT) on growth kinetics of entropy stabilized oxides.

Peter was chosen to receive the Graduate Excellence in Materials Science (GEMS) award from the American Ceramic Society (ACerS). Congratulations!

Peter gives a talk on the magnetism of entropy-stabilized oxides at MS&T 2018 in Columbus, OH, entitled Structurally Driven Magnetic Disorder in Entropy-Stabilized Oxides.

Abstract: A unique benefit to entropic stabilization is the increased solubility of elements, which opens a broad compositional space with subsequent local chemical and structural disorder resulting from different atomic sizes and preferred coordinations of the constituents. In the antiferromagnetic entropy-stabilized oxides studied here, we see that by tuning the chemistry, and thus the concentration of local structural distortions, we can either induce or reclaim a large degree of frustration in the magnetic lattice of the material. This effect can then be engineered to enhance the strength of the magnetic exchange field by a factor of 10x in ferromagnetic/antiferromagnetic heterostructures, when compared to a “normal” antiferromagnetic oxide, such as CoO. Our results reveal that the unique characteristics of entropy stabilized materials can be utilized to engineer and enhance magnetic functional phenomena in oxide thin films, as well as offer a powerful platform for the study of defects and functional properties.