Oxides and the high entropy regime: A new mix for engineering physical properties
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MRS Advances © 2020 Materials Research Society . This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. DOI: 10.1557/adv.2020.295
Oxides and the high entropy regime: A new mix for engineering physical properties P. B. Meisenheimer1 and J. T. Heron1 1
University of Michigan, Department of Materials Science and Engineering, 2300 Hayward St, Ann Arbor, MI, USA, 48109
ABSTRACT
Historically, the enthalpy is the criterion for oxide materials discovery and design. In this regime, highly controlled thin film epitaxy can be leveraged to manifest bulk and interfacial phases that are non-existent in bulk equilibrium phase diagrams. With the recent discovery of entropy-stabilized oxides, entropy and disorder engineering has been realized as an orthogonal approach. This has led to the nucleation and rapid growth of research on high-entropy oxides – multicomponent oxides where the configurational entropy is large but its contribution to its stabilization need not be significant or is currently unknown. From current research, it is clear that entropy enhances the chemical solubility of species and can realize new stereochemical configurations which has led to the rapid discovery of new phases and compositions. The research has expanded beyond studies to understand the role of entropy in stabilization and realization of new crystal structures to now include physical properties and the roles of local and global disorder. Here, key observations made regarding the dielectric and magnetic properties are reviewed. These materials have recently been observed to display concerted symmetry breaking, metal-insulator transitions, and magnetism, paving the way for engineering of these and potentially other functional phenomena. Excitingly, the disorder in these oxides allows for new interplay between spin, orbital, charge, and lattice degrees of freedom to design the physical behavior. We also provide a perspective on the state of the field and prospects for entropic oxide materials in applications considering their unique characteristics. 1
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INTRODUCTION High-entropy materials, popularized by the discovery of high-entropy metal alloys ~20 years ago1–4, typically include 5 or more atomic species and are kinetically frozen into a metastable solid solution phase that is stable at high temperature5,6. These materials are characterized by both rule-of-mixtures and cocktail effect behavior and are nominally disperse on an atomic scale1,7,8. In an entropy-stabilized material, the configurational entropy contribution to the Gibbs’ free energy drives the formation of a single phase solid solut
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