Novel electronic and magnetic properties of high Entropy Oxides

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This project is devoted to explore novel high-entropy oxides (HEOx) with exotic electronic and magnetic properties. HEOx is a novel class of materials discovered recently (C. M. Rost et al, Nat. Commun. 2015), by extending to oxides the concept of high-entropy materials that is well-known for metallic alloys. They comprise at least five different cations and are obtained by heating at high temperature and subsequent quenching. When the temperature is large enough, the entropy of configuration becomes dominant in the Gibbs energy, which drives the formation of a metastable solid-solution instead of enthalpy driven phases. Such entropy-stabilized oxides can be frozen at room temperature by quenching. The first synthesized HEOx, (MgCoNiCuZn)O, has a simple rocksalt structure. Interestingly, some of the constituting binary oxides do not crystallize in the rocksalt structure and even do not form solid solutions. Other types of HEOx with perovskite and spinel structures have been reported, suggesting a huge variety of materials to be explored. Unfavorable cationic dopants such as Zn2+ in an octahedral site may be able to be introduced in the HEOx. These make HEOx quite promising in the development of new oxide materials. We will explore new HEOx with perovskite, pyrochlore, spinel and related structures in this project.HEOx serve not only as a mine of materials but also as a mine of unexpected functionalities, partly due to the local lattice distortions with unconventional cation arrangement. In the rocksalt-type HEOx, we have discovered unexpected functions such as a colossal dielectric response, a high ion mobility over 1 mS/cm for lithium and a long-range antiferromagnetic ordering. Such novel properties and functions should not be limited in the rocksalt HEOx. By introducing the concept to correlated electrons in transition-metal oxides, unprecedented electronic and magnetic properties can be anticipated. The correlated transition metal oxides often host competing electronic states and the unique ground state is selected from them with the help of subtle lattice distortion. In HEOx-type correlated oxides, the structural disorder may not allow the system to choose its ground state, leading to exotic fluctuating phases and functionalities. The expected phases/functions include a giant magnetoelectric effect, a high-performance thermoelectricity and an exotic magnetism such as a quantum spin-liquid, which will be the targets of this project.Through the collaboration of two teams, the pioneer of entropy-stabilized materials (the French partner) and the leading group in correlated electron physics (the German partner), we aim to open a new arena in the research on electronic and functional oxides.