Novel electronic and magnetic properties of high Entropy Oxides

本项目致力于探索具有奇异电子和磁性质的新型高熵氧化物（HEOx）。HEOx是最近发现的一类新型材料（C. M. Rost等人，Nat. common . 2015），通过将金属合金中众所周知的高熵材料概念扩展到氧化物。它们包含至少五种不同的阳离子，并通过高温加热和随后的淬火获得。当温度足够大时，构型熵在吉布斯能量中占主导地位，这驱动了亚稳态固溶体的形成，而不是焓驱动相。这种熵稳定氧化物可以通过淬火在室温下冷冻。第一个合成的HEOx (MgCoNiCuZn)O具有简单的岩盐结构。有趣的是，一些二元氧化物在岩盐结构中不结晶，甚至不形成固溶体。其他类型的钙钛矿和尖晶石结构的HEOx已被报道，这表明有大量的材料有待探索。不利的阳离子掺杂剂，如八面体中的Zn2+，可能会被引入HEOx中。这使得HEOx在新型氧化物材料的开发中具有很大的前景。我们将在本项目中探索含有钙钛矿、焦绿石、尖晶石及相关结构的新型HEOx。HEOx不仅是一个材料的宝库，也是一个意想不到的功能宝库，部分原因是由于非常规阳离子排列的局部晶格扭曲。在岩盐型HEOx中，我们发现了意想不到的功能，如巨大的介电响应，锂的高离子迁移率超过1 mS/cm，以及远程反铁磁有序。这些新的特性和功能不应局限于岩盐HEOx。通过在过渡金属氧化物中引入相关电子的概念，可以预测前所未有的电子和磁性能。相关的过渡金属氧化物通常具有竞争的电子态，并且借助细微的晶格畸变从它们中选择出独特的基态。在heox型相关氧化物中，结构无序可能使系统无法选择其基态，从而导致奇异的波动相和功能。预期的相/功能包括一个巨大的磁电效应，一个高性能的热电和一个奇异的磁性，如量子自旋液体，这将是这个项目的目标。通过两个团队的合作，即熵稳定材料的先驱（法国合作伙伴）和相关电子物理的领导小组（德国合作伙伴），我们的目标是在电子和功能氧化物的研究中开辟一个新的领域。

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.