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的新型高熵氧化物(HEOx)。HEOx是最近发现的一类新材料(C.M.Rost等人，NAT.交警。2015)，通过将金属合金中众所周知的高熵材料的概念扩展到氧化物。它们由至少五种不同的阳离子组成，通过高温加热和随后的淬火获得。当温度足够大时，组态熵在吉布斯能中占主导地位，从而驱动亚稳固溶体的形成，而不是热焓驱动相的形成。这种熵稳定的氧化物可以通过淬火在室温下冻结。首次合成的HEOx为(MgCoNiCuZn)O，具有简单的岩盐结构。有趣的是，一些构成二元氧化物的化合物不在岩盐结构中结晶，甚至不形成固溶体。其他类型的具有钙钛矿和尖晶石结构的HEOx也有报道，这表明有大量的材料有待探索。可能会在HEOx中引入不利的阳离子掺杂剂，如八面体位置上的锌。这使得HEOx在开发新的氧化物材料方面非常有前途。在这个项目中，我们将探索具有钙钛矿、焦绿石、尖晶石和相关结构的新型HEOx。HEOx不仅是材料的矿藏，也是意想不到的功能矿藏，部分原因是局部晶格扭曲和非传统的阳离子排列。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1ms/cm，以及长程反铁磁有序。这种新的性质和功能不应该局限于岩盐HEOx。通过将这一概念引入过渡金属氧化物中的相关电子，可以预见前所未有的电子和磁性。相关的过渡金属氧化物经常含有竞争电子态，唯一的基态是通过微妙的晶格扭曲来选择的。在HEOx类型的相关氧化物中，结构无序可能不允许系统选择其基态，从而导致奇异的波动相和功能。预计的相/功能包括巨磁电效应、高性能热电和量子自旋液体等奇异磁性，这将是该项目的目标。通过两个团队的合作，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。这两个团队是熵稳定材料的先驱(法国合作伙伴)和关联电子物理领导小组(德国合作伙伴)。

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.