High Entropy Oxides: Structure and Properties

Abstract

Since the origin of humankind numerous approaches have been employed to develop new materials. Of these approaches, changing the composition of a given system, typically referred as alloying for metallic and doping for non-metallic systems, is undoubtedly the most common way of designing new materials. Conventionally, alloying or doping implies introduction of relatively small amounts of secondary elements to a base system. The base system typically consists of one major component, e.g., Fe for steels, while in yttria-stabilized zirconia, ZrO2 is considered as the base system. The concept of high entropy materials (HEMs) can be considered as an extreme adaption of alloying or doping, where five or more elements all in (nearly) equal proportions are incorporated into a system. Hence, there is no base element (“baseless”) as such in HEMs. Their unexpected tendency to form single phase solid solutions despite the high chemical complexity makes HEMs unique. Essentially, the combination of several elements in near equiatomic proportion enhances the configurational entropy of HEMs. It is believed, in some cases proven, that this enhanced configurational entropy drives the formation of a single phase solid solution. Due to these distinctive features, the high entropy based design concept is often considered as an original approach, and not a mere extension to alloying or doping. The subject of studies in this doctoral thesis is high entropy oxides (HEOs). HEOs are phase-pure solid solutions arising from the inclusion of five or more elements into the cationic sub-lattice(s) of oxide materials. Building upon the initial reports on HEOs, the first one published in 2015, the main objective of this work is to investigate the unexplored regions of oxide compositions and structures offered by the high entropy based design approach. The initial task was the identification and optimization of a suitable synthesis technique for fabrication of HEOs with rocksalt and fluorite structures. In this regard, several techniques, each possessing certain advantages and disadvantages, were explored. Out of these considered ones, aerosol based nebulized spray pyrolysis (NSP) was found to be the most versatile technique for preparation of HEOs on a laboratory scale and was used as the primary synthesis tool in this thesis. The exploration of new HEO systems with different compositions and crystallographic structures was the next challenge. Perovskite type HEOs (P-HEOs) were developed, in which up to 10 different cations in equiatomic proportion can be homogeneously incorporated into a single-phase orthorhombic structure. Besides the synthesis aspect, emphasis was placed on comprehensive understanding of the underlying phase stability mechanisms in different crystal types of HEOs, such as rocksalt, fluorite and perovskite. It was observed that the governing principles were rather distinct for different types of HEOs. In some cases, such as in rocksalt-HEOs (R-HEOs), an entropy-driven phase transformation is dominant, whereas in the other HEOs, aspects like tolerance factors, oxidation state of the cations and related internal charge compensations play determining roles. Apart from these structural investigations, a major part of this doctoral work is dedicated to explore the functional properties of HEOs. Oxides, in general, show rich structure-composition-property relationships. Hence, the properties of HEOs were explored based on their crystal structure and composition characteristics. Three different classes of properties were investigated: electrochemical, optical and magnetic. Transition metal (TM) based R-HEO was probed as electrode material for secondary Li-ion batteries (LIBs). Highly reversible lithium storage capacities (above 600 mAh/g for more than 900 cycles) were observed. A major part of the capacity is drawn from electrochemical reactions below 1 V (vs Li+/Li), which warrants its possible use as an anode in LIBs. Importantly, a unique electrochemical reaction mechanism, possibly stemming from an entropy effect, was discovered. Rare earth (RE) based fluorite-HEOs (F-HEOs), on the other hand, showed interesting optical properties like narrow band gap of ∼2 eV, which could be reversibly tuned (from 2 – 3.2 eV) by conducting heat treatments under different atmospheres. Element specific techniques, like X-ray absorption spectroscopy (XAS) and energy electron loss spectroscopy (EELS), allowed to disentangle the individual effects of the constituent cations in complex F-HEOs and identify the relevant features in the electronic band structure underpinning the observed reversible changes in the optical behavior. The reason behind the change in band gap is closely associated with the presence of redox active multivalent cations, like Pr3+,4+, which result in the formation of intermediate unoccupied energy states. Finally, P-HEOs comprising of multiple RE cations on the A-site and/or multiple TM cations on the B-site exhibited an interesting interplay between the magnetic exchange interactions and the high degree of chemical disorder in the systems. Additional ferromagnetic interactions in otherwise predominant antiferromagnetic environment leading to exchange anisotropy were observed in phase-pure P-HEOs, wherein the former could be attributed to either small ferromagnetic clusters or spin canting. In brief, this doctoral work highlights the versatility of the high entropy based design concept in oxides by demonstrating the structure-property relationships in three different crystal structure types of HEOs. As the research on HEOs is still in its early state, a plethora of fundamental aspects of HEOs are yet to be explored to assess their full potential for practical applications.