Compositionally complex materials – whose structure, stability and physical characteristics are determined by a disordered arrangement of atomic species within a regular crystalline lattice – have been at the centre of considerable attention in the last few decades. They offer virtually unlimited possibilities to design new materials with properties tailored for specific applications. Whereas high-entropy alloys have been studied extensively, far less is known about their non-metallic counterparts: the high-entropy ceramics. Dr Emanuel Ionescu, at the Technische Universität Darmstadt, is leading the Compositionally Complex Ceramics (C3) project, which aims to deepen our understanding of the physics of these fascinating materials and to develop efficient routes to their synthesis.
Most of the synthetic materials with which we come into contact in everyday life have a relatively simple microscopic structure, in which atoms, or groups of atoms, are arranged according to highly symmetric three-dimensional sequences, called crystal lattices. The physical properties of a given material are determined by how these elementary repeating units interact with each other. It has been known for centuries that adding impurity elements to a metallic material, which converts a pure metal into an alloy, can modify its properties, sometimes leading to highly desirable outcomes. For instance, adding small concentrations of carbon or manganese to iron (to obtain steel) drastically increases the material’s strength and resistance compared to the pure metal. This is related to the fact that the added atomic species hinder the relatively free motion of the iron atoms, increasing the rigidity of its lattice.
High-entropy materials
In standard alloys, like steel, one of the atomic species is by far the most abundant, and added elements appear in the lattice at relatively low concentrations in the form of impurities. In the early 2000s, an alternative alloying strategy was proposed, in which multiple elements are combined with similar concentrations to create new materials characterised by a much higher degree of compositional complexity than traditional alloys.
Compositionally complex nitride- and carbonitride-based ceramics.
Considering, for instance, five metallic elements combined to form one of these so-called high-entropy alloys and the number of available metallic elements from the periodic table, millions of potential alloy compositions may be accessed, each of which has its own structure and physical properties. This number grows to billions when six-component alloys are considered, even if only a small subset of the elements of the periodic table are included in the alloy formulation. A few high-entropy alloys have been shown to exhibit exceptional properties, far exceeding those of conventional alloys, and a virtually unlimited multi-dimensional compositional space remains to be explored. This could potentially uncover novel materials with unprecedented mechanical, electrical and optical properties.
Compositionally complex ceramics
Whereas high-entropy metallic alloys have been studied extensively since the early 2000s, high-entropy non-metallic inorganic compounds (or compositionally complex ceramics) remain relatively unknown and scarcely studied. Similar to high-entropy alloys, compositionally complex (or high-entropy) ceramics consist of a mixture of several elements in a single crystal phase.
“High-entropy materials provide a means to access a tremendously large compositional space and to explore systems with novel physico-chemical properties.”
‘As for high-entropy alloys’, explains Dr Ionescu, ‘the design concept of compositionally complex ceramics involves the preparation of materials possessing crystalline lattices that are governed by configurational disorder. This means that there are various species (atoms in high-entropy alloys and ions in compositionally complex ceramics), which are randomly distributed within the same crystalline lattice.’ The disorder generated by these microscopic random configurations of elements, which appear in nearly equivalent amounts, has a strong contribution to the stability of the material, and is referred to as entropic stabilisation. In addition, unique physical and chemical properties emerge from this inherent disorder of the material’s crystalline structure.
Structural diversity and novel materials properties
The members of the compositionally complex ceramic family have been steadily increasing since the concept of entropic stabilisation in multicomponent oxidic systems was demonstrated in 2015 (Rost et al, 2015). These now also include borides, nitrides, carbides and silicides, which have received increasing attention in recent years. In contrast to high-entropy alloys, compositionally complex ceramics are typically semiconductors or insulators, which makes them potentially useful in the development of new functional materials, i.e., materials engineered to exhibit well-defined physical and chemical properties. For instance, compositionally complex chalcogenides have been shown to act as thermoelectric materials with improved performance compared to competing materials.
Similarly, compositionally complex oxides are showing promise in the development of materials with specific dielectric properties and thermal conductivity, and in their application to the development of Li-ion batteries. The data collected on compositionally complex materials has also shown the existence of a wide variety of lattice structures capable of supporting entropy-stabilised ceramics. Currently, various structural types have been identified, corresponding to a specific feature of the underlying crystal lattices (e.g., rocksalt-type, spinel, perovskite, fluorite, pyrochlore, bixbyite etc.), and even more will likely be reported in the near future.
Design of compositionally complex ceramics
The central challenge in the development of new compositionally complex materials is the virtually unlimited number of compositions that must be analysed to identify stable materials with well-defined properties. Exploring this tremendously large compositional space calls for innovative approaches to materials discovery. ‘As obviously these compositions cannot be synthesised individually,’ says Dr Ionescu, ‘in silico approaches, based on theoretical or computational methods, need to be developed in order to screen those compositions with respect to their stability and property profiles. Based on the modelling outcome, selected compositions can be identified and experimentally synthesised.’
“Understanding the link between properties and composition in high-entropy ceramics is crucial for the discovery of new functional materials.”
Computational screening thus involves developing mathematical models that can be used to predict the stability of a given material’s compositions for a given crystal structure, or to explore a large combinatorial space of compositions to identify stable structures. At variance with the case of high-entropy alloys, the design of compositionally complex ceramics is, however, still at a very early stage. Typically, two approaches to in silico screening are adopted in the search for new stable structures. One of these is based on the use of descriptors, i.e., sets of empirical parameters that attempt to capture features that are shared among stable structures. The other is based on the direct calculation of the Gibbs free energy, using sophisticated quantum-mechanical methods, which provides a quantitative measure of the stability of a given set of hypothetical structures by accounting specifically for the stabilisation via configurational entropy.
New synthetic approaches to high-entropy ceramics
Whereas substantial work has been devoted in recent years to the study of oxidic ceramics, far less information is currently available on the preparation and properties of non-oxidic entropy-stabilised inorganic materials, such as nitrides and carbonitrides. This is largely a consequence of the fact that the typical synthesis processes used to produce high-entropy ceramics involve high temperatures (well above 1000oC) and that nitride- and carbonitride-based ceramics tend to release nitrogen when exposed to high temperatures.
Example of a synthetic procedure towards high-entropy nitrides / carbonitrides.
The work of the Compositionally Complex Ceramics (C3) group, led by Dr Ionescu, is focusing on understanding the fundamental aspects of potential synthetic routes to gain access to stable compositionally complex nitride and carbonitride ceramics. The work of the C3 project involves developing straightforward and reliable synthetic protocols to produce compositionally complex metal nitrides, oxynitrides and carbonitrides based on low-temperature molecular preparative routes.
The materials obtained using these approaches then undergo extensive structural characterisation, with the aim of pinpointing the role of entropic stabilisation from an experimental perspective. Moreover, a detailed analysis of selected physical and chemical properties of a series of multicomponent equiatomic non-oxidic ceramic samples is also used to examine the role of configurational entropy in driving the appearance of specific properties, with the aim of gaining insight into potential ways to enhance and tune specific properties of new multicomponent materials.
The ability to control and fine-tune the properties of new high-entropy ceramic materials by changing their composition and lattice structure is one of the most intriguing and far-reaching characteristics of these materials. What are the most promising technological fields that could be most impacted by the novel synthetic routes that your group is developing? This class of exciting materials may be indeed useful for various applications, e.g., energy conversion and storage, high and ultrahigh temperature exposure, thermoelectrics, (electro)catalysis, optical and photonic applications, etc. Moreover, I do expect a high relevance and suitability of high-entropy materials formulations within the context of anticipating the use of raw materials with fluctuating compositions, as well as when considering recycling purposes and aspects related to the use secondary raw materials.
References
Ionescu, E, Bernard, S, Lucas, R, Kroll, P, Ushakov, S, Navrotsky, A, Riedel, R, (2019). Polymer-Derived Ultra-High Temperature Ceramics (UHTCs) and Related Materials. Advanced Engineering Materials, 1900269.
Liu, J, Shao, G, Liu, D, Chen, K, Wang, K, Ma, B, Ren, K, Wang, Y, (2020). Design and synthesis of chemically complex ceramics from the perspective of entropy. Materials Today Advances, 8, 100114.
Rost, C, Sachet, E, Borman, T, et al, (2015). Entropy-stabilised oxides. Nature Communications, 6, 8485. Available at: doi.org/10.1038/ncomms9485
Xiang, H, Xing, Y, Dai, F, Wang, H, Su, L, Miao, L, Zhang, G, Wang, Y, Qi, X, Yao, L, Wang, H, Zhao, B, Li, J, Zhou, Y, (2021). High-entropy ceramics: Present status, challenges, and a look forward. Journal of Advanced Ceramics, 10, 385-441.
Zhang, R-Z, Reece, M, (2019). Review of high entropy ceramics: design, synthesis, structure and properties. Journal of Materials Chemistry A, 7, 22148.
Dr Emanuel Ionescu explores the complexity of high-entropy ceramics.
Funding
German Research Foundation (Deutsche Forschungsgemeinschaft), Bonn, Germany.
Collaborators
Prof Hans-Joachim Kleebe, TU Darmstadt
Prof Ravi Kumar, IIT Madras
Dr Maren Lepple, DECHEMA
Prof Sanjay Mathur, University of Cologne
Prof Branko Matovic, University of Belgrade
Bio
Emanuel Ionescu has been Heisenberg Research Fellow and Docent at Technical University Darmstadt since 2019. He studied Chemistry and Physics at the University Bucharest and Technical University Braunschweig and received his PhD in Inorganic Chemistry in 2005 from the University of Bonn. His scientific background and interests relate to the development of advanced ceramics with tailor-made chemical/phase compositions, morphologies, microstructures, and property profiles for structural applications as well as for energy-related, environmental or biomedical purposes.
Emanuel Ionescu
Contact
Technische Universität Darmstadt
Institut für Materialwissenschaft
Fachgebiet Disperse Feststoffe
