Researchers from AGH University are seeking order in entropy of oxide spinels

The advantage of high-entropy materials, which is a wide range of possibilities related to shaping their parameters, also poses a tremendous challenge. Due to a very large number of possible combinations, predicting the influence of different additives on material properties is significantly complicated. Satisfactory results are usually achieved through trial and error, experimenting with various concentrations of elements. There is a lack of general rules that would describe the relationships between them at the atomic level, which would allow for a more intentional shaping of the process.

A team led by Professor Cieślak aims to change this, at least partially. The team also includes researchers from the Faculty of Materials Science and Ceramics and the AGH Academic Centre for Materials and Nanotechnology. The scientists intend to closely examine oxide spinels, which currently hold great promise for creating high-entropy materials. Although the concept of high-entropy oxide systems was proposed relatively recently in 2015, the number of examples of their applications in various fields is incredibly wide. They can be used as magnetic materials, catalysts, or in energy conversion technologies, including solid oxide fuel cells or materials for Li-ion batteries. The researchers employ experimental and numerical methods to identify patterns that will enable a conscious shaping of the properties of newly synthesised compounds.

What are oxide spinels? 

Spinels are a group of compounds that often occur naturally as minerals. They typically take the form of AB204, where A and B are metal atoms at the second and third oxidation states respectively. Oxygen anions form a regular structure in which metal cations occupy tetrahedral and octahedral interstices. In the elementary unit cell of a spinel (8 AB204), there are 64 tetrahedral and 32 octahedral interstices. Eight atoms with a +2 oxidation state arrange themselves in tetrahedral interstices, while 16 atoms with a +3 oxidation state occupy octahedral interstices. However, it happens that half of the latter ones, along with the metals in the +2 oxidation state, occupy tetrahedral interstices, resulting in an inverted spinel structure.

"Spinels that occur naturally in nature are rarely perfectly pure. Because they formed under natural conditions, various other elements could join them. Many studies compare the properties of spinels extracted from different locations, and they show that even very small additions can affect their properties," describes Professor Cieślak.

In the case of synthetic spinels, their stoichiometry can be precisely controlled. Some cations of one metal can be replaced by cations of another, or several others, as is the case of high-entropy materials.

As an example, the AGH scientist points to magnetite (Fe304), where both cationic sublattices are occupied by iron atoms. "Iron has very good magnetic properties, but in magnetite, the magnetic moments on both sublattices are oriented in opposite directions, and as a result they significantly cancel out. So what we observe is only the difference between two large magnetisations. When we replace iron atoms on one of the sublattices with, for example, non-magnetic zinc, the total magnetism becomes much more pronounced. In the case of two- and three-component materials, we can usually predict the consequences of such modifications. However, when we enter the realm of high entropy, arranging five elements on both sublattices can be done in many different ways, and each of these configurations may exhibit different properties," explains Professor Cieślak.

Dialogue of experiment with theory 

As part of their ongoing work, scientists from the AGH University will be synthesising new high-entropy spinels and investigating their properties. Their methodology is based on a close relationship between experiments and computational tools rooted in quantum Density Functional Theory (DFT).

Professor Cieślak explains, "We systematically modify the additives in such systems and study how their structure and properties evolve. If the calculations indicate that certain concentrations of additives yield interesting effects, we create additional samples. Then, we subject them to further analysis, and the results are incorporated into subsequent calculations. The results from the calculations influence the interpretation of measurement outcomes, and the measurement results inspire further calculations."

Unlike many other research teams around the world, the goal of the AGH researchers is not to find one perfect recipe for a cocktail of elements for a specific application. Instead, based on their observations, they aim to establish general rules that in the future will allow for the selection of cocktail components in a way that achieves the desired outcome.

"The end result will be far from scenes straight out of science fiction films, where a machine can instantly determine the elemental composition of a material with desired parameters. Instead, equations describing various properties will be developed, for example, how a magnetic moment on a specific sublattice will change in a certain way depending on a particular parameter. Another equation may describe the electrical resistance in relation to a different set of parameters. It won't be just one formula but rather a set of rules, which, compared to the current state of knowledge, represents significant progress," concludes Professor Cieślak.

The project "Designing functional properties of high-entropy spinels by analysing their structure at the atomic level" has been funded by the National Science Centre under the OPUS 23 programme.