Study reveals profound impact of synthesis method on disordered materials

A new study has revealed for the first time how different synthesis methods can profoundly impact the structural and functional properties of high-entropy oxides, a class of materials with applications in everyday electronic devices. The study is published this week in the journal Journal of the American Chemical Society.

“The specific material we studied here is a high-entropy oxide with a spinel crystal structure, which is a mixture of five different transition metal oxides. Much of the excitement we see around this class of materials is their electrochemical properties,” said Alannah Hallas, a materials scientist at the Blusson Quantum Matter Institute and the Department of Physics and Astronomy at the University of British Columbia.

“The reason these high-entropy systems are so promising from this perspective is that they have enormous chemical flexibility. When synthesizing these materials, we have many different buttons that we can turn, which provides unlimited possibilities for building them.”

The researchers prepared the same samples using five different synthesis methods: solid-state, high-pressure, hydrothermal, molten salt, and combustion synthesis. The methods involve different ways of heating the material, different rates at which the material is cooled to room temperature, and different chemical conditions under which the heating can occur.

“Our results confirm that the synthesis method is very important. We found that while the average structure remains unchanged, samples vary considerably in their local structures and microstructures, with combustion synthesis producing the most homogeneous samples.”

The main difference between the synthesis methods lies in the driving mechanism that forms the material, said the study’s lead author, Mario Ulises González-Rivas, who mastered the art of preparing samples using the different synthesis methods during his PhD in Hallas’ group.

In the solid-state method, metal oxides are mixed and then heated, similar to baking a cake. The high-pressure method adds external pressure during heating, which can influence the formation of the material. The hydrothermal method mimics mineral formation in the Earth’s core by heating metal salts in water inside a pressure vessel, creating a flow that promotes crystal growth.

The molten salt method uses molten metal salts that form a thick liquid that, as it cools, allows the crystals to precipitate. Finally, the combustion method involves dissolving the metal salts in water, forming a gel that ignites, quickly producing the desired material through a rapid combustion reaction.

“Some of these materials have great potential to address energy challenges. The technological implementation of these materials for energy systems is deeply influenced by the type of structural variations we observe in this study,” González-Rivas said. “Our results effectively provide a new optimization axis to consider when implementing these materials in an applied setting.”

The study is a collaboration between Hallas’ team at UBC Blusson QMI, Robert Green, a UBC Blusson QMI affiliated researcher at the University of Saskatchewan, and Hidenori Takagi of the Max Planck Institute for Solid State Research.