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Sometimes, less really is more. By removing oxygen during synthesis, a team led by materials scientists at Penn State has created seven new high-entropy oxides (HEOS), a class of ceramics composed of five or more metals with potential for applications in energy storage, electronics and protective coatings.

In the process of synthesizing the new materials, the researchers created a framework using principles of future materials. The team has its work in Nature Communications.

"By carefully removing oxygen from the atmosphere of the tube furnace during synthesis, we stabilized two metals, iron and manganese, into the ceramics that would not otherwise stabilize in the ambient atmosphere," said corresponding and first author Saeed Almishal, research professor at Penn State working under Jon-Paul Maria, Dorothy Pate Enright Professor of Materials Science.

Almishal first had success stabilizing a manganese- and iron-containing material by controlling oxygen in a composition that he named J52, which consisted of magnesium, cobalt, nickel, manganese and iron. Then, using newly developed machine learning—a subtype of artificial intelligence—capabilities that screen thousands of material compositions in seconds, Almishal identified six additional combinations of metals forming HEOs.

With the help of a team of undergraduate students who processed, fabricated and characterized the samples in the lab, Almishal synthesized bulk ceramic pellets of the seven novel, stable and potentially functional metal HEO compositions.

"In a single step, we stabilized all seven compositions that are possible given our current framework," Almishal said. "Although this was previously treated as a complex problem in the field of HEOs, the solution was simple in the end. With a careful understanding of the fundamentals of material and ceramic synthesis science—and particularly the principles of thermodynamics—we found the answer."

Stabilizing those materials means to "coerce" the manganese and iron atoms to remain in the 2+ oxidation state, or rock salt structure, where they each only bind to two oxygen atoms, Almishal explained. If researchers tried to synthesize the materials at room-levels of oxygen, the materials would not stabilize because manganese and iron would continue to collect oxygen atoms and move to a higher oxidation state. Removing the oxygen from the atmosphere in the tube furnace limits the oxygen available to the material, allowing the material to stabilize in the desired rock salt structure.

"The main rule we followed in synthesizing these materials is the role that oxygen plays in stabilizing such ceramic materials," Almishal said.

To make sure that manganese and iron in each new material were stable in the target oxidation state, Almishal collaborated with researchers from Virginia Tech. They performed an advanced imaging technique to measure how X-rays are absorbed by the atoms in the material. By analyzing the resulting data, researchers could determine the oxidation state of specific elements and confirm the stability of manganese and iron in the new materials.

In the next phase of research, the researchers said they will test all seven new materials for their magnetism. They also aim to apply their thermodynamic framework for controlling oxygen during synthesis to other material classes currently considered unstable and challenging to synthesize.

"This paper, which has already been accessed online thousands of times, seems to resonate with researchers because of its simplicity," Almishal said. "Although we focus on rock salt HEOs, our methods provide a broad adaptable framework for enabling uncharted, promising chemically disordered complex oxides."

As a result of his extensive lab work on the new materials, co-author and undergraduate materials science and engineering major Matthew Furst was invited to present the research at the —an honor usually reserved for faculty or senior graduate students—which took place Sept. 28 through Oct. 1 in Columbus, Ohio.

"I am so grateful for the opportunities that I have had on this project and to be involved in every step of the research and publication process," Furst said. "Being able to present this material to a broad audience as an invited talk reflects my involvement and the excellent guidance I have received from my mentors. It means a lot to me to develop important communication skills as an undergraduate student, and I look forward to pushing myself further in the future."