Lang’s Lab Uncovers New Path to Metastable Materials
Photo printers don’t contain hundreds of ink colors. Instead, they place tiny dots of just four pigments—yellow, magenta, cyan, and black. From a distance, those four pigments can look like any color in the world.
Alex Solomon
Alexandre Solomon (NE/PhD ’23) discovered a similar phenomenon when he investigated the atomic-scale structure of a material called zirconia. Now, that discovery is poised to disrupt the field of metastable materials science.
“Some material phases have desirable traits, like high-temperature superconductivity or exceptional strength, at very high pressures,” said Department of Nuclear Engineering (NE) Professor Maik Lang, Solomon’s former graduate advisor. “If those phases could be ‘locked in’ so their unique properties remain after pressure is reduced to ambient conditions, these materials could be harnessed for technological applications.”
Zirconia is a material that has several metastable phases. Above 1,000 degrees Celsius, the atoms align with tetragonal (rectangular prism) symmetry, conveying many desirable traits including excellent electrical properties and high strength.
Recovering that tetragonal form at room temperature and pressure usually requires inducing strain on the zirconia, but the details of the resulting atomic configuration, and how that strain ‘locked in’ the useful phase, were not well understood.
“In a few instances, previous researchers could see that the atomic-scale structure of metastable zirconia contained something unexpected, but their techniques were not sensitive enough to explain what it was,” said Lang.
In research published earlier this month in Science Advances, Solomon, Lang, Research Assistant Professor Eric O’Quinn, and their coauthors revealed the reason behind metastable zirconia’s strange behavior: its atomic scale is not tetragonal, but orthorhombic (made of prisms where the height, width, and length are all different).
Like the dots of red and yellow ink in a photograph of an orange, these different orthorhombic prisms average out to create the macroscopic tetragonal structure of metastable zirconia. It may be the same ‘color’—the same overall shape—as the high-temperature zirconia phase, but its underlying structure is completely different.
“It’s awesome,” Lang said. “It is rare that graduate students publish in such impactful, broad-interest journals, and given the fundamental nature of our findings, I think this will be one of my lab’s most impactful publications ever.”
International Collaboration
Solomon joined Lang’s lab to study simple oxide metastable materials, including zirconia, in 2018. During his PhD studies, the group received a grant from the Department of Energy’s Synthesis and Processing Science program to research new methods for stabilizing metastable materials.
The team planned to use the new funding to investigate how metastability is ‘locked in’ at the atomic scale. Solomon suggested focusing on zirconia, a well-studied system that nevertheless had plenty of mysteries left to discover.
“I think it’s important that graduate students take on early control of their own research. That’s part of becoming an independent scientist,” Lang said. “Professor O’Quinn and I provide guidance and regular meetings to make sure research is progressing, but in general, our graduate students control their own research.”
Maik Lang working on a project in his lab
Lang said Solomon showed incredible initiative during the research, overseeing the development of metastable zirconia via multiple methods, including nanoscale preparation—for which Solomon joined Igor Gussev (NE/PhD, ‘23), another of Lang’s former students and one of the paper’s coauthors, at Paris-Saclay University in France.
Solomon also conducted multiple ion-beam exposure experiments, collaborating with neutron scattering scientist Joerg Neuefeind at Oak Ridge National Laboratory (ORNL) and scientists at the GSI Helmholtz Center in Darmstadt, Germany, one of the largest ion accelerator facilities in the world.
“This study could only be accomplished through the complementary expertise of all our co-authors,” Lang said. “This was truly a collaborative effort of the University of Tennessee team with other colleagues in the US and abroad.”
A New Approach to Defect Engineering
Stabilizing materials like zirconia at ambient temperature while maintaining their most desirable traits requires “defect engineering”—carefully introducing structural defects that force the materials to maintain structures that typically aren’t stable at ambient conditions.
Solomon and his coauthors determined that metastable tetragonal zirconia is composed of nanometer-sized orthorhombic domains with four types of atomic arrangements. Like the four ink colors in a printer blending to create a complex image, these four arrangements average out to form the macroscopic tetragonal phase.
Eric O’Quinn
The team also demonstrated that the mechanism of stabilization is not related to material strain, as was previously assumed, but instead relates to the structure of these domains and the boundaries between them.
“It is extremely challenging to investigate such domain walls, but we believe that they are essential in the stabilization process and also exist in many other metastable materials,” Lang said. “This will keep us busy for years to come.”
In the meantime, other teams in the field can start applying Solomon’s discovery to other materials with unique properties that hold promise in next-generation technologies, but have not yet been stabilized at ambient conditions.
“We have discovered how nature works at a very fundamental level,” Lang said. “I think our findings will provide a new way of thinking about, and creating, metastable material phases.”
Contact
Izzie Gall (865-974-7203, egall4@utk.edu)