Mapping metastable phases among lanthanide sequioxides

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Schematic representation of calculated lanthanide sesquioxides phase diagrams at the ground state (change in stored energy equals zero) and different values of change in stored energy, along with the corresponding metastable phases of lanthanide sesquioxides, as function of change in energy and temperature. The five different phases are represented by the five different colors. 

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Lanthanide sesquioxides are materials with many different applications, from corrosion resistant materials to solid oxide fuel cells to biomedical applications. The compounds have many different metastable phases — states of matter that are not thermodynamically stable — where the chemistry remains the same, but the crystal structure, or the atomic ordering, changes. To enable efficient and effective fabrication of next-generation technologies, scientists need the ability to predict how to access these different phases. In research described in Materials Advances, a Los Alamos National Laboratory research team linked computer modeling and experimental science to formulate a predictive metastability map, useful for understanding how imparting a certain amount energy on the lanthanide sesquioxide crystal structure can lead to phase changes.

Synthesizing metastable phases currently depends on a trial-and-error approach. The research team sought to understand the amount of stored energy — the metastability threshold — needed to form metastable phases. Lanthanide sesquioxides make an ideal case study to understand metastable phase formation, given their rich polymorphism with various properties and the ease with which they transform to different polymorphs. Using density functional theory to determine the metastability threshold of all relevant phases, the team was able to generate metastable phase diagrams. That work helped understand the formation of metastable phases and predict the specific conditions that can contribute to the formation of particular phases.

The team predicted a series of metastable phase transformations and then showed for the first time, via irradiation, that those phases could be accessed in sequence, correlated with their metastability threshold. The metastability limit for lanthanide sesquioxides, approximately 8 millielectronvolts per atom, was identified, answering questions about the inability to synthesize metastable phases from decomposition of salts or hydrothermal synthesis. Above that threshold, metastable phases cannot be synthesized from decomposition of salts or hydrothermal synthesis. That insight may inform approaches for lanthanide sesquioxide production.

The results offer useful insight into lanthanide sesquioxide metastability that can form the basis for future experimental studies. The same approach can be used to identify the metastable phases that form under non-equilibrium conditions in other materials as well.

The research team notes the development of the metastable phase diagram approach as described in a recent Nature Communications publication by members of Argonne National Laboratory’s Center for Nanoscale Materials.

Funding and mission

The research was supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. The work was performed, in part, at the Center for Integrated Nanotechnologies, a DOE Office of Science User Facility, and also used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility located at Lawrence Berkeley National Laboratory. The work supports the Global Security mission area and the Materials for the Future capability pillar.

Reference

“Predicting and accessing metastable phases,” Materials Advances, 4, 1101 (2023); DOI: 10.1039/D2MA00995A. Authors: V. Kocevski, J.A. Valdez, B.K. Derby, Y.Q. Wang, G. Pilania and B.P. Uberuaga (Los Alamos National Laboratory).

See also, “Machine learning the metastable phase diagram of covalently bonded carbon,” Nature Communications, 13 (2022); DOI: 10.1038/s41467-022-30820-8. Authors: Srilok Srinivasan, Rohit Batra, Duan Luo, Troy Loeffler, Sukriti Manna, Henry Chan, Jianguo Wen, Pierre Darancet and Subramanian K.R.S. Sankaranarayanan (Argonne National Laboratory); and Liuxiang Yang and Wenge Yang (Center for High Pressure Science and Technology Advanced Research, China).

Technical contacts: Benjamin Derby (MPA-CINT) and Blas Uberuaga (MST-8)