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September 2013

Experiments Make Missing Materials Predicted by Theory

First experimental realization of the theoretically predicted new ternary compound

Vladan Stevanović

Experimental realizations of TaCoSn and TaCo2Sn: (a) ternary Ta–Co–Sn composition space with two new ternary compounds TaCoSn and TaCo2Sn (green pluses) and other known phases (blue checkmarks). Experimental and theoretical X-ray diffraction patterns of (b) TaCoSn and (c) TaCo2Sn thin films demonstrating agreement between the theoretically predicted and experimentally realized crystal structures.

Browsing the standard databases of known inorganic solids shows that a relatively large number of chemically reasonable compounds are missing. In the case of compounds composed of three or more chemical elements, this number can exceed 50 percent. Scientists from the Center for Inverse Design developed a computational approach to address the problem of existence of these missing compounds; that is, their stability with respect to decomposition. This approach, developed iteratively between theory and experiment, was used to predict the existence of more than 300 new compounds with ABX and A2BX4 chemical formulas. In their article in the Journal of the American Chemical Society, the scientists report on the first experimental confirmation of the theoretically predicted 18-electron TaCoSn ternary compound, a new semiconductor material with potential application in the thin-film photovoltaics.

Importance of guiding experiment in discovering missing materials. Over the second half of the 20th century, solid-state physics and quantum mechanics slowly transformed from purely scientific endeavors into powerful engineering disciplines. Ex post facto explanations and trial-and-error discovery were gradually replaced with powerful quantitative and predictive models. The phrase rational materials design is now frequently used in the context of finding new material solutions to various types of problems. As already mentioned, there is a vast space of yet undiscovered materials and, as the history of science teaches us, new materials often bring new and surprising properties, such as superconductivity or ferroelectricity. Therefore, theoretical and computational tools that predict the existence of unreported compounds would radically accelerate the discovery of new and potentially game-changing materials.

Theoretical approach. In the case of unreported compounds, predicting their stability relative to competing phases is done in two steps. In the first step, the likely structure for a given A-B-X combination has to be determined. This is done by computing the total energy of a list of candidate structure types and sorting out the lowest energy one. This result is subsequently verified with an unbiased, but computationally more demanding, Global Space Group Optimization Method. In the second step, the compound is predicted to be stable with respect to decomposition into competing phases if its energy to form is lower than the formation energy of all known competing phases including pure elements, competing binary and ternary compounds, and their combinations.

Experimental results and approach. The team targeted the hypothetical -- but predicted stable -- TaCoSn compound for experimental synthesis because of its potential as a new thin-film photovoltaic material. Experimental efforts include both bulk and combinatorial approaches. Both of these approaches resulted in synthesizing the target TaCoSn compound with pure enough samples to confirm both the composition and the theoretically predicted structure. Further, combinatorial experiments across the entire Ta-Co-Sn space revealed the existence of another previously unreported ternary compound TaCo2Sn, which was subsequently added to list of competing phases considered by theory. Further improvements in the purity of samples are needed for the experiment to fully confirm the predicted electronic structure properties of the new TaCoSn compound.

Synthesis of the theoretically predicted TaCoSn compound is the first demonstration and the proof of concept that the theoretical models and computational tools currently at our disposal are now mature enough to be used for predicting the existence of new solid compounds. This is an important step forward in accelerating the discovery of new materials, and demonstrating that rational materials design is a truly viable paradigm for future research.

More experiments are in progress, synthesizing newly predicted ABX and A2BX4 compounds. The results of these experiments will provide more thorough feedback on the strengths and the limitations of the theory, and will ultimately lead to more robust predictive models, and improve our understanding of the structural preferences and stability in solid-state systems.

More Information

Zakutayev A, X Zhang, A Nagaraja, L Yu, S Lany, TO Mason, DS Ginley, and A Zunger. 2013. "Theoretical Prediction and Experimental Realization of New Stable Inorganic Materials Using the Inverse Design Approach." Journal of the American Chemical Society 135:10048-10054. DOI: 10.1021/ja311599g

Acknowledgments

This work is supported by the Center for Inverse Design funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. TaCoSn bulk XRD patterns were collected and refined at the J. B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University. X.Z. also acknowledges the administrative support of REMRSEC under an NSF grant, Colorado School of Mines, Golden, Colorado.

About the author(s):

  • Vladan Stevanović received his Ph.D. from the Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, in 2009 in studying transition metal clusters for application in heterogeneous catalysis. After receiving his PhD, Vladan moved to National Renewable Energy Laboratory in Golden, CO, and currently holds a Research Assistant Professor position at Colorado School of Mines. Vladan is member of the Center for Inverse Design, and his research is mostly dedicated to developing predictive computational tools to address materials properties of relevance for renewable energy applications.

Materials Are "Missing" No More

Inverse design allows targeting of previously unknown compounds of interest for solar cells

Using inverse design, scientists realized three metal materials that were missing from materials databases. Copyright 2013: American Chemical Society.

To replace fossil fuels with renewable energy demands new compounds that will radically improve solar cell efficiency. The old trial-and-error approach is too slow and expensive. Scientists devised a new theoretical and computational approach that predicts existence, i.e. stability with respect to decomposition, of any compound, including those with desirable traits for solar cells and other types of devices. Using this approach, they predicted and later synthesized a new material composed of three metals: tantalum, cobalt and tin. They confirmed theoretical predictions by growing the predicted material in the lab, which was the first experimental realization of this compound. The inverse approach is accelerating material discovery demonstrating that rational design can benefit future research. The Center for Inverse Design, led by the National Renewable Energy Laboratory, did the research.

More Information

Zakutayev A, X Zhang, A Nagaraja, L Yu, S Lany, TO Mason, DS Ginley, and A Zunger. 2013. "Theoretical Prediction and Experimental Realization of New Stable Inorganic Materials Using the Inverse Design Approach." Journal of the American Chemical Society 135:10048-10054. DOI: 10.1021/ja311599g

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.