We know that when atoms or molecules come together to form a solid, some atomic arrangements are more favorable than others. These different atomic arrangements, known by their crystal structures, each have different energies, and the most favorable crystal structure is the one with the lowest energy. It’s also the most thermodynamically stable structure.
The Materials Project uses the power of supercomputing to provide open Web-based access to computed information on known and predicted materials as well as powerful analysis tools to inspire and design novel materials. (Image source: The Materials Project)
To date, materials science has primarily focused on the design and engineering of stable structures. However, many materials can exist for extended periods in what’s known as “metastable structures,” which are not the lowest-energy crystalline arrangement, usually because there is a barrier to transforming to a more stable form.
Some metastable structures can be technologically useful. Diamonds, which are a metastable arrangement of carbon atoms (the stable form is graphite), have spectacular properties, including high hardness and thermal conductivity (and they’re also quite pretty). Increasingly, researchers are looking for ways to identify other useful metastable materials.
Researchers from the Department of Energy's (DOE) Lawrence Berkeley National Laboratory are forging a path toward an easy way to design and create promising next-generation materials for use in everything from semiconductors to pharmaceuticals to steels, according to Wenhao Sun, one of the researchers on the project.
The research was published last month in the journal Science Advances.
“The first step to designing new metastable materials is to understand the differences in energy between metastable structures and their stable structures,” Dr. Sun told Design News. “And that is what our study aimed to do. We measured the thermodynamic scale of metastability for all known inorganic solids, by a large-scale data-mining of our computed materials property database, named the Materials Project.”
Because investigations of metastable materials have been limited to date, and the information that did exist was scattered, materials scientists have had a relatively poor understanding of which material chemistries and compositions can be metastable, and how metastable they can be. Sun calls the new research a way to begin “building intuition” when it comes to metastable materials.
“By studying the metastability of existing materials, we can better predict which new metastable materials can be made,” he said. “Our data-mining study revealed some new trends in metastable materials, that could be used to help a materials designer estimate whether a predicted metastable material could be made or not.”
Thanks to quantum-mechanical methods used to directly compute materials properties, the team was essentially able to calculate the properties of all known inorganic materials. The calculated properties are put into extremely large databases – the Materials Project being one – and this enables the team to make very broad and general observations about metastability.
A better understanding of metastability will open many new avenues in materials science. While metastability in materials like steel are better understood because steel has a long history of being engineered for new properties, the new knowledge can help materials researchers build new properties into it.
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“More recently, people have become interested in amorphous metals (glassy materials with no long-range crystal structure), which are metastable; and there is another very exciting class of steels, known as high-entropy dual-phase alloys, that can be metastable and can exhibit both strength and toughness, whereas traditional steels must be either one or the other,” Sun told Design News.
There are also promising implications for pharmacology when it comes to metastable materials. For drug molecules to be bioactive, they must be dissolvable in the stomach. The simplest and most preferable way to deliver drugs is in pill form, wherein drug molecules can be packed into different crystal structures. Sometimes the lowest-energy structure is so stable that the pill can’t dissolve fast enough to release the drug molecules. In this case it’s preferable to have a metastable, higher-energy structure that dissolves more readily.
According to Berkeley Lab researchers, the most exciting potential for the work is how it will predict promising new technological materials by computation rather than expensive and time-consuming trial-and-error.
“Sometimes, the promising materials we identify are metastable – so, how do we go to the laboratory to make them, instead of the stable structure?” Sun said. “It’s well-known that the metastable structure can actually become the stable structure under different thermodynamic conditions (temperatures, pressures, size, electrochemical voltage, etc.) We propose to synthesize predicted metastable structures under conditions where they are stable, and then retain them as ‘remnants’ of those conditions. We termed this concept ‘remnant metastability.’ ”
Dr. Sun noted that there is still a long way to go, and the researchers will continuously work with experimentalists to compare their predicted theories with experimental observations. The research group is involved in a large theory/experiment collaboration across numerous universities, called the Center for Next Generation Materials by Design: Incorporating Metastability. They will attempt to synthesize new metastable materials while refining their understanding of fundamental synthesis science.
Tracey Schelmetic graduated from Fairfield University in Fairfield, Conn. and began her long career as a technology and science writer and editor at Appleton & Lange, the now-defunct medical publishing arm of Simon & Schuster. Later, as the editorial director of telecom trade journal Customer Interaction Solutions (today Customer magazine) she became a well-recognized voice in the contact center industry. Today, she is a freelance writer specializing in manufacturing and technology, telecommunications, and enterprise software.