Crystal lovers rejoice – researchers have created the largest database of elemental crystal surfaces and shapes to date.
There is an incredibly large variety of crystals in nature. When you add in the ones humans designed themselves, the possibilities are simply staggering. Dubbed Crystallium, this new open-source database can go a long way towards helping researchers design new materials – especially where crystal surface orientation
“This work is an important starting point for studying the material surfaces and interfaces, where many novel properties can be found. We’ve developed a new resource that can be used to better understand surface science and find better materials for surface-driven technologies,” said Shyue Ping Ong, a nanoengineering professor at UC San Diego and senior author of the study.
Crystals found in rocks typically range in size from a fraction of a millimetre to several centimetres across, although exceptionally large crystals are occasionally found. But this database is less about geology and more about material design. Size is not often important when trying to use crystals to design materials but the other geometrical parameters are. For instance, fuel cell performance is significantly influenced by molecules of hydrogen and oxygen reacting on the surface of metal catalysts. The orientation of the building blocks of that surface is key here. Similarly, the interfaces between the electrodes and electrolyte in a rechargeable lithium-ion battery can increase or reduce the battery’s performance. This is where the database steps in – it makes this kind of information much more accessible to everyone.
“Researchers can use this database to figure out which elements or materials are more likely to be viable catalysts for processes like ammonia production or making hydrogen gas from water,” said Richard Tran, a nanoengineering PhD student in Ong’s Materials Virtual Lab and the study’s first author. Tran did this work while he was an undergraduate at UC San Diego.
The surface resistance is particularly important in such cases. Surface energy describes the stability of a surface, it’s basically a measure of the excess energy of atoms on the surface relative to those in the bulk material. The surface resistance is always important for designing nanoparticles and catalysts.
At this point, you might think this is a trivial task. After all, what’s so special in publishing a crystal database?
Well, in the past, researchers have experimentally measured the surface energies elements in their crystal. This is a complex process which traditionally involves melting the crystal, measuring the resulting liquid’s surface tension at the melting temperature, then extrapolating that value back to room temperature – and all this must be done on a perfectly smooth surface, which is pretty difficult to obtain. Some crystals had already been measured this way, but the problem is that the results were averaged values and thus lacked the specific resolution that is necessary for design, Ong said. Furthermore, the surface energy is not a simple number, because it depends on the crystal’s orientation
“A crystal is a regular arrangement of atoms. When you cut a crystal in different places and at different angles, you expose different facets with unique arrangements of atoms,” explained Ong, who teaches the Crystallography of Materials course at UC San Diego.
There was no place with such information for all elemental crystals, so Ong and his team developed sophisticated automated workflows to calculate surface energies virtually, using the open-source Python Materials Genomics library and FireWorks workflow codes of the Materials Project. Their virtual laboratory setup was excellent, thanks to the powerful supercomputers at the San Diego Supercomputer Center and the National Energy Research Scientific Computing Center. It simulates the experiments with accuracy, offering all the required information without any hassle.
“This is one of the areas where the ‘virtual laboratory’ can create the most value–by allowing us to precisely control the models and conditions in a way that is extremely difficult to do in experiments.”
At the moment, the database covers only chemical elements, but the team is already working on expanding it to multi-element compounds like alloys, which are made of two or more different metals, and binary oxides, which are made of oxygen and one other element.
