When a team led by Nicola Marzari examined databases of different materials, they were amazed. Researchers from the École Polytechnique Fédérale de Lausanne found a pattern they couldn't explain. Nearly 60% of the approximately 80,000 electronic structures of inorganic compounds apparently follow the “rule of four”: the basic building blocks of their crystal structure consist of multiples of four atoms, a report said. Experts in April 2024 in the specialized magazine “npj Computational Materials”.
The atoms of solids and many inorganic compounds are arranged in a regular crystal lattice. The smallest building block that can be used to create the periodic network is called a primitive unit cell. In theory, these could contain a variety of different atoms. To better understand the structure of unit cells, Marzari researchers examined two large materials databases: Project materials database And Material cloud 3D crystal structures source database Each contains about 80,000 entries for existing and projected material. The team realized that most primitive unit cells consisted of multiples of four atoms.
Since they could not explain the rule of four directly, experts feared that the data collected contained errors. But they quickly managed to dispel suspicions. They then investigated a possible chemical explanation: Since silicon can bond four atoms to itself, they investigated whether the base of the four compounds consisted of silicates, a very common class of silicon-based materials. But this is not always the case. The researchers also found no connection between the symmetry or energy states of the crystal structures, which obey the mysterious rule of four.
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