Publication date: Jul 27, 2023
Why are materials with specific characteristics more abundant than others? This is a fundamental question in materials science and one that is traditionally difficult to tackle, given the vastness of compositional and configurational space. We highlight here the anomalous abundance of inorganic compounds whose primitive unit cell contains a number of atoms that is a multiple of four. This occurrence - named here the 'rule of four' - has to our knowledge not previously been reported or studied. Here, we first highlight the rule's existence, especially notable when restricting oneself to experimentally known compounds, and explore its possible relationship with established descriptors of crystal structures, from symmetries to energies. We then investigate this relative abundance by looking at structural descriptors, both of global (packing configurations) and local (the smooth overlap of atomic positions) nature. Contrary to intuition, the overabundance does not correlate with low-energy or high-symmetry structures; in fact, structures which obey the 'rule of four' are characterized by low symmetries and loosely packed arrangements maximizing the free volume. We are able to correlate this abundance with local structural symmetries, and visualize the results using a hybrid supervised-unsupervised machine learning method.
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|155.6 MiB||SOAP vector representation of the 79,854 entries of the Materials Cloud 3-dimensional crystal structures "source" (MC3D-source) database, along with outcomes of the classification algorithms and calculated geometric descriptors used in the analysis of the paper.|
|222.4 MiB||Structure files and SOAP vector representation of the 83,989 entries of the Materials Project crystal structures (MP) database, along with outcomes of the classification algorithms and calculated geometric descriptors used in the analysis of the paper.|
|2.5 MiB||Full list of database versions and IDs for each structure obtained from the three databases (MPDS, ICSD, COD) composing the MC3D-souce database. The MC3D-source database cannot be released due licensing constraints.|
|1.7 KiB||Complete description of the contents of the archive.|