Atomic-Environment Classification of the Chemical Elements

Of the 103 known chemical elements, 96 crystallize in 52 different crystal structures. Included in these 52 structures are those which are stable under normal (298K, 100 kPa) conditions (NC) as well as high or low temperature (HT/LT) and high pressure (HP) conditions, but with the O-, N-, and C-stabilized structures excluded as well as structures observed in thin films. Of the 275 experimentally found and reported element modifications, 95% can be classified with just seven different atomic environment types (AETs), all belonging to the single-environment type. A single-environment type encompasses all structures in which all atoms present in the structure have the same AET. The seven AETs observed are the octahedron, the cubo-octahedron, the twinned cubo-octahedron and the rhombic dodecahedron for metals, and the dumb-bell, the double link and the tetrahedron for non-metals. With modified structural stability diagrams (SSDs), it is shown that under normal conditions the elements are simply separated into different AET domains. With the HT/LT and HP crystal structure data included in the SSDs, the AET modification (so far unknown) to be found under HP or HT/LT conditions was predicted for 31 elements based on the SSD-AET domains. Finally, the investigation includes a comprehensive, critically evaluated crystallographic data set for all element modifications as well as a recommended atomic volume table. These volumes are calculated from the unit cell volume divided by the number of atoms per unit cell.