Metallic Aluminum Forms Tetrahedrons

Atoms in aluminum metal are bound together in a tetrahedral geometry, a finding by researchers at Australia’s Monash University that better explains some of bulk aluminum’s properties (Science, DOI: 10.1126/science.1198543). The traditional view of a metal as an array of ions in a mobile sea of electrons works well to explain properties such as conductivity. But it doesn’t explain some mechanical qualities, such as why aluminum is more elastic in some directions than others, which would arise from directional bonding. In the sea of electron density, however, it is difficult to pick up localized electron distribution to identify bonds between atoms. Monash’s Philip N. H. Nakashima and colleagues used convergent-beam electron diffraction to map electron distribution in aluminum and observed that the atoms are bound together in a tetrahedral geometry, rather than in an octahedral or other fashion. The results agree with aluminum’s mechanical properties: The metal’s strongest axis passes through areas of highest bonding electron density within the tetrahedrons, whereas its weakest axis corresponds to holes in the electron density.