Chemical interactions that govern the structures of metals*

Most metals adopt simple structures such as BCC, FCC, and HCP in specific groupings across the Periodic Table, and many undergo transitions to surprisingly complex structures on compression, not expected from conventional free-electron-based theories of metals. First-principles calculations have been able to reproduce many observed structures and transitions, but a unified, predictive theory that underlies this behavior is not yet in hand. Discovered by analyzing the electronic properties of metals in various lattices over a broad range of sizes and geometries, a remarkably simple theory shows that the stability of metal structures is governed by electrons occupying local interstitial orbitals and their strong chemical interactions. The theory provides a basis for predicting new structures in solid compounds and alloys over a broad range of conditions.