Gallium and indium under high pressure

Ga and In crystallize in unusual open ground-state crystal structures. Rece nt experiments have discovered that Ga under high pressure transforms into a close-packed structure, while this has so far not been observed for In. R esults from first principles calculations explain in a simple way this diff erence in behavior We predict a so far undiscovered transition of In to a c lose-packed structure at extreme pressures and show that the structure dete rmining mechanism originates from the degree of s-p mixing of the valence o rbitals. Group-III elements are shown to strongly disobey the standard corr esponding-state rule.