Crystal Structures and Exotic Behavior of Magnesium under Pressure

The high-pressure structures of magnesium (Mg) have been extensively explored through our newly developed particle swarm optimization algorithm on crystal structural prediction. Two structures with face-centered cubic (fcc) and simple hexagonal (sh) symmetries are discovered to be stable at 456-756 GPa and above 756 GPa, respectively. Especially, the sh structure, which is known to occur at high pressure in P, Si, and Ge, is reported here for Mg. This structure can be derived from the fcc lattice by distortion of the alpha and gamma angles from 60 degrees to 90 degrees. More intriguingly, the calculated valence electron localization function reveals an electride nature of the fcc and sh structures with valence electrons localized in the interstitial regions, analogous to what was recently reported in Li, Na, K, and Ca under high pressure. However, what makes Mg unique is that it remains metallic. The temperature-pressure phase diagram of Mg has also been explored using quasi-harmonic approximation, and the finite-temperature phase boundaries of the fcc and sh structures are determined.