Structural richness of ionic binary materials: An exploration of the energy landscape of magnesium oxide

We search for polymorphs of magnesium oxide (MgO) close in energy to the rocksalt global minimum by using variants of the basin hopping approach. Our exploration clearly demonstrates that the energy landscape of this nominally octahedrally coordinated, densely packed solid is much more complicated than suggested by previous work. Specifically, we find that MgO has a dense spectrum of as yet undiscovered low-energy polymorphs, many of which have a much lower density than the rocksalt structure (as also recently shown for tetrahedrally coordinated binary solids (e. g., ZnO) [Phys. Rev. Lett. 104, 175503 (2010)]). Unlike in the case of tetrahedrally coordinated materials, we further find low-energy crystal structures with four-, five-, or six-coordinated atoms. Strikingly, some of these structures also have pores that are arguably large enough for small molecular species to enter, while others are found to be quasi-amorphous. We also discuss the problem of searching for systems that are currently too large to study by basin hopping (or similar approaches) in a tractable fashion, and the implications of our findings for other solids.