Theoretical and computational study of high-pressure structures in barium

Recent high-pressure work has suggested that elemental barium forms a high-pressure self-hosting structure (Ba IV) involving two "types" of barium atom. Uniquely among reported elemental structures it cannot be described by a single crystalline lattice, instead involving two interpenetrating incommensurate lattices. In this Letter we report pseudopotential calculations demonstrating the stability and the potentially disordered nature of the "guest" structure. Using band structures and nearly free electron theory we relate the appearance of Ba IV to an instability in the close-packed structure, demonstrate that it has a zero energy vibrational mode, and speculate about the structure's stability in other divalent elements.