THE STRUCTURE OF BARIUM IN THE HEXAGONAL CLOSE-PACKED PHASE UNDER HIGH-PRESSURE

Recent experimental results on two hcp phases of barium under high pre ssure show interesting variation of the lattice parameters. They are h ere interpreted in terms of electronic structure calculations by using the linear muffin-tin orbital method and generalized pseudopotential theory with a nearly-free-electron plus tight-binding-bond approach. I n phase II (5.5-12.6 GPa) the dramatic drop in c/a is an instability a nalogous to that in the group II metals Mg to tig but with the transfe r of s to d electrons playing a crucial role in Pa Meanwhile in phase V (45-90 Cpa), the instability decreases a lot due to the core repulsi on at very high pressure. The c/a ratio in phase V is somewhat less th an the expected ideal value due to some admixture of 5d and 4f compone nts in the wave function.