Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany;
Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK

Recent experimental results on two hcp phases of barium under high pressure show interesting variation of the lattice parameters. They are here 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. In phase II (5.5-12.6 GPa) the dramatic drop in c/a is an instability analogous to that in the group II metals Mg to Hg but with the transfer of s to d electrons playing a crucial role in Ba. Meanwhile in phase V (45-90 GPa), the instability decreases a lot due to the core repulsion at very high pressure. The c/a ratio in phase V is somewhat less than the expected ideal value due to some admixture of 5d and 4f components in the wave function.

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Journal of Physics: Condensed Matter, 9 3489-502, 1997.