| Summary: | Barium is also known for having two hexagonally close packed phases at lower and higher pressures than the hotel structure. Although the two hexagonally close packed phases have different properties we show that barium would transform continuously between these phases but for the intercession of the hotel phase. The differences between the two are predominantly caused by the progressive transfer of electrons from s-type orbitals to d-type orbitals. At ambient pressures the group-V elements have a rhombohedral structure which is a distortion of a simple-cubic arrangement. As the pressure is increased, the distortion decreases and the rhombohedral structure transforms continuously back to simple cubic. Experimentally, however, a first-order discontinuous transformation is seen in arsenic. In bismuth and antimony, transitions to completely different structures occur before the rhombohedral to simple cubic transition can occur. A Landau-type model is developed to describe the rhombohedral distortion. This model predicts a gradual transfer to simple cubic, not only with increasing pressure but also with increasing temperature. Although this prediction is confirmed with some molecular dynamics calculations there is unfortunately no experimental data. As well as <i>ab initio </i>calculations we also present a many-body interaction potential which is based on that of Finnis and Sinclair but which describes separately s- and d-type electrons. With this potential it is possible to get a transfer of electrons from s-type orbitals to d-type orbitals as the pressure is increased. It is able to produce an iso-structural transition with a volume collapse. By considering spin bands, the model can also be modified to model magnetic systems.
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