Metalinsulator transition induced by pressure in chemically bonded solids

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Abstract
A review is given of metal-insulator transitions induced by pressure in chemically bonded solids. After an introduction to energy band overlap mechanisms, which appear very useful to characterize the insulator-metal transitions in some solids (the rare gases He and Xe, the iso-electronic sequence of Xe, H2 and I2), some discussion is given of electron correlation in relation to molecular dissociation. However, it is to be emphasized that such dissociation is a sufficient, but not a necessary, condition for metallization in H. and I2. For N2, the above considerations, which focussed dominantly on ground-state properties as a function of pressure, need transcending because the observation of metallization in this system plainly involves thermal effects in an important way. The final part of the review considers the way various low-dimensional solids (polyacetylene, TTF-TCNQ, and appropriate phases of H2 and I2) are altered by application of high pressures. Present understanding leads to the conclusion that there should be an interesting interplay between Hubbard on-site electron correlation, bond-charge repulsion and near-neighbour interaction, as a function of thermodynamic state.