Transitions to Metallic States in Ionic Crystals, with Particular Reference to Cesium Iodide

Shock-wave experiments by Alder and Christian have indicated that metallic transitions might occur in certain ionic crystals. Therefore, one heavy ionic crystal, CsI, has been studied by the cellular method of band theory, both for the normal lattice parameter and for a reduced spacing corresponding to an estimated pressure of ∼250 000 atmospheres. Our principal conclusion is that the energy gap between valence and conduction bands varies little with pressure in the range we consider. A gap of 6 or 7 ev is obtained with our relatively crude potential. The width of the valence band is estimated at 3-4 ev, and with our potential, and neglect of spin-orbit effects, a state at the zone edge along [100] appears to be the highest in this band, while the conduction band minimum is at k=0. The results under normal conditions are in good agreement with the photoemission experiments of Philipp, Taft, and Apker, both for the gap and the valence bandwidth, but the shock-wave experiments of Alder and Christian cannot be understood along the present lines. Later static pressure experiments by Griggs et al., are not, however, in conflict with our findings.