Instability of the CsCl Structure in Ionic Solids at High Pressures

It is shown by a simple model calculation that the ionic solids in the CsCl phase transform discontinuously to a tetragonal phase under high pressures at a fractional volume in the neighborhood of 0.5. This general result follows as a consequence of competing Madelung energy and repulsive energy terms. The $\frac{c}{a}$ variation with pressure and transition volume fraction are in approximate agreement with the recent ultrahigh pressure experiments on cesium halides.