POLYHEDRAL IONIC MOLECULES

Small isolated clusters of alkali halides and analogous divalent speci es composed of closed-shell atomic ions are known to prefer densely pa cked structures similar but not identical to the repent units found in the solid state. Beyond the simple cube structure that occurs, for ex ample, in alkali halide tetramers, no other ionic polyhedral structure s containing only atomic ions are known. In this article, we explore t he possibility of other three-dimensional structures, and we show comp utationally that two molecules, Mg2Al4O8 and Na4Mg4O6, have very stabl e distorted rhombic dodecahedron structures. We therefore suggest that a wide variety of stable polyhedral structures could be constructed f rom ions, and we offer a simple electrostatic model for predicting whi ch polyhedral ionic structures should be stable and which should not. We Feint out that several recently observed inorganic solid state mate rials contain core structures that fit our predictions.