STRUCTURE AND BONDING IN SMALL NEUTRAL ALKALI-HALIDE CLUSTERS

The structural and bonding properties of small neutral alkali halide c lusters, (AX)(n) with n less than or equal to 10, A = Li+,Na+,K+,Rb+, and X=F-,Cl-,Br-,I-, are studied using the ab initio perturbed ion (PI ) model and a restricted structural relaxation criterion. A trend of c ompetition between rocksalt and hexagonal ringlike isomers is found an d discussed in terms of the relative ionic sizes. The main conclusion is that an approximate value of r(C)/r(A) = 0.5 (when r(C) and r(A) ar e the cationic and anionic radii) separates the hexagonal from the roc ksalt structures. The classical electrostatic part of the total energy at the equilibrium geometry is enough to explain these trends. The ma gic numbers in the size range studied are n = 4, 6, and 9, and these a re universal since they occur for all alkali halides and do not depend on the specific ground-state geometry. Instead those numbers allow fo r the formation of compact clusters. Full geometrical relaxations are considered for (LiF)(n) (n = 3-7) and (AX)(3) clusters, and the effect of Coulomb correlation is studied in a few selected cases. These two effects. preserve the general conclusions achieved thus far.