The structures and stability of BnNn clusters with octagon(s)

Abstract

The structures and stability of (BN)n clusters with alternate B and N atoms containing squares, hexagons and octagons ((BN)n-F4F6F8) are investigated by using density functional theory. The results demonstrate that the isomers of (BN)n-F4F6F8 clusters generally satisfy the isolated-square rule (ISR) and the square adjacency penalty rule (SAPR). The energetically favorable isomers generally have fewer square–square bonds, larger HOMO–LUMO gaps, lower sphericity and asphericity, as well as lower pyramidalization of B and N atoms than other structures. As a whole, the stability of (BN)n-F4F6F8 clusters decreases with the number of octagons. However, four isomers containing one or two octagons in four isomeric clusters (i.e. (BN)n-F4F6F8 (n = 19, 20, 23, and 24) are more thermodynamically stable than their (BN)n-F4F6 counterparts. Further structural analysis demonstrates that octagon(s) of (BN)n-F4F6F8 clusters can release the strain energy by decreasing the pyramidalization angles of the corresponding vertex. Finally, the entropy effect is examined to evaluate the relative stability of (BN)n-F4F6F8 (n = 19, 20, 23, and 24) clusters at high temperatures.