ROLE OF LARGE BUT DEFECTIVE DELTAHEDRA IN THE STRUCTURAL CHEMISTRY OF VERY COMPLEX SOLID BORIDES AND GALLIDES

Large (icosahedra and higher) deltahedra are usually found in extended solid-state arrays with some of their sites not fully occupied, thus giving rise to nonstoichiometric materials. How the electronic characteristics of such deltahedra change with increasing size, the electronic reasons behind the tendency to lose such atoms, and the resulting changes in the electronic structure of the defect deltahedra are described. It is shown that the Wade rule breaks down if two or more adjacent vacancies are created on a deltahedron, and how this influences the electron counting for fused deltahedra is described. Some structural aspects of complex borides and gallides including SiB6, AlB12, beta-tetragonal boron, Li2Na5Ga19.6, Rb0.6Na6.25Ga20.02, and K3Li9Ga28.83 are discussed. An interesting correlation is shown between the sign of the computed atom-atom pair potentials and site occupancy for the twinned icosahedron.