Structure and preparation of boron‐rich borides

The B12 icosahedral compounds may be classified into 8 types: 1. α‐rhombohedral boron type; 2. β‐rhombohedral boron type; 3. α‐tetragonal boron type; 4. β‐tetragonal boron type; 5. AlB10 (or AlC4B26) type; 6. NaB15 type; 7. YB66 type; and 8. γ‐AlB12 type. Both α‐AlB12 and γ‐AlB12 consist of B12 icosahedra and B20 units, and have a B12 icosahedral framework analogous to that in β‐rhombohedral boron. Charge balance among the boron polyhedral units in these phases was investigated by counting the number of contacts between the metal atoms and the boron units. Precision X‐ray diffraction studies on B13 C2, α‐AlB12 and LiAlB14 revealed the difference electron densities, demonstrating three‐center and two‐center bonds in the B12 icosahedral structures. A net charge of (B12)1.2− was observed for LiAlB14. The single crystal growth of some B12 icosahedral borides from metal solution is discussed.