Electronic Properties of Boron Octahedral Cluster.

摘要

A series of lithium-boron alloys have been developed in recent years at the Naval Surface Weapons Center. Principally through x-ray and NMR analysis, in the composition range, Li/B = 80/20 approx. 30/70 atomic %, two cubic phases, Li2B3 and LiB2 have been tentatively identified. In these compounds, boron essentially exists as a six-atoms octahedral cluster much similar to the boron cluster in recently found LiB6 as well as in other MB6 metal-boride compounds. However, the principal difference between Li2B3, LiB2 (Hereafter to be referred to as the alloy phases) and LiB6 is that the boron cluster are directly linked to one another in LiB6 whereas the boron clusters are insulated from one another by lithium or non-clustered boron atom in the alloys. Physically, the alloy phases are also dramatically different from LiB6; LiB6 is brittle and stable in air whereas the alloy phases are ductile and react with air. The picture of the electronic properties of the B6 octahedron that emerges from the present study is in reasonable agreement with much of the earlier work using simplified LCAO formalisms. It has been possible to classify the B6 boron cluster orbitals within the cluster as whether internal or external in nature and to further divide these classes into bonding and anti-bonding combinations. That fourteen electrons are required to fill the internal bonding orbitals of the B6 octahedron is confirmed and in agreement with the conventional picture. The response of the B6 cluster orbitals to different local lattice structures has also been investigated.