Boron boride

All known structures of elemental boron contain icosahedral B₁₂ clusters. Recently, a new form of elemental boron called γ-B₂₈ has been characterised. Its structure resembles that of a sodium chloride, with the B₁₂ icosahedra and B₂ pairs playing the roles of ‘anions’ and ‘cations’, respectively.

“The example of γ-B₂₈ shows that significant ionicity can occur in elemental solids”, the authors wrote. “Because of the charge transfer between these two components, the new phase can be regarded as a boron boride (B₂)^δ+(B₂₈)^δ–.”