Page "Boron" Paragraph 10

Boron is similar to carbon in its capability to form stable covalently bonded molecular networks.

Even nominally disordered ( amorphous ) boron contains regular boron icosahedra which are, however, bonded randomly to each other without long-range order.

Crystalline boron is a very hard, black material with a high melting point of above 2000 ° C.

It exists in four major polymorphs: α, β, γ and T. Whereas α, β and T phases are based on B < sub > 12 </ sub > icosahedra, the γ-phase can be described as a rocksalt-type arrangement of the icosahedra and B < sub > 2 </ sub > atomic pairs.

It can be produced by compressing other boron phases to 12 – 20 GPa and heating to 1500 – 1800 ° C ; it remains stable after releasing the temperature and pressure.

The T phase is produced at similar pressures, but higher temperatures of 1800 – 2200 ° C.

As to the α and β phases, they might both coexist at ambient conditions with the β phase being more stable.

Compressing boron above 160 GPa produces a boron phase with an as yet unknown structure, and this phase is a superconductor at temperatures 6 – 12 K.