Allotropes of boron

In special circumstances, boron can also be synthesized in the form of its α-tetragonal (α-T) and γ-orthorhombic (γ) allotropes.

The latter allotrope is a very hard[n 1] grey material, about ten percent lighter than aluminium and with a melting point (2080 °C) several hundred degrees higher than that of steel.

[6] Elemental boron has been found in star dust and meteorites, but does not exist in the high oxygen environment of Earth.

[7][8] Very pure boron, for use in the semiconductor industry, is produced by the decomposition of diborane at high temperatures, followed by purification via zone melting or the Czochralski process.

Pure α-tetragonal can only be synthesized as thin layers deposited on an underlying substrate of isotropic boron carbide (B50C2) or nitride (B50N2).

[11][33][34][35][36][37] The β phase was produced in 1960 by hydrogen reduction of BBr3 on hot tungsten, rhenium or tantalum filaments at temperatures 1270–1550 °C (i.e. chemical vapor deposition).

It can be produced by compressing other boron phases to 12–20 GPa and heating to 1500–1800 °C, and remains stable at ambient conditions.

While this allotrope is occasionally mentioned in the literature,[46] no subsequent work appears to have been published either confirming or discrediting its existence.

Donohue (1982) commented[47] that the number of atoms in the unit cell did not appear to be icosahedrally related (the icosahedron being a motif common to boron structures).