is not a molecular solid, but the icosahedra in it are connected by strong covalent bonds. α-tetragonal boron Pure α-tetragonal can only be synthesized as thin layers deposited on an underlying substrate of isotropic boron carbide (B50C2) or nitride (B50N2). Most examples of α-tetragonal boron are in fact boron-rich carbide or nitrides. β-rhombohedral boron β-rhombohedral boron has a unit cell containing 105–108 (ideally exactly 105) atoms. Most atoms form B12 discrete icosahedra; a few form partially interpenetrating icosahedra, and there are two deltahedral B10 units, and a single central B atom. For a long time, it was unclear whether the α or β phase is most stable at ambient conditions; however, gradually a consensus was reached that the β phase is the most thermodynamically stable allotrope. β-tetragonal boron 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). Further studies have reproduced the synthesis and confirmed the absence of impurities in this phase. γ-boron The γ-phase can be described as a NaCl-type arrangement of two types of clusters, B12 icosahedra and B2 pairs. 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. There is evidence of significant charge transfer from B2 pairs to the B12 icosahedra in this structure; in particular, lattice dynamics suggests the presence of significant long-range electrostatic interactions. This phase was reported by Wentorf in 1965; however, neither structure nor chemical composition were established. The structure was solved using ab initio crystal structure prediction calculations and confirmed using single crystal X-ray diffraction. Cubic boron Sullenger et al. (1969) and McConville et al. (1976) reported a cubic allotrope of boron, obtained in argon plasma experiments, with a unit cell of 1705±3 atoms and a density of 2.367 g/cm3. While this allotrope is occasionally mentioned in the literature, no subsequent work appears to have been published either confirming or discrediting its existence. Donohue (1982) commented 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). High-pressure superconducting phase Compressing boron above 160 GPa produces a boron phase with an as yet unknown structure. Contrary to other phases, which are semiconductors, this phase is a metal and becomes a superconductor with a critical temperature