Explain the allotropes of boron.
Boron can be prepared in several crystalline and amorphous forms. Well known crystalline forms are α-rhombohedral, β-rhombohedral, and β-tetragonal. In special circumstances, boron can also be synthesized in the form of its α-tetragonal, and γ-orthorhombic allotropes. Two amorphous forms, one a finely divided powder and the other a glassy solid, are also known. Although at least 14 more allotropes have been reported, these other forms are based on tenuous evidence or have not been experimentally confirmed, or are thought to represent mixed allotropes, or boron frameworks stabilized by impurities. Whereas the β-rhombohedral phase is the most stable and the others are metastable, the transformation rate is negligible at room temperature, and thus all five phases can exist at ambient conditions. Amorphous powder boron and polycrystaline rhombohedral β-boron are the most common forms. The latter allotrope is a very hard grey material, about ten percent lighter than aluminium and with a melting point (2080 °C) several hundred degrees higher than that of steel.
α-rhombohedral boron α-rhombohedral boron has a unit cell of twelve boron atoms. The structure consists of B
12 icosahedra in which each boron atom has five nearest neighbors within the icosahedron. If the bonding were the conventional covalent type then each boron would have donated five electrons. However, boron has only three valence electrons, and it is thought that the bonding in the B
12 icosahedra is achieved by the so-called 3-center electron-deficient bonds where the electron charge is accumulated at the center of a triangle formed by three adjacent atoms.
The isolated B
12 icosahedra are not stable; thus boron 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 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 . 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 confirmed using single crystal X-ray diffraction.
Boron can be prepared in several crystalline and amorphous forms. Well known crystalline forms are α-rhombohedral, β-rhombohedral, and β-tetragonal. In special circumstances, boron can also be synthesized in the form of its α-tetragonal, and γ-orthorhombic allotropes. Two amorphous forms, one a finely divided powder and the other a glassy solid, are also known. Although at least 14 more allotropes have been reported, these other forms are based on tenuous evidence or have not been experimentally confirmed, or are thought to represent mixed allotropes, or boron frameworks stabilized by impurities. Whereas the β-rhombohedral phase is the most stable and the others are metastable, the transformation rate is negligible at room temperature, and thus all five phases can exist at ambient conditions. Amorphous powder boron and polycrystaline rhombohedral β-boron are the most common forms. The latter allotrope is a very hard grey material, about ten percent lighter than aluminium and with a melting point (2080 °C) several hundred degrees higher than that of steel.
α-rhombohedral boron has a unit cell of twelve boron atoms. The structure consists of B
12 icosahedra in which each boron atom has five nearest neighbors within the icosahedron. If the bonding were the conventional covalent type then each boron would have donated five electrons. However, boron has only three valence electrons, and it is thought that the bonding in the B
12 icosahedra is achieved by the so-called 3-center electron-deficient bonds where the electron charge is accumulated at the center of a triangle formed by three adjacent atoms.
The isolated B
12 icosahedra are not stable; thus boron is not a molecular solid, but the icosahedra in it are connected by strong covalent bonds.
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 boronβ-rhombohedral boron has a unit cell containing 105–108 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 boronThe β phase was produced in 1960 by hydrogen reduction of BBr3 on hot tungsten, rhenium or tantalum filaments at temperatures 1270–1550 °C . Further studies have reproduced the synthesis and confirmed the absence of impurities in this phase.
γ-boronThe 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 confirmed using single crystal X-ray diffraction.
