Black crystalline /3-rhombohedral boron

Boron (B), the second hardest element, is the only allotropic element in Group 13. It is second only to carbon (C) in its ability to form element-element bonded networks. Thus, in addition to amorphous boron, several different allotropes of boron are known, of which three are well characterized. These are red crystalline a-rhombohedral boron, black crystalline /3-rhombohedral boron (the most thermodynamically stable allotrope), and black crystalline /3-tetragonal boron. All are polymeric and are based on various modes of condensation of the Bj2 icosahedron (Figure 2). 

The determination of precise physical properties for elemental boron is bedevilled by the twin difficulties of complex polymorphism and contamination by irremovable impurities. Boron is an extremely hard refractory solid of high mp, low density and very low electrical conductivity. Crystalline forms are dark red in transmitted light and powdered forms are black. The most stable ()3-rhombohedral) modification has mp 2092°C (exceeded only by C among the non-metals), bp 4000°C, d 2.35 gcm (a-rhombohedral form 2.45gcm ), A77sublimation 570kJ per mol of B, electrical conductivity at room temperature 1.5 x 10 ohm cm- . 

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