Elementary boron and related borides

The crystal structures of 0-812 and iS-Bios- (a) and (b), packing of 812 units in 0-812. (c) Section through the structure of 0-812 perpendicular to a 3-fold axis showing the 3-centre bonds, (d) The 834 unit of 3-Bios showing the central icosahedron and six only of the half-icosahedra , one of which is attached to each B atom of the central Bi 2 group. 

There are many compounds with the same general type of structure as B4C and rhombohedral Bj 2 though the details of their structures are not in all cases certain. The C3 or CBC chains may be replaced by S in B12 S, by P—B-P in B13 P2 (B-P in chain, 1-87 A, P-Bjj g, 2 03 A), 0—B—0 in B13O, or by 2 Si in B2.s9Si. Since there would appear to be room for only 2 Si replacing 3 C there is apparently some (statistical) replacement of B by Si in the icosahedra, giving a composition close to Bar Si 11 (i.e. B3iSi5. Si6 or 63.39 Si). 

There is a closely related (orthorhombic) phase Al2.iCgB51 (variously described as AlBi 0, AlBj 2, or AIC4B24) in which there is distorted hexagonal closest packing of Bj 2 icosahedra and linear CBC chains, in some of which A1 replaces B. As in B4C each Bi2 group is directly bonded to six others via B-B bonds (1-77 A) so that six B atoms of each icosahedron form 6 B—B bonds and the remainder are bonded to 5 B + C. The bond lengths are similar to those in B4C  

The close relation between these two structures is shown by the fact that the A1 compound undergoes a topotactic transition by loss of A1 at 2000°C to form B4C, a process involving a change from h.c.p. to c.c.p. Bj 2 icosahedra. 

In the isostructural NaB 1 j and MgAlB 14 the B12 icosahedra are packed in layers which are stacked vertically above one another, so that only two direct pentagonal pyramidal contacts between icosahedra result (perpendicular to the layers). In the layers each B[ 2 is bonded to four others at the same distance (1-75 A) but there is also bonding through Mg, Al, and additional B atoms. The interstitial atoms obviously play an essential part in stabilizing this mode of packing of the icosahedra. 

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