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The boron framework of YB 41 Si 1.2 can be described as a layered structure where two boron networks (figures 9a,b) stack along the z-axis. One boron network consists of 3 icosahedra I1, I2 and I3 and is located in the z = 0 plane; another network consists of the icosahedron I5 and the B 12 Si 3 polyhedron and lies at z = 0.5.
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 increasing from 6 K at 160 GPa to 11 K at 250 GPa. [ 48 ]
The crystal twinning with icosahedral shapes also occurs in crystals, especially nanoparticles. [29] Many borides and allotropes of boron such as α-and β-rhombohedral contain boron B 12 icosahedron as a basic structure unit. [30]
a The number n in the atom designation Bn,n refers to the B 12-nth icosahedron to which the Bn,n belongs. Si6.n and B6.n belong to the B 12 Si 3 unit. b,c,d The Si and B sites are in the same interstice, which is assumed to be fully occupied by both Si and B atoms with occupancies of Occ.(Si) and Occ.(B), respectively, where Occ.(Si)+Occ.(B) = 1.
The borides can be classified loosely as boron rich or metal rich, for example the compound YB 66 at one extreme through to Nd 2 Fe 14 B at the other. The generally accepted definition is that if the ratio of boron atoms to metal atoms is 4:1 or more, the compound is boron rich; if it is less, then it is metal rich.
The dodecaborate(12) anion, [B 12 H 12] 2−, is a borane with an icosahedral arrangement of 12 boron atoms, with each boron atom being attached to a hydrogen atom. Its symmetry is classified by the molecular point group I h.
Boron (III) trifluoride structure, showing "empty" boron p orbital in pi-type coordinate covalent bonds. The trihalides adopt a planar trigonal structures, in contrast to the behavior of aluminium trihalides. All charge-neutral boron halides violate the octet rule, hence they typically are Lewis acidic.
Boron arsenide also occurs as subarsenides, including the icosahedral boride B 12 As 2.It belongs to R 3 m space group with a rhombohedral structure based on clusters of boron atoms and two-atom As–As chains.