Layers borides

In borides of the type M3B4 the boron atoms form double chains in which the central bonds are apparently much stronger than those along the length of the chain (Fig. 24.4). The TasB4 structure is in this way intermediate between the chain and layer borides. 

In AlBi the B atoms are arranged in layers with layers of A1 interleaved between them. The structure of a B layer is the same as that of a layer in the graphite structure (p. 734). Each B is here equidistant from three other B atoms (at T73 A), the next nearest neighbours being a set of six A1 at the vertices of a trigonal prism (Fig. 24.3(b)). Most of the layer borides have the AlBj structure, but in the Mo-B and W—B systems there are also phases with the ideal composition M Bs,... 

To produce wear-resistant or hardened surfaces, thin layers of borides can be prepared on metal surfaces by reaction and diffusion (see Metal SURFACE treatments). Boride powders can be formed iato monolithic shapes by cold pressing and sintering, or by hot pressiag. 

Borides of Group Via. As with the borides of Group Va, the incorporation of free metal in the Group Via borides is difficult to avoid. Both tungsten and molybdenum borides are obtained at high temperature by the hydrogen reduction of the mixed bromides.Bonding appears a more effective method to form these borides in thin layers (see Sec. 2.2 above). 

Borides are relatively inert, especially to non-oxidizing reagents. They react violently with fluorine, often with incandescence. Reaction with other halogens is not as violent and may require some heat. Resistance to oxidation, acids, and alkalis is summarized in Table 17.5. In oxidation conditions, a layer of boric oxide is formed on the surface which passivates it to some degree. Boric oxide melts at 450°C and vaporizes at 1860°C. It offers good protection up to 1500°C in a static environments but it has low viscosity at these temperatures and tends to flow under stress and the protection it offers is limited.f k l... 

Nickel atoms in BajNi B form distorted, puckered 3.6.3.6-kagome nets stacked in six layers perpendicular to the c axis. The densely packed framework of trigonal-Ni prisms again result in boron-pair formation, although Ba atoms are too large to be sandwiched between two Ni layers, and only four Ba can be accommodated within six Ni layers. Superconductivity is found for ( a, Sr, Ba)2pt9Bg borides with a structure related to Ba2Ni9Bfi and e o,B2 however, with respect to crystal chemistry and boron coordination, only the subcell is derived so far. 

Planar hexagonal boron layers are also found in a lower boride structure Pr5, Co2+xBg (0 < X < 1), where B and Co atoms are substituting each other to some extent. 

Fibers of titanium diboride can be prepared by reaction (a) at 400°C in an electrical discharge. Adherent layers of certain metal borides on metal substrate surfaces are obtained by thermal decomposition of metal (Mo, W, Nb, Ta) halides and BBr3 on a metallic substrate using a solar furnace or induction heating ... 

Structures of the lanthanide nitridoborates appear as layered structures with approximate hexagonal arrangements of metal atoms, and typical coordination preferences of anions. As in many metal nitrides, the nitride ion prefers an octahedral environment such as in lanthanum nitride (LaN). As a terminal constituent of a BNx anion, the nitrogen atom prefers a six-fold environment, such as B-N Lns, where Ln atoms form a square pyramid around N. Boron is typically surrounded by a trigonal prismatic arrangement of lanthanide atoms, as in many metal borides (Fig. 8.10). All known structures of lanthanide nitridoborates compromise these coordination patterns. 

Moreover, stable liquid systems made up of nanoparticles coated with a surfactant monolayer and dispersed in an apolar medium could be employed to catalyze reactions involving both apolar substrates (solubilized in the bulk solvent) and polar and amphiphilic substrates (preferentially encapsulated within the reversed micelles or located at the surfactant palisade layer) or could be used as antiwear additives for lubricants. For example, monodisperse nickel boride catalysts were prepared in water/CTAB/hexanol microemulsions and used directly as the catalysts of styrene hydrogenation [215]. 

The hardest of the transition-metal borides are the diborides. Their characteristic crystal structure (Figure 10.6) consists of plane layers of close-packed metal atoms separated by plane openly-patterned layers of boron atoms ( chicken-wire pattern). If the metal atoms in the hexagonal close-packed layer have a spacing, d, then the boron atoms have a spacing of d/V3. 

The process of forming boron-boron bonds is carried on further in aluminum boride, AIB, which has a very simple hexagonal structure, consisting of hexagonal layers of boron a,toms, like the layers of carbon atoms in graphite, with aluminum atoms in the spaces between the layers (Pig. 11-15). The B—B bond length is 1.73 A, corresponding to n = 0.66 that is, two valence electrons per boron atom are used in the B—B bonds, which are two-thirds bonds. 

Fig. 4.9. Formation of a molybdenum boride layer between Mo and B-containing phase (a) and between Mo and MoB (b)289 Annealing temperature 1450°C. Time (a), 9000 s (2.5 h) (b), 7200 s (2 h). Photographs kindly provided by Prof. W. Lengauer. Reprinted with permission from Elsevier Science.

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