Metal borides forms

The phase equilibria in the ternary systems Fe-B-N, Co-B-N, and Ni-B-N have been determined from X-ray powder diffraction data and isothermal sections at 900°C and 1 atm Ar have been constructed. In all three systems the binary transition metal borides form two-phase equilibria with a-BN [28]. The phase equilibria are depicted in Fig. 4-29 to 4-31. 

Boron forms B—N compounds that are isoelectronic with graphite (see Boron compounds, refractoryboron compounds). The small size also has a significant role in the interstitial alloy-type metal borides boron forms. Boron forms borides with metals that are less electronegative than itself including titanium, zirconium, and hafnium. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

Table 1 fists many metal borides and their observed melting points. Most metals form mote than one boride phase and borides often form a continuous series of solid solutions with one another at elevated temperatures thus close composition control is necessary to achieve particular properties. The relatively small size of boron atoms facilitates diffusion. 

The formation of higher transition-metal borides depends on the competition and the statistical weight of the d and states of the metal atoms. Consequently, the acceptor metals Ni, Pd and Pt are expected to form metal-rich borides only i.e., besides the known PtBo.7 (anti-NiAs), Pt forms two borides, Pt 2B and Pt 4B, which... 

Thorium is one of the largest metals to form Mt MjB4 borides. The ThMoB4 type, however, has a different arrangement of the five- and seven-membered B rings... 

Little information is available on homogeneity ranges and defect structures in the dodecaborides. The only variation from stoichiometry in these borides is for YB,2i the limiting phase determined by density measurements is Yq92B,2. This result can be attributed to the size of Y which is the maximum for metals that form the dodecaborides. Attempts to prepare DyB,2 with a nonstoichiometric composition are conclusive. ... 

A number of metals that form MB4 and MB borides, and virtually all of those that contribute to the MB12 and MBg series also form MB2 borides (Table 1). 

The actinide borides must be considered a special case. The metallic radii exhibited in the borides are correlated with the abilities of the metals to form various... 

Among the tetraborides, UB4 has the smallest volume and hence the smallest effective radius. Thus an actinide element having a metallic radius of 1.59 A (Pu) or smaller forms a diboride, while those having larger radii do not. As in the rare-earth series, the actinides able to form MB4, MBg and MB,2 borides form also MB2 diborides (Table 1). 

Borides of Al, Ti, Zr and Hf are prepared according to reaction (a), although even with careful measuring of reactants it is not always possible possible to form the metal boride in stoichiometric ratio, and free metal B may be deposited . This method is not suitable when the free metal deposits at T below those required for boride formation, as in the cases of Nb, Ta, Mo and W. 

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. 

A large number of metal borides have been prepared and characterized. Several hundred binary metal borides M Bj, are known. With increasing boron content, the number of B-B bonds increases. In this manner, isolated B atoms, B-B pairs, fragments of boron chains, single chains, double chains, branched chains, and hexagonal networks are formed, as illustrated in Fig. 13.3.1. Table 13.3.1 summarizes the stoichiometric formulas and structures of metal borides. 

There are also important classes of metallic compounds in which one component is not metallic in its elemental forms. The transition metal borides and carbides are such compounds, and TiC, which has a simple structure, has been selected for discussion in some detail. Indeed AuSn as well would be such a case if a—Sn were the only form of that element. In TiC X-ray absorption and emission and photoelectron results have been obtained for the core levels of both Ti and C. Ramqvist and co-workers have analyzed these data by comparing the shifts of different core levels in the same atom (22). This approach avoids difficulties caused by uncertainties both in the dep terms and in Fiatt, but it requires dealing with the small differences between already small individual level shifts. The relevant atomic and molecular volumes are tabulated in Table 4. If diamond, the carbon analog of a—Sn, is used as the volume characteristic of C, one has... 

Compounds of boron and the less electronegative elements (metals and metalloids) are loosely referred to as borides, a very large class encompassing hundreds of known compounds. Well-characterized binary transition metal borides number over 120, and include compounds of all of the d-block elements except Zn, Ag, Cd, Au, and Hg (Cr alone forms nine known borides). 

---