Borides stoichiometry

The crystal stmeture and stoichiometry of these materials is determined from two contributions, geometric and electronic. The geometric factor is an empirical one (8) simple interstitial carbides, nitrides, borides, and hydrides are formed for small ratios of nonmetal to metal radii, eg, < 0.59. 

The various stoichiometries are not equally common, as can be seen from Fig. 6.5 the most frequently occurring are M2B, MB, MB2, MB4 and MBfi, and these five classes account for 75% of the compounds. At the other extreme RunBg is the only known example of this stoichiometry. Metal-rich borides tend to be formed by the transition elements whereas the boron-rich borides are characteristic of the more electropositive elements in Groups 1-3, the lanthanides and the actinides. Only the diborides MB2 are common to both classes. 

Figure 6.S Frequency of occurrence of various stoichiometries among boride phases ...

The structures of metal-rich borides can be systematized by the schematic arrangements shown in Fig. 6.6, which illustrates the increasing tendency of B atoms to catenate as their concentration in the boride phase increases the B atoms are often at the centres of trigonal prisms of metal atoms (Fig. 6.7) and the various stoichiometries are accommodated as follows ... 

Hydrides of the types AnHi (An = Th, Np, Pu, Am, Cm) and AnHs (Pa —> Am), as well as ThaHis (i.e. ThHs.yj) have been so obtained but are not very stable thermally and are decidedly unstable with respect to air and moisture. Borides, carbides, silicides and nitrides (q.v.) are mostly less sensitive chemically and, being refractory materials, those of Th, U and Pu in particular have been studied extensively as possible nuclear fuels.Their stoichiometries are very varied but the more important ones are the semi-metallic monocarbides, AnC, and mononitrides, AnN, all of which have the rock-salt structure they are predominantly ionic... 

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. ... 

Line compounds. These are phases where sublattice occupation is restricted by particular combinations of atomic size, electronegativity, etc., and there is a well-defined stoichiometry with respect to the components. Many examples occur in transition metal borides and silicides, III-V compounds and a number of carbides. Although such phases are considered to be stoichiometric in the relevant binary systems, they can have partial or complete solubility of other components with preferential substitution for one of the binary elements. This can be demonstrated for the case of a compound such as the orthorhombic Cr2B-type boride which exists in a number or refractory metal-boride phase diagrams. Mixing then occurs by substitution on the metal sublattice. 

The crystal structure and stoichiometry of these materials is determined from two contributions, geometric and electronic. The geometric factor is an empirical one (8) simple interstitial carbides, nitrides, borides, and hydrides are formed for small ratios of nonmetal to metal radii, eg, rx / rM < 0.59. When this ratio is larger than 0.59, as in the Group 7—10 metals, the structure becomes more complex to compensate for the loss of metal—metal interactions. Although there are minor exceptions, the H gg rule provides a useful basis for predicting structure. 

A large number of binary metal borides have been prepared and characterized. Their stoichiometries vary from M5B to MBioo, but the most common are M2B, MB, MB2, MB4, MBe, and MB12. The structures of metal borides vary from isolated boron atoms in borides from M4B to M2B, to chains of boron atoms in MB and M3B4, to two-dimensional networks of boron atoms in MB2 and M2B5, to complex three-dimensional arrays of boron atoms in MB4, MBe, and MB12. In general, metal borides exhibit characteristics of typical metals, for... 

With the exception of some special cases, two major classes of structure can be distinguished. In the first, the metal-rich borides possess boron atoms at the centers of trigonal prisms of metal atoms, and the boron atoms interact in one- or two-dimensional homonuclear networks. This class covers the range of stoichiometries from M3B to MB2. Presumably these structures, particularly those containing the highest metal-to-boron ratio, are controlled by the requirements of the metal... 

Fig. 18. The Hf-B-C isothermal ternary cross sections at three different temperatures. Points A and B represent two possible stoichiometries for processing hafnium boride/hafnium carbide composites. After Rudy [57],...

In this section, details of an easily controllable, safe method for producing high-purity Hz gas are described. This method of generating Hz gas is particularly suitable for providing a clean source of Hz gas for use as an anodic fuel in fuel cells or as a fuel for internal combustion engines in transportation applications. This compact, portable Hz generator is based on a non-pressurized, aqueous solution of alkaline sodium borohydride (NaBH, tetrahydroborate). As found by Schlesinger et al., when aqueous NaBH, solutions contact selected metal (or metal boride) catalysts, these solutions hydrolyze to yield Hz gas and water-soluble, sodium borate. Overall reaction stoichiometry can be represented in a simplified form as ... 

Phosphides resemble in many ways the metal borides (p. 145), carbides (p. 297), and nitrides (p. 417), and there are the same difficulties in classification and description of bonding. Perhaps the least-contentious procedure is to classify according to stoichiometry, i.e. (a) metal-rich phosphides (M/P > 1), (b) monophosphides (M/P =1), and (c) phosphorus-rich phosphides (M/P < 1) ... 

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