Structures of borides

Nonclassical type structures of borides involving different basic units of B atoms, e.g., the B5 pentagonal pyramid in MgB4, the B octahedron in Sm2B3 and finally the B 2 octahedra in Mg2Bi4, MgAlBi4 and NaxBogBi4, are considered here. 

From the discussion in Section II,C it should be clear that there is a relationship between the structures of borides and boranes as far as the boron networks are concerned. A comparison of the literature shows that the known structural diversity of the boranes is much greater than that of the borides. This may well be due to the rigorous conditions required for boride preparation that is, only the most stable boron networks are formed. TTie low-temperature routes illustrated in the foregoing work suggest an approach to presently unknown compounds with novel boride networks. Hence, the multitude of known metalla-boranes constitute a stockpile of potential precursors to metal borides with new and perhaps useful properties. 

FIG. 24.2. (a) Layers of Ni atoms in NiaB. At the right is shown a portion of a layer parallel to the plane (loll) in hexagonal close-packing, (b) Layer of metal atoms in borides with the CuAlj structure, (c) The structure of borides Mj M B. ... 

Similar to silicates, the crystal structures of borides can easily be classified according to the arrangement of the boron atoms. Boron may occur as an isolated atom or form B-B bonds with an increasing degree of interconnection in the chains, double chains, layers and frameworks and combinations thereof (Fig. 1). Due to the strong covalent bonding between the boron atoms and the electron deficiency of the three-center bond a number of complex and unique structures result which... 

Following the previous description of the atomic structure of boride, carbide and nitride ceramics, Table 2 lists physical crystalline stmctural differences of a variety of UHTCs along with respective density and melting point. Note that density increases with increasing mass of the metal atom. Note also the differences in melting points between materials whereby the carbides typically have the highest melting points, above borides or nitrides of the same metal constituent. 

Borides, carbides, hydrides — Boride, Carbide, Hydride 3.1 Table of the structures of borides - Tabelle der Strukturen der Boride (System containii B, B-C and B-H but not O, N or a halogen- B-, B-C- nnd B-H-haltige Systeme ohne O, N oder Halogen )) ... 

Fig. 4. Micro structure of borided layer on the EN CIS steel consisting of FeB (dark) and FezB (light) phases.

Boe] Boehmann, G., Klaus, E., Fritsche, G., Wagnre, W., Phase Structure of Boride Layers on Iron Substrates with Eleetroplated Cobalt Layers , Crysi Res. Techn., 22(7), 961-967 (1987) (ExperimentaL Morphology, 14)... 

The structural complexity of borate minerals (p. 205) is surpassed only by that of silicate minerals (p. 347). Even more complex are the structures of the metal borides and the various allotropic modifications of boron itself. These factors, together with the unique structural and bonding problems of the boron hydrides, dictate that boron should be treated in a separate chapter. 

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 ... 

The structures of boron-rich borides (e.g. MB4, MBfi, MBio, MB12, MBe6) are even more effectively dominated by inter-B bonding, and the structures comprise three-dimensional networks of B atoms and clusters in which the metal atoms occupy specific voids or otherwise vacant sites. The structures are often exceedingly complicated (for the reasons given in Section 6.2.2) for example, the cubic unit cell of YB e has ao 2344 pm and contains 1584 B and 24 Y atoms the basic structural unit is the 13-icosahedron unit of 156 B atoms found in -rhombohedral B (p. 142) there are 8 such units (1248 B) in the unit cell and the remaining 336 B atoms are statistically distributed in channels formed by the packing of the 13-icosahedron units. 

MP2, MAs2 and MSb2 all have a compressed form of the marcasite structure, while the carbides MC have trigonal prismatic coordination in the WC structure. Several borides are known MB2 has nets of boron atoms. RunBg has branched chains while RU7B3 has isolated borons. 

Borides, in contrast to carbides and nitrides, are characterized by an unusual structural complexity for both metal-rich and B-rich compositions. This complexity has its origin in the tendency of B atoms to form one- two-, or three-dimensional covalent arrangements and to show uncommon coordination numbers because of their large size (rg = 0.88 10 pm) and their electronic structure (deficiency in valence electrons). The structures of the transition-element borides are well established " . 

The stabilizing influence of small amounts of B (M/B > 0.25) in the voids of the metal host lattice varies with the mode of filling (partial or complete) of the interstitial, mostly O, sites and whether the compounds develop from the binary-intermetallic host lattice. The structures of B-rich compounds (M/B < 4) are mainly determined by the formation of regular, covalent B polyhedra (O, icosahedron) and the connections between them (B frame structures). Typical metal (M) borides therefore are found within a characteristic ratio of metal to boron 0.125 < M/B < 4. 

The crystal structures of the borides of the rare earth metals (M g) are describedand phase equilibria in ternary and higher order systems containing rare earths and B, including information on structures, magnetic and electrical properties as well as low-T phase equilibria, are available. Phase equilibria and crystal structure in binary and ternary systems containing an actinide metal and B are... 

Formation of tetrahedral My4 clusters is the structural unit common to all known MRgMT4B4-type phases with strong M.,—B, but weak B-M g interaction in accord to the characterization of borides. Involving two remote Mre atoms, the B... 

Table 1. Correlation of Metallic Radii with the Existence of the Various Type Structures of the Higher Borides...

Table 2. The Ranges in Appropriate TO THE Several Structural Type (Borides OF THE Actinides Are Excluded)...

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