Structure borides

Boron is unique among the elements in the structural complexity of its allotropic modifications this reflects the variety of ways in which boron seeks to solve the problem of having fewer electrons than atomic orbitals available for bonding. Elements in this situation usually adopt metallic bonding, but the small size and high ionization energies of B (p. 222) result in covalent rather than metallic bonding. The structural unit which dominates the various allotropes of B is the B 2 icosahedron (Fig. 6.1), and this also occurs in several metal boride structures and in certain boron hydride derivatives. Because of the fivefold rotation symmetry at the individual B atoms, the B)2 icosahedra pack rather inefficiently and there... 

It is helpful to divide the boride structures into four partially overlapping groups based primarily on the arrangement of B atoms ... 

Metal Boride Structures (Isolated Boron Atoms). 

Table 1. Structure Types, Boron Coordination and Representatives of Metal Boride Structures Containing Isolated B Atoms (B—B > 200 pm)...

Borides with Isolated Boron Atoms 6.7.2.1.3. Metal Boride Structures (Isolated Boron Atoms). 

Table 1. Metal Borides Structure Types and Representatives with Boron Pairs... 

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. 

The cubic UB, 2-type boride structure with space group Fm3m can be described on the basis of a B,2-cubooctahedron (see Fig. 1) . The association of the B,2-poly-hedra by oriented B—B bonds gives rise to a three-dimensional skeleton with boron cages. Formally, the arrangement of the B,2-units and of the metals atoms is of the NaCl-type. Each metal is located in the center of a B24-cubooctahedron. 

Fig. 1.12 The iron boride structure type. From Wells (1986).

It has been reported that ScBi2 has a tetragonal modification of the UBi2-type structure with lattice parameters of a = 5.22 A and c = 7.35 A with the space group of I4/mmm (Hamada et al., 1993 Paderno and Shitsevalova, 1995). Due to the small size of scandium as compared to the other rare earth atoms, scandium phases have been observed to form anomalous higher boride structures compared to the heavy lanthanides and yttrium, as will be discussed later in Sections 9 and 11. Small amounts of metal replacement for Sc in Sci xMxBi2 (x as small as 0.1, M = Y, Tm, Lu) have been reported to stabilize the structure in the normal cubic UBi2-type. 

As noted in previous sections, the scandium phases of compounds have sometimes been observed to have different structures compared to the other rare earths, because of the relatively small size of scandium. The scandium higher borides have been found to be a particular fertile ground for the formation of new higher boride structures. 

Table 5-1 summarizes the main trends in metal boride structures. As can be... 

Owing to the great number of structure types represented among the binary and ternary borides, it is not possible to present in this article even a simple description of all known structure types. The following presentation deals mainly with binary boride structmes. Ternary boride structures show many similarities with those of binary boride phases but cannot be treated in any detail in the present paper because of space limitations. Binary and ternary boride structures are treated in more detail elsewhere. It is convenient to describe here... 

The proper metal boride structures with isolated boron atoms include approximately 20 structure types with many ternary representatives. The boron coordination is trigonal prismatic (in the Fe3C, TisP, ResB ThvFeB structure types), octahedral (in RhsB4), or Archimedian prismatic (in the CuAl2-type structure with many M2B representatives). 

A presentation of binary and ternary boride structure types, which are bmlt up of a rigid three-dimensional boron network, is given in Table 3. Many ternary boride phases also crystallize in the structure types collected in Table 3. It is noticeable that the B/M ratio ranges as much as from 2.5 to 66. 

The metal borides are one of the five major classes of boron compounds (1). In the following we review the geometric and electronic structural data with an emphasis on the transition metal borides. Because the structures of transition metals and elemental boron provide end points, we begin by reviewing the solid state structures of these elements. A brief survey of the range of metal boride structures in general is followed by some more detailed consideration of the problems of electronic structure raised by the geometries of the transition metal borides. 

Metal-Rich Borides Structural Arrangement of the Boron Atoms... 

Fehlner, T.P. Molecular models of solid state metal boride structure. J. Solid State Chem. 154, 110-113 (2000)... 

Kuzma, Yu.B. Bilonizhko, N.S. (1974). New boride structural types in the homologous series based on the CaCus and CeCos types. Sov. Phys. Crystailogr. 18,447-449. 

Among the remarkably diversified variety of binary and ternary meta boride structures, the triangular prismatic coordination MgB has the dominating role, whereas among metal phosphides, sillcides and carbides this is merely true for a limited series of structure types. 

Differing from the compounds described above, a second group of R B Si compounds are found to form when silicon atoms are inserted or replace boron atoms in rare earth boride structures, or boron is inserted or replaces sihcon in rare earth sUicides. 

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