Metal borides boron atom arrangement

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. 

The electrical conductivity of these alloys is metallic i.e., inverse proportional to the temperature between 4 to 300 K and is about 7 x 106 (Qm) 1 at room temperature. The crystal structure determination [7] shows the B atomic arrangement to consist of coplanar 4B clusters (isolated from one another) with three B atoms at the vertices and one B at the center of a triangle as shown in Fig. 5. This isolated 4B atomic arrangement has no precedent either in metal-borides [14] or boron hydrides [15]. 

Fig. 9. (a) Arrangement of trigonal prisms in a close-packed structure showing a boron atom in one trigonal prismatic hole and the locations of the next nearest neighbor holes p, q, and r. (b) A three dimensional representation of the boron atom environment in (a), (c) A representation of the structure of the boride MB2, where all holes are filled and the boron atoms form a hexagonal net within the close-packed metal lattice. 

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

Borides with isolated boron atoms. These include most of those with low B to M ratios such as M4B, M3B, M2B, M5B2 and M7B3. In the M4B and M2B structures, boron atoms lie in triangular-prismatic or square-antiprismatic holes between multiple layers of metal atoms. In the others, the metal atoms are arranged in approximately close-packed arrays, with the boron atoms in triangular-prismatic interstices. 

Rogl and Nowotny (1978) have observed that the type of homonuclear linkage in borides can be correlated in simple form with the metal boron ratio of the compound. All ternary rare earth-transition metal borides are listed in table 20, arranged according to decreasing metal boron ratio, and the observed homonuclear linkage of the B atoms. One notes that if the ratio is larger than 1.75 only isolated B atoms occur. Boron dumbbells are found with values between 1,3 and 1.0. Two homonuclear bonds occur first with a metal to boron ratio of 1 and three... 

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. 

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