Crystal structure borides

Carbide, nitride, boride Crystal structure Clay... 

Figure 6.1 The icosahedron and some of its symmetry elements, (a) An icosahedron has 12 vertices and 20 triangular faces defined by 30 edges, (b) The preferred pentagonal pyramidal coordination polyhedron for 6-coordinate boron in icosahedral structures as it is not possible to generate an infinite three-dimensional lattice on the basis of fivefold symmetry, various distortions, translations and voids occur in the actual crystal structures, (c) The distortion angle 0, which varies from 0° to 25°, for various boron atoms in crystalline boron and metal borides.

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... 

Silico- and phosphorus borides crystallizing with ordered W5Si3-type structures, such as Ni4ftSi2B and Cr. 45P2B, are closely related structurally. ... 

Many ternary borides crystallize with the ordered BaAl4 or ThCr2Si2 structure ... 

Table 2. Crystal Structures and Boron Coordination of Platinum Metal Borides WITH Isolated B Atoms (Owing to Defect Boron Sublattice)...

Boron is as unusual in its structures as it is in its chemical behavior. Sixteen boron modifications have been described, but most of them have not been well characterized. Many samples assumed to have consisted only of boron were possibly boron-rich borides (many of which are known, e.g. YB66). An established structure is that of rhombohedral a-B12 (the subscript number designates the number of atoms per unit cell). The crystal structures of three further forms are known, tetragonal -B50, rhombohedral J3-B105 and rhombohedral j3-B 320, but probably boron-rich borides were studied. a-B50 should be formulated B48X2. It consists of B12 icosahedra that are linked by tetrahedrally coordinated X atoms. These atoms are presumably C or N atoms (B, C and N can hardly be distinguished by X-ray diffraction). 

The prototype hard metals are the compounds of six of the transition metals Ti, Zr, and Hf, as well as V, Nb, and Ta. Their carbides all have the NaCl crystal structure, as do their nitrides except for Ta. The NaCi structure consists of close-packed planes of metal atoms stacked in the fee pattern with the metalloids (C, N) located in the octahedral holes. The borides have the A1B2 structure in which close-packed planes of metal atoms are stacked in the simple hexagonal pattern with all of the trigonal prismatic holes occupied by boron atoms. Thus the structures are based on the highest possible atomic packing densities consistent with the atomic sizes. 

The hardest of the transition-metal borides are the diborides. Their characteristic crystal structure (Figure 10.6) consists of plane layers of close-packed metal atoms separated by plane openly-patterned layers of boron atoms ( chicken-wire pattern). If the metal atoms in the hexagonal close-packed layer have a spacing, d, then the boron atoms have a spacing of d/V3. 

Kiessling, R. The Crystal Structures of Molybdenum and Tungsten Borides. 

Hagg, G. 1931. Regularity in crystal structure in hydrides, borides, carbides and nitrides of transition elements. Z. Physik. Chem. 12B 33-56. 

Parth6, E. and Chabot, B. (1984) Crystal structures and crystal chemistry of ternary rare earth-transition metal borides, silicides and homologues. In Handbook on the Physics and Chemistry of Rare Earths, ed. Gschneidner Jr., K.A. and Eyring, L. (North-Holland, Amsterdam), Vol. 6, p. 113. 

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. 

According to literature sources mainly oxides, nitrides, borides and carbides are used as ceramic raw materials of chemical and structural applications and the most common elements in these compounds are Be, Mg, Ca, Ti, V, Cr, Y, Zr, La, Hf, W, B, Al, Si and Sn. These elements can be found in rather small area of the periodic table, i.e. in the groups 2 up to and including 6, 13 and 14. So apparently relatively few ingredients are used in this branch of ceramics to produce a wide range of products. The tricks of the trade are in the preparation. The properties are determined by a number of factors, such as the nature of the building blocks, the kind of bonds, the strength of the bonds, the crystal structure and the reactivity of the material. 

The CuA12 tetragonal crystal structure, D j, 14/m cm is found for many AM2 type intermetallic and boride compounds (see Table 9.6). The tetragonal cell has four molecules per unit cell, a = 6.067 and... 

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]. 

E. Parthe and B. Chabot, Crystal structures and crystal chemistry of ternary rare earth-transition metal borides, silicides and homologues 113... 

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