Boron tetragonal

The physical properties of elemental boron are significantly affected by purity and crystal form. In addition to being an amorphous powder, boron has four crystalline forms a-rhombohedral, P-rhombohedral, a-tetragonal, and -tetragonal. The a-rhombohedral form has mp 2180°C, sublimes at approximately 3650°C, and has a density of 2.45 g/mL. Amorphous boron, by comparison, has mp 2300°C, sublimes at approximately 2550°C, and has a density of 2.35 g/mL. 

The a-tetragonal form of boron has a unit cell B qC2 or B qN2 it always has a carbon or nitrogen in the crystal. The cell is centered a single-boron atom is coordinated to four icosahedrons (4Bj2 + 2B). The -tetragonal form has a unit cell of 192 boron atoms but is not, as of this writing, totally defined. 

The stmcture of boron phosphate prepared under normal atmospheric conditions consists of tetragonal bipyrimids analogous to the high cristobahte form of siUca. Both the boron and phosphoms are tetrahedraHy coordinated by oxygen. Similar siUcalike stmctures ate found for BAsO and TaBO (156). A quart2like form of boron phosphate can be prepared by heating the common form to 500°C at 5.07 GPa (50,000 atm) (157). 

Figure 6.4 Crystal structure of ar-tetragonal boron. This was originally thought to be B50 (4Bi2 + 2B) but is now known to be either B50C2 or B50N2 in which the 2C (or 2N) occupy the 2(b) positions the remaining 2B are distributed statistically at other vacant sites in the lattice. Note that this reformulation solves three problems which attended the description of the or-tetragonal phase as a crystalline modification of pure B ...

The radius of the 24-coordinate metal site in MBs is too large (215-225 pm) to be comfortably occupied by the later (smaller) lanthanide elements Ho, Er, Tm and Lu, and these form MB4 instead, where the metal site has a radius of 185-200 pm. The structure of MB4 (also formed by Ca, Y, Mo and W) consists of a tetragonal lattice formed by chains of Bs octahedra linked along the c-axis and joined laterally by pairs of B2 atoms in the xy plane so as to form a 3D skeleton with tunnels along the c-axis that are filled by metal atoms (Fig. 6.11). The pairs of boron atoms are thus surrounded by trigonal prisms of... 

HojNijjBj P2./C Tetragonal prism, isolated borons + B pairs ... 

Figure 3. Relation between the orthorhombic structure types of CrB and UBC (projected along [100]) as well as between the tetragonal structure types of a-MoB and ThBC (projected along [100]). Boron atoms are at the centers of trigonal metal prisms, carbon atoms at the centers of MjB-Oj,. The numbers given indicate the heights in projection.

In the crystal structure of these phases with tetragonal symmetry (P4/mbm, D h) the boron covalent sublattice is formed by chains of octahedra, developing along the c axis and by pairs of B atoms, bonding the octahedra in the xOy plane (see Fig. 1). The resulting three-dimensional skeleton contains tunnels parallel to the c axis that are filled by metal atoms . ... 

The solubility of transition elements in o-rh boron is low. The purity of a-rh boron prepared by thermal decomposition of BI3 is high, although that of -rh prepared from the same reactants is low. Crystallization of a-rh boron from a Pt melt requires pure chemicals. The solubility of transition and inner transition elements in -tetragonal boron is not known, but that in j8-rh boron is deseribed in 6.7.2.5.2 and 6.7.2.5.3. 

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). 

There are several ways of arranging the almost spherical B12 units. Three of these result in the forms of boron known as tetragonal, a-rhombohedral, and (3-rhombohedral. All of the structures are very rigid, which results in boron having a hardness of 9.3 compared to the value of 10.0 for diamond (Mohs scale). 

Pentaborane, B5H9, is unusual in having a tetragonal C4v skeleton, rather than the icosahedral fragment geometry that is typical of other boron hydrides. The unique apex atom B1 makes four short B—B bonds to the base (1.695 A), whereas the four basal B—B bonds are of more typical length (1.798 A). 

Pentaborane(9) is a volatile, colorless liquid, m.p. —46.9 °C, b.p. 60.1 °C, which ignites in contact with air. It is fairly stable hydrolytically, but reacts rapidly with hot water. It has a tetragonal pyramidal structure with four 3c2e BHB bridge bonds at the basal boron atoms. 

The a tetragonal form, firstly reported as a boron allotrope, has been reformulated as B50C2 or B50N2, depending on the preparative conditions it never forms in absence of carbon or nitrogen. 

For example in the Fe-Cr-B system (Fig. 5.5) Cr2B extends across the diagram at a constant stoichiometry of boron as (Cr,Fe)2B (Villars and Calvert 1985). Also shown is the corresponding extension of the tetragonal Fe2B compound. Such phases are commonly called Tine compounds . Generally, sublattice occupation can be described as... 

A characteristic feature of the structuie of most electron-deficient substances is that the atoms have ligancy that is not only greater than the number of valence electrons but is even greater than the number of stable orbitals.66 Thus most of the boron atoms in the tetragonal form of crystalline boron have ligancy 6. Also, lithium and beryllium, with four stable orbitals and only one and two valence electrons, respectively, have structures in which the atoms have ligancy 8 or 12. All metals can be considered to be electron-deficient substances (Chap. 11). 

The structure of tetragonal boron has been determined with care.68 There are 50 boron atoms in the unit of structure. All but two of them are in icosahedral groups of 12, as shown in Figure 10-1. In the Ba icosahedron each boron atom forms five bonds with adjacent atoms. The icosahedra and the two additional boron atoms per unit (interstitial atoms) are arranged in such relative positions that each icosahedral boron atom also forms one more bond, extending in the direction directly out from the center of the icosahedron. Thus each of the... 

Fig. 10-1.—Structure of tetragonal boron as viewed in the direction of the c axis. One unit cell is shown. Two of the icosahedral groups (light lines) are centered at z , and the other two (heavy lines) at. The interstitial boron atoms (open circles) are at (0, 0, 0) and (j, ). Numbers identify...

Boron nitride is a voluminous white powder which under the microscope shows no sign of crystallization. The sp. gr. depends on the temp, to which it has been ignited. F. M. Jager and H. G. K. Westenbrink said that the X-radiogram shows that the space-lattice is either cubic with a=7 44 A., or tetragonal with o=4 295 A., and c—5 176 A., or a c= 1 1 2052. In the first case, the elementary cell contains 20 molecules, which is improbable and in the second case, 4 or 5... 

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