Basal planes distances between

The height (between two opposite edges) of a phosphate tetrahedron is about 1.7 A. If the same distance were preserved between the next oxygen layers, the basal-plane distance along the fiber axis would be 3.4 A. This value is the spacing observed for the principal meridional reflection. 

Figure 6.2 Basal plane of a-rhombohedral boron showing close-packed arrangement of B 2 icosahedra. The B-B distances within each icosahedron vary regularly between 173-179 pm. Dotted lines show the 3-centre bonds between the 6 equatorial boron atoms in each icosahedron to 6 other icosahedra in the same sheet at 202.5 pm. The sheet-s are slacked so that each icosahedron is bonded by six 2-centre B-B bonds at 171 pm (directed rhombohedral ly, 3 above and 3 below the icosahedron). B12 units in the layer above are centred over 1 and those in the layer below are centred under 2.

Orthoboric acid, B(OH)3, is the normal end product of hydrolysis of most boron compounds and is usually made ( 160 000 tonnes pa) by acidification of aqueous solutions of borax. Price depends on quality, being 805 per tonne for technical grade and about twice that for refined material (1990). It forms flaky, white, transparent crystals in which a planar array of BO3 units is joined by unsymmetrical H bonds as shown in Fig. 6.25. In contrast to the short O—H O distance of 272 pm within the plane, the distance between consecutive layers in the ciystal is 318 pm, thus accounting for the pronounced basal cleavage of the waxy, plate-like ciystals, and their low density (1.48 g cm ). B(OH)3 is a very weak monobasic acid and acts exclusively by hydroxyl-ion acceptance rather than proton donation ... 

Figure 7.8 The structure of InCls " showing square-pyramidal (C4 ) geometry. The In-Clapex distance is significantly shorter than the In-Clbase distances and In is 59 pm above the basal plane this leads to a Clapex-In-Clbase angle of 103.9° which is very close to the theoretical value required to minimize Cl Cl repulsions whilst still retaining C4, symmetry (103.6°) calculated on the basis of a simple inverse square law for repulsion between ligands. [NEt4]2[TlCl5] is isomoiphous with [NEt4]2[InCl5] and presumably has a similar structure for the ardon.

From the interatomic distances the conclusion is to be drawn that the bonds in the hexagonal layers of atoms in these metals are stronger than those between layers. This conclusion is substantiated by the properties of the crystals, which show basal cleavage and have larger values of the compressibility, coefficient of thermal expansion, and electrical resistance in the direction perpendicular to the basal plane than in this plane. Moreover, measurements of the intensities of... 

Consider the diffusion of a randomly walking diffusant in the h.c.p. structure, which is composed of close-packed basal planes stacked in the sequence ABABA. The lattice constants are a and c. The probability of a first-nearest-neighbor jump within a basal plane (jump distance = a) is p, and the probability of a jump between basal planes (jump distance = /a2/3 + c2/4) is 1 — p. If axes X and X2 are located in a basal plane, derive the following expressions for the diffusivities Du and D33 ... 

Since the four B-B distances between the apex and the basal plane are equal, we have to invoke resonance among structures in which the (2c, 2e) and (3c, 2e) bonds interchange. 

Hhere z is the distance from the graphite surface, A is the distance between the Kraphite layers (0.335 nm) and p is the number of carbon atoms per unit volume m4nm 3). The derivation of this 10-4-3 potential function involved integration ver the basal plane and summation over the successive layers. The 10- and 54- terms, therefore, represent the repulsive and attractive interactions with the psal plane, while the 3- term takes care of the summation over the remaining layers. This form of potential function has been favoured in recent computer Ifoulation studies of the adsorption of molecules by porous carbons (Nicholson, 996,1997). 

---