Body-centred structure

Figure 6.1 The placement of ions in the face-centred or body-centred structures, and the location of interstitial particles on the [111] cell diagonal directions...

This progression fits well experimental data collected so far. In the binary glycerol monooleate-water system there is conclusive experimental evidence for the presence of a body-centred structure of symmetry laSd at lower water contents and a primitive cubic bilayer (symmetry Pn m) at higher water contents [28] (Fig. 4.15). 

Other lipophilic molecules can be incorporated into the lipid bilayer of the cubic monoolein-water phase, for example gliadin. An electron micrograph showing the Cp structure of a lipid-protein-water phase is shown in Fig. 5.3. The body-centred structure characteristic of the P-surface is evident from the displacement of adjacent fracture planes. 

A2, cubic body-centred structure. X, other more complex structures. 

A substance is said to be polymorphous when it is capable of existing in two or more forms with different crystal structures. We have already encountered numerous instances of this phenomenon as, for example, carbon, selenium, some of the metallic elements, zinc sulphide, ferric oxide, silica, and many others. In some of these examples one form alone is found under a given set of physical conditions and a reversible transition between forms is brought about by a change in these conditions, in which case the forms are said to be enantiotropic. Thus iron has the cubic close-packed structure between the temperatures 906 and 1401 °C, and the cubic body-centred structure at temperatures outside this range. Even so, the rate at which the transition takes place may vary between wide limits at the one extreme it may be virtually instantaneous and at the other it may be so slow that a form is capable of indefinite existence under conditions in which it is, strictly speaking,... 

Fig. 9.02. Four unit cells of the cubic body-centred structure of a-iron showing the all-face-centred tetragonal unit cell in terms of which the structure may alternatively be described.

Figure 8.11 Proposed body-centred structure in lipid systems. Two structure elements are shown to the left (consisting of planar hexagons above and deformed to correspond to an infinite periodic minimum surface below). The tube network shown to the right separates two water channel systems.

Figure Al.3.23. Phase diagram of silicon in various polymorphs from an ab initio pseudopotential calculation [34], The volume is nonnalized to the experimental volume. The binding energy is the total electronic energy of the valence electrons. The slope of the dashed curve gives the pressure to transfomi silicon in the diamond structure to the p-Sn structure. Otlier polymorphs listed include face-centred cubic (fee), body-centred cubic (bee), simple hexagonal (sh), simple cubic (sc) and hexagonal close-packed (licp) structures.

In the face-centred cubic structure tirere are four atoms per unit cell, 8x1/8 cube corners and 6x1/2 face centres. There are also four octahedral holes, one body centre and 12 x 1 /4 on each cube edge. When all of the holes are filled the overall composition is thus 1 1, metal to interstitial. In the same metal structure there are eight cube corners where tetrahedral sites occur at the 1/4, 1/4, 1/4 positions. When these are all filled there is a 1 2 metal to interstititial ratio. The transition metals can therefore form monocarbides, niU ides and oxides with the octahedrally coordinated interstitial atoms, and dihydrides with the tetrahedral coordination of the hydrogen atoms. 

We begin by looking at the smallest scale of controllable structural feature - the way in which the atoms in the metals are packed together to give either a crystalline or a glassy (amorphous) structure. Table 2.2 lists the crystal structures of the pure metals at room temperature. In nearly every case the metal atoms pack into the simple crystal structures of face-centred cubic (f.c.c.), body-centred cubic (b.c.c.) or close-packed hexagonal (c.p.h.). 

Compounds with Sc, Y, lanthanoids and actinoids are of three types. Those with composition ME have the (6-coordinated) NaCl structure, whereas M3E4 (and sometimes M4E3) adopt the body-centred thorium phosphide structure (Th3P4) with 8-coordinated M, and ME2 are like ThAsi in which each Th has 9 As neighbours. Most of these compounds are metallic and those of uranium are magnetically ordered. Full details of the structures and properties of the several hundred other transition metal-Group 15 element compounds fall outside the scope of this treatment, but three particularly important structure types should be mentioned because of their widespread occurrence and relation to other structure types, namely C0AS3,... 

Figure 13.2 The cubic structure of skutterudite (C0AS3). (a) Relation to the ReOs structure (b) unit cell (only sufficient Co-As bonds are drawn to show that there is a square group of As atoms in only 6 of the 8 octants of the cubic unit cell, the complete 6-coordination group of Co is shown only for the atom at the body-centre of the cell) and (c) section of the unit cell showing CoAsg octahedra comer-linked to form AS4 squares.

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