Twin structures

Electron diffraction studies are usually limited to transferred films (see Chapter XV), One study on Langmuir films of fatty acids has used cryoelectron microscopy to fix the structures on vitrified water [179], Electron diffraction from these layers showed highly twinned structures in the form of faceted crystals. 

Initiation of growth may also proceed by formation of metastable structures when nucleation is inhibited. Multiply twinned structures have been observed for a number of metals. Their presence indicates an icosohedral or decahedral precursor cluster which has decomposed to a multiply twinned crystal at a critical size [117, 118], Another example of metastable intermediate structures was reported by Dietterle et al. 

Finally, one should be cautioned that, occasionally, substances form chiral single crystals of nearly racemic composition. For example, hexahelicene crystals grown from racemic solutions apparently undergo spontaneous resolution, displaying the enantiomorphic space group P2[2,2, however, the e.e. in the crystal is only —2%. This material (and probably others as well) has a lamellar, twinned structure in which alternating layers, 20 p,m thick, of optically pure (/ )-( + )-and (M)-( — )-hexahelicene are perfectly aligned to build up the observed crystal (266). 

There has been considerable work in the literature on the structure of very small particles and clusters. Interest in this field has been primarily due to Ino s (1966) early experimental studies of normally fee metals prepared by vapour condensation which showed that a sizable portion of the particles exhibited non-crystallographic structures. These non-crystallographic atomic clusters or polycrystalline nuclei have been observed to consist of pentagonal bi-pyramid or icosahedra form of twinned structures and are known as multiply twinned particles (MTPs). EM studies of supported transition metal catalyst systems have indicated that MTPs sinter faster in catalytic reactions leading to the loss of surface area and are not beneficial to catalysis (Gai 1992). We describe the structure and the role of MTPs in catalysis in the following sections. 

Fig. 2.11 Structure in the vicinity of the shear plane for the shear operation (/ifcZ) [011] of a TiOj-type structure, (a) (121)5[0il] (b) (132) [0il] (c) (011)2[0il] the structure obtained after only operation (3) on TiO2 without the elimination of an oxygen-only plane. The structure is called an APB (anti phase boundary) or twin structure, and is similar to the shear structure of (110)j[li0] of ReOj (see Fig. 2.6(a)). Note the atomic arrangement in the zones framed by dotted lines (see also Fig. 2.113).

We shall discuss first the micro-twin structure derived from an HCP lattice. Figure 2.54(a) shows a projection of HCP on (OOI)hcp. In Fig. 2.54(b) the ortho-hexagonal axes c<,, ) are shown as well as the usual hexagonal... 

Fig. 2.56 Twin structure of hexagonal close packing projected on (OlO)oh - Twin plane is (101),. Note that a new type of interstice (trigonal prism) appears on the twin plane.

Compounds with micro-twin structure derived from... 

Next we discuss the micro-twin structure derived from a CCP lattice. Using the hexagonal expression, the CCP structure is composed of layer stacking... ABCABC. .. along the Ch(, -axis (= The ortho-hexagonal... 

It is convenient to draw micro-twin structures derived from the micro-twinning of a CCP lattice projected on the normal plane to the pillars of the trigonal prisms. Let us consider a twin structure derived from a NaCl-type structure, in which both anions and cations have a CCP arrangement. Figure 2.67 shows the structure of a NaCl-type (a) is projected on (OOl)Naci and (b) on (110). It can be easily shown that the twin plane (203) ,j in the ortho-hexagonal expression corresponds to the (113) plane in the cubic one. Figure 2.68 shows a twin structure of NaCl-type,... 

Fig. 2.68 Twin structure of NaCl-type (CCP of anions and cations) projected on (HO)Naci- Twin plane is (113)Naci> corresponding to (203) . ...

Fig. 2.69 Reconstruction of twinning structure of (llSjuaci. Projected on (llO)jsiaci (a) Structure of twinning. Note that...

In Fig. 2.73 is shown the structure of CrB, which is a typical example of (1, IjccpCCrP B ). The compound YOOD has an anti-CrB type structure as expressed by (1, l)ccpY O OP D, where half of the prismatic sites are occupied by Y. Table 2.3 shows some examples of micro-twin structure related to CCP. 

We shall now present studies relating to the micro-twin structure performed by B. G. Hyde, who is a main proposer of the concepts of shear and micro-twin structures. One study is on the mineral humite family,the composition of which is generally expressed by nMg2SiO4-Mg(OH)2 (this has to be altered to nMg2SiO4-Mg(OH, F)2, because (OH) is often replaced by F in the minerals). In Table 2.4, the mineral name, together with the compositions and micro-twin structures proposed by the authors, are collected, the space group is Pmcn (orthorhombic) for n = odd and... 

Minerals ofhumite family with micro-twin structure ... 

White and Hyde extended their observations to the manganohumite family (Mg is replaced by Mn in humite) and leucophoenicite 3Mn2SiO4-Mn(OH)2 and obtained fruitful results from the viewpoint of the micro-twin structure. 

The structures described in this chapter, especially on micro-twin structure, are referred to in Inorganic crystal structures by B. G. Hyde and S. Andersson. ... 

---