Micro-twins

B) The model of De Wolff disorder gives no explanation for the line broadening of reflections which are not affected by this type of lattice disorder. Chabre and Pannetier ascribed this effect to a micro twinning of the ramsdel-lite/rutile lattice on the planes [0 2 1] and [0 6 1], These faces are believed to be growth planes of EMD [45, 46],... 

As for the structures of ammonium compounds apparently no certainty has been attained. Bode and Foss followed the views of earlier authors and accepted the (NH4)sFeF6-type 245) with a linear array of ions as an example of the ideal cryolite structure. The discrepancy between the observed lattice constants and the ionic radii sum is indeed smaller in the case of ammonium-compounds. But this applies also to the compound (NH4)3GaFe, of which a structure analysis has been performed recently by Schwarzmann 303). After a detailed discussion of the possibilities of arranging the fluoride ions in this compound, Schwarzmann shows, that there must be either micro-twinning in the space group Pa3 or a statistic occupation of the 192-fold position of space group FmSw by 24 fluoride ions, as characteristic of the KsFeFs-type. 

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... 

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,... 

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 ... 

Fig. 2.77 Lattice image of clinohumite (3, 2 )ccp On the right is shown the lattice image of clinohumite, in the top left the diffraction pattern and in the bottom left the corresponding structure model (micro-twin model of Mg lattice). In the middle of the lattice image, the (3, 2 ) structure intergrows in the clinohumite (3,2 ) matrix.

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. 

Fig. 2.101 Micro-twinning operation to UjOg-type structure. The structures are represented by a chain of pentagonal bipyramids for simplicity. The structures with a (/, /)ij,OB typc twinning operation are shown, where I is the number of pentagonal bipyramids between the twin planes (hatched ones), (a) UjOg-type structure (mother structure) (b) / = 0 (same as the mother structure) (c) I = 1 (d) I = 2 (e) / = 3 (f) Z = 4. The chain lines indicate the folding planes.

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. ... 

Fig. 16. (b) Isolated wedge micro-twin (T) in a B-Sm203 crystal. 

Mah] Mahajan, S., Jin, S., Brasen, D., Micro-twinning in a Spinodally Decomposed Fe-Cr-Co Alloy , Acta Metall, 28, 971-977 (1980) (Experimental, Phase Relations, 24)... 

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