Twinned nucleus

The appearance of a twin nucleus associated with phase transitions occurs preferentially, in many cases, on free surfaces, internal surfaces of inclusions, boundary surfaces of solid inclusions, or in dislocations where strain is concentrated. 

The core structure of the 1/2 [112] dislocation is shown in Fig. 4. This core is spread into two adjacent (111) plames amd the superlattice extrinsic stacking fault (SESF) is formed within the core. Such faults have, indeed, been observed earlier by electron microscopy (Hug, et al. 1986) and the recent HREM observation by Inkson amd Humphreys (1995) can be interpreted as the dissociation shown in Fig. 4. This fault represents a microtwin, two atomic layers wide, amd it may serve as a nucleus for twinning. Application of the corresponding external shear stress, indeed, led at high enough stresses to the growth of the twin in the [111] direction. 

Wolf, S.S., Jones, D.W, Knable, M.B., Gorey, J.G., Lee, K.S., Hyde, T.M., Coppola, R., and Weinberger, D.R. (1996) Tourette syndrome prediction of phenotypic variation in monozygotic twins by caudate nucleus D2 receptor binding. Science 273 1225-1227. 

If two individuals conjugate on an r or a z face, a nucleus of a Japanese twin with 1122 as the composition plane is formed. This indicates that a Japanese twin is not a penetration twin (i.e. not the upper half of an X-shape), but a contact twin (i.e. the upper half of a Y-shape). Horizontal banding in geode agate (see Section 10.9) appears through grain size variation due to gravitational sedimentation, and con-... 

Twinning. Two or more crystals of the same species are sometimes found joined together at a definite mutual orientation, this orientation of the individual crystals being constant in different examples of any one species. Such crystals are said to be twinned. Certain species show this phenomenon frequently, and some species invariably. The most frequent type of twinning is that of calcium sulphate dihydrate (gypsum), which is often found in the form shown in Fig. 41 a. The two crystals appear to be joined at the 100 plane. At the junction there is presumably a sheet of atoms common to the two individuals when the crystal nucleus was formed, two lattices were probably built by deposition on opposite sides of this common sheet of atoms. 

Sometimes twinned crystals appear to be interpenetrating, as in the calcium fluoride twin illustrated in Fig. 41 6. Here we may imagine (in the crystal nucleus) a common 111 sheet of atoms, the symmetry of which is trigonal the crystal on one side of it is rotated 60° with respect to the one on the other side. The twin plane is not always respected during subsequent growth one individual may encroach on the domain of the other, so that the junction surface in the final crystal is irregular. 

Let us return to Siegel-cyclohexatriene (30) and inspect the relationship between the geometries of the ground state and the twin excited states. The model in Figure 11a (also eq 11) predicts that an attractive 77-curve, displaced to the left of the a-minimum, will reduce the bond alternation induced by the er-strain. Since the 77-curve of the twin excited state is attractive, this state will lose the bond alternation of the ground state and the benzene nucleus will regain its local D%h symmetry. 

The frequency exaltation of the Kekule mode is mirrored by the structural manifestations in the twin states, discussed with reference to Figures 16 and 17. Thus, the repulsive jr-curve in the ground state softens the potential and thereby enables the ground-state molecule to distort along the Kekule mode when angular strain is exerted. In contrast, the attractive jr-curve in the twin excited state stiffens the potential and restores the local Deh symmetry of the benzene nucleus. The two physical effects are in perfect harmony and find a natural reflection in the VB model. 

Figure 6.4. (a) Schematic view of a penetrating twin (b) cross section of the nucleus of a penetrating twin viewed along the [110] direction. The dashed lines are dimer bonds. Surface-bonded hydrogen atoms are not depicted [99],... 

These are cantankerous twins because they arose together along with the metaphysical conception of substance. According to Bachelard, for the metaphysical realist existence is a one-toned function, that is, everything is real or unreal in the same way, in that it either exists or does not exist— the electron, the nucleus, the atom, the molecule, the colloidal particle, the mineral, the planet, the star, the nebula (Bachelard 1968, 46). 

The current information on size, structure and chemistry of diamond nuclei is primarily speculative, with a small number of conclusive results. It has been proposed that diamond nuclei may be multiple twinned particles, likely containing some of the structures related to the boat-boat conformer of bicyclodecane (10 carbon atoms) or boat-chair-chair-boat tetracyclo octadecane (18 carbon atoms) within higher molecular weight compounds formed by the partial hydrogenation of graphitic or polyaromatic hydrocarbons. The diameter of a critical nucleus of diamond is presumably around 3 nm. 

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