Centrosymmetric arrangements

Single crystals of ferroelectric TGS grown from solution generally contain domains of the two enantiomorphous phases (110). This juxtaposition of enantiomorphous domains may be explained in terms of the minor deviations of the crystal structure from a centrosymmetric arrangement. The relative concentrations of the two enantiomorphous phases in a single crystal may be determined by means of anomalous dispersion of X-rays (111,112). 

Above diffraction patterns of (a) mask representing molecule of durene, 1,2,4,5 tetra-methyl benzene, (6) the same ma k with an extra hole at the centre positive regions are strengthened, negative regions weakened. (Lipson and Cochran, 1953.) Below diffraction patterns of (c) the centrosymmetric arrangement of holes shown inset, and (d) the non-centrosymmetric arrangement of holes shown inset (C. A. Taylor, 1952)... 

Figure 2-33. Left The centrosymmetric arrangement of acetanilide molecules of the crystal resulting in centrosymmetric crystal habit Right. The p-acetanilide molecules are aligned in a head-tail orientation resulting in the occurrence of a polar axis of the crystal habit [39], Both are reprinted with permission from 1981 American Chemical Society and D. Y. Curtin and I. C. Paul.

Figure 9. Crystal packing diagram of the complex 59 (bis-dichloromethane solvate), showing the centrosymmetric arrangement of the complexes (solvent omitted) and the ir-stacking interactions [139]...

E(ind) = —x E + X E - - ). Hence, only molecules that are non-centrosymmetric can have a non-zero (3 and only non-centrosymmetric materials can have non-zero x (i.e., a centrosymmetric arrangement of non-centrosymmetric molecules will have zero xM-... 

Figure 2-44. (a) The centrosymmetric arrangement of acetanilide molecules in the crystal,... 

An alternative approach is to prepare so-called poled polymer samples. An NLO compound is dispersed into a polymer at a temperature above Tg for the polymer. A large voltage (for example, 10 V/cm) is placed across the material. Since the NLO molecules have large dipole moments, they will tend to align with the field. After adequate alignment is achieved, the temperature is dropped below Tg, locking in the now non-centrosymmetric arrangement of NLO chromophores. This fairly simple procedure can be used to prepare samples with quite acceptable x values. 

In a preceding paragraph we have shown that a necessary condition for second order NLO activity in a material is the absence of the symmetry center. This a stringent requirement and it is perhaps the most difficult feature the chemistry of these materials must face with. To get a non-centrosymmetric arrangement of molecules on a macroscopic scale is a difficult task, and even more difficult is to get a non-centrosymmetric polar structure. A polar structure is that in which there is at least one macroscopic direction (polar axis) which is not changed in the opposite direction by symmetry transformations allowed for the material. With reference to crystals, out of the 32 crystal classes only the following 20 correspond to non-centrosymmetric crystals 1,2, m, 222, mm2,4, -4,422,4mm, -42m, 3, 32, 3m, 6, —6, 622, 6mm, —62m, 23, —43m... 

The statistically non-centrosymmetric polar order of the chromophore units reached during the poling procedure is thermodynamically stable only with the electric field on. Once cooled down at room temperature and switched off the field, that polar order becomes metastable and it will relax to the more stable centrosymmetric arrangement, which is not NLO active. So, an other fundamental issue is to reach an high time stability of the polar order. Industrial standards for device testing require stability of NLO performances for 10 years at 85°C, that are quite severe. 

Figure 5. Electron selected area diffraction diagram produced by two layers (mutually disoriented) of nontronite (Pfaffenreuth). Of the group (11) (02), only (02) spots are observed (arrows on the figure). This diagram is characteristic of the centrosymmetrical arrangement of octahedral atoms (group c2 mm Figure 1).

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