Lattice poles

When used in combination with gerbs, saxons can contribute to spectacular set-pieces such as lattice poles and mosaics, with the gerbs providing overlapping crosses of hre while the saxons hll in the gaps with vivid circles of hame and sparks. 

If the p labels refer to lattice sites j, this matrix reduces to 6(k) in the KKR matrix M(k) and Eq. (15) can be shown to reduce to Eq. (14). The evaluation of is hindered by the free-electron poles in the b matrices. This has formed a barrier for electronic structure calculations of interstitial impurities, but in some cases this problem was bypassed by using an extended lattice in which interstitial atoms occupy a lattice site. For the calculation of Dingle temperatures [1.3] and interstitial electromigration [14] the accuracy was just sufficient. Recently this accuracy problem has been solved [15, 16]. 

A further example illustrating the importance of lattice effects on the SCO behaviour of these trinuclear compounds is given by [Fe3(iptrz)f,(H20)6] (CF3S03)6 (iptrz = 4-(isopropyl)-1,2,4-triazole). A strong influence of the ST of the central Fe(II) ion on both external Fe(II) ions has been found by Mossbauer spectroscopy, as detected by the perturbation of their quadru-pole interactions [15]. The nature of this phenomenon has been proposed to... 

Figures 3a and 3a depict the weak bond of an O2 molecule with the lattice. It is formed by an electron being drawn from an ion of the lattice to an O2 molecule. Owing to the greater electron aflSnity of the O2 molecule, the electron may be considered completely transferred from the lattice to the molecule as a result, a molecular ion 02 is formed and a localized hole appears in the lattice attached to the ion Oi, The entire system (the adsorbed O2 molecule + adsorption center) acquires a noticeable dipole moment with negative pole directed outward, but remains electrically neutral as a whole. The bond is effected without the participation of a free lattice electron. The transition to a strong acceptor bond entails the localization of an electron, or, what amounts to the same thing, the delocalization of a hole. Such a strong acceptor bond is depicted in Figs. 3b and 3b. ...

Another successful approach involves the cross-linking of the side chain NLO polymer, after poling, at multiple sites by a different type of polymerisation mechanism. Subsequent curing and hardening produces a lattice that locks in the poled dipole. . One such process is outlined in Figure 5.32. 

Figure 5.32 Cross-linking and poling of NLO chromophore into a hardened lattice.

It is often necessary to find the angle between two crystallographic poles of known Miller indices hkl) and (h k V). For the cubic lattice, it is given by... 

It still has to be explained why the boiling points of NH3 and HF are not also high. This is due to the fact that water, with its two positive poles, can easily form a lattice, whereas HF molecules, with only one positive and one negative pole in each, can at most only form chains or rings. In ammonia, which has three positive poles, a structure similar to that of water cannot be formed and it is again apparent that it is only a very special combination of circumstances which gives rise to the properties of water. 

The presence of phase transitions at 19 and 30°C provides an opportunity to test the proposed deformation model. Below 19°C the lattice contracts into a triclinic structure witli strong intermolecular interaction. 5,26 sjamplcs deformed below 19°C should develop off-c-axis orientation while samples deformed above 30°C should not. Figures 1.12 and 1.13 show inverse pole figures for samples deformed at 2 and 70°C. The observed orientation agrees with our proposed model. - With tlris set of experiments, it is possible to activate the oblique slip process or, alternatively, to deactivate it in the high-temperature phase above 30°C. 

Yamada et al. [9,10] demonstrated that the copolymers were ferroelectric over a wide range of molar composition and that, at room temperature, they could be poled with an electric field much more readily than the PVF2 homopolymer. The main points highlighting the ferroelectric character of these materials can be summarized as follows (a) At a certain temperature, that depends on the copolymer composition, they present a solid-solid crystal phase transition. The crystalline lattice spacings change steeply near the transition point, (b) The relationship between the electric susceptibility e and temperature fits well the Curie-Weiss equation, (c) The remanent polarization of the poled samples reduces to zero at the transition temperature (Curie temperature, Tc). (d) The volume fraction of ferroelectric crystals is directly proportional to the remanent polarization, (e) The critical behavior for the dielectric relaxation is observed at Tc. 

Figure 24.9 Stereographic representation of undistorted plane produced by the lattice-invariant deformation and lattice deformation illustrated in Fig. 24.8. Traces h and h" represent initial and final positions of the undistorted plane, respectively. Their poles are at h and h". Figure referred to f.c.c. axes. After Wayman [5],...

Because of the four-fold symmetry of the [001] pole figures in Figs. 24.6-24.9, additional symmetry-related invariant planes can be produced. Also, further work shows that additional invariant planes can be obtained if a lattice-invariant shear corresponding to a = 7.3° rather than a = 11.6° (see Fig. 24.8) is employed [5]. Multiple habit planes are a common feature of martensitic transformations. 

Chromophores must be thermally robust enough to withstand temperatures encountered in electric field poling and subsequent processing of chromophore/polymer materials. Chromophore decomposition temperatures can be assessed by techniques such as thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA and DSC measurements on neat chromophore samples in air will tend to yield decomposition temperatures lower than those for the same chromophores in hardened polymer lattices. Typically, to be useful for development of device quality materials, a chromophore must exhibit thermal stability of 250 °C or higher (with thermal stability defined as... 

If the host polymer does not have a high glass transition temperature then the structures of polymer and chromophore must be such that post-poling lattice hardening can be carried out to assure thermally stable electro-optic activity [2, 5,50,63,64,110,132-150]. 

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