Non-centrosymmetric

Yu N-T, Nie S and Lipscomb L 1990 Surface-enhanced hyper-Raman spectrosocpy with a picosecond laser. New vibrational information for non-centrosymmetric carbocyanine molecules adsorbed on colloidal silver J. Raman Spectrosc. 21 797-802... 

The preparation and structural characterization of the ions HX2 has been an important feature of such work/ As expected, these H-bonded ions are much less stable than Hp2 though crystalline salts of all three anions and of the mixed anions HXY (except HBrI ) have been isolated by use of large counter cations, typically Cs+ and NR4+ (R = Me, Et, Bu") — see pp. 1313-21, of ref. 23 for further details. Neutron and X-ray diffraction studies suggest that [C1-H - C1] can be either centrosymmetric or non-centrosymmetric depending on the crystalline environment. An example of the latter mode involves interatomic distances of 145 and 178 pm respectively and a bond angle of -168 (Cl- -Cl 321.2pm).( >... 

Fig. 9.7 Two cubic arrangements for Li2ZnGe centrosymmetric Fm3m (a), non-centrosymmetric F43m (b).

Fig. 9.8 Band structure and densities of states calculated for the compound Li2ZnGe in the non-centrosymmetric F43m cubic arrangement. Zn 3d inert orbitals (flat levels at -8 eV in the band structure) are not represented in the DOS.

For this scheme, m, n, and p have been defined for minerals as being even numbers. Notice also that the formula for bismuthite and aikinite have been doubled in keeping with the reported model where only (l) = Cu and (ll) = Pb are known [78]. Also defined are the terms A (A=m/4) and B (B=n/2). It has been observed that for most mineralogical systems, when A-i-B is even, the corresponding space group was the non-centrosymmetric, Pmc2i, and when A-vB is... 

To demonstrate the utilities of salt inclusion, we review the selected zeoUte-like transition-metal-containing open frameworks (TMCOFs) and then describe the structures of non-centrosymmetric solids (NCSs) and, finally, report crystalline solids containing a periodic array of transition metal nanostructures. In particular, we will address the issues concerning the role that molten salt has in... 

These structure-function relationships provide extremely useful guidance for the future rational design of molecules and polymers with even higher optical nonlinearities. For non-centrosymmetric molecules such as 95, very high first hyperpolarizabilities /3 that determine the second-order nonfinear optical properties were also measured [140]. 

W-Li absorption edges for WO3 and for several WOx-ZrOa samples calcined at 1073 K are shown in Figure 7. The absorption edge reflects the excitation of W 2s electrons by X-ray photons. A pre-edge feature at -5 eV is caused by 2s to 5d dipole-forbidden transitions, which become detectable in non-centrosymmetric compounds because of d-p orbital mixing [28]. 

Hyper-Raman spectroscopy is not a surface-specific technique while SFG vibrational spectroscopy can selectively probe surfaces and interfaces, although both methods are based on the second-order nonlinear process. The vibrational SFG is a combination process of IR absorption and Raman scattering and, hence, only accessible to IR/Raman-active modes, which appear only in non-centrosymmetric molecules. Conversely, the hyper-Raman process does not require such broken centrosymmetry. Energy diagrams for IR, Raman, hyper-Raman, and vibrational SFG processes are summarized in Figure 5.17. 

Crystals can only be piezoelectric when they are non-centrosymmetric. In addition, they may not belong to the crystal class 4 32. The effect is thus restricted to 20 out of the 32 crystal classes. 

Hoier, R., Bakken, L.N., Marthinsen, K. and Holmestad, R. (1993) Structure factor determination in non-centrosymmetric crystals by a two-dimensional CBED-based parameter refinement method, Ultramicroscopy, 49, 159-170. 

However, recent theoretical studies [90] indicate that systems exhibiting attractive interactions of the A and D blocks generally should favour an -ABCDABCD- sequence leading to a non-centrosymmetric lamellar phase. As the theoretical phase diagram (Fig. 28) predicts that the annealing process goes through a mixed centrosymmetric phase (MCS BC) before the fully separated centrosymetric one (CS) is reached, the prediction may not be at... 

Fig. 28 Mean-field phase diagram for ABCD tetrablock quaterpolymer melts with 0=1/4. Flory-Huggins parameters are xab = X except /Ad- Phases abbreviations MCS BC mixed centrosymmetric NCS non-centrosymmetric CS centrosymmetric. From [90]. Copyright 2004 American Chemical Society...

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