Antiphase structure

A 90° Gaussian pulse is employed as an excitation pulse. In the case of a simple AX spin system, the delay t between the first, soft 90° excitation pulse and the final, hard 90° detection pulse is adjusted to correspond to the coupling constant JJ x (Fig- 7.2). If the excitation frequency corresponds to the chemical shift frequency of nucleus A, then the doublet of nucleus A will disappear and the total transfer of magnetization to nucleus X will produce an antiphase doublet (Fig. 7.3). The antiphase structure of the multiplets can be removed by employing a refocused ID COSY experiment (Hore, 1983). 

I h. correct values of coupling constants were attributed to the appropriate protons by the analysis of antiphase structures of cross-peaks in DQF-COSY spectrum. 

Higgins, J. B., Ribbe, P. H. Nakajima, Y. (1982) An ordering model for the commensurate antiphase structure of yoderite. Amer. Mineral., 67,76-84. 

Fig. 5. Magiutude mode P, Ag HSQC spectra of [Ag(m-Ph2PCH=CHPPh2)2jN03 measured with (A) and without (B) 180° pulse during t,. The coupling with four phosphorus atoms gives rise to a quintet splitting along FI in (B) whose central line is missing because of the antiphase structure. Reproduced, with permission, from ref. 87. Copyright 1990 John Wiley Sons.

It has also been mentioned that TOCSY results in the net transfer of in-phase magnetisation, meaning the cancellation effects from antiphase multiplet fine-structure associated with COSY are not a feature of TOCSY. Such cancellation can be problematic for molecules that posses large natural linewidths, for example (bio)-polymers, but may also prevent the observation of COSY peaks in the spectra of small molecules that have complex multiplet structures which may cancel under conditions of poor digital resolution. In these cases the TOCSY experiment may be viewed as the more sensitive option because of the greater crosspeak intensities. The lack of antiphase structure also means spectra... 

The interatomic interaction is described by an EAM potential specifically developed for NiAl in the B2 structure [12]. Compared to the older potential [16], which was used in most of the previous atomistic studies, our new potential gives considerably higher antiphase boundary (APB) energies = 0.82 J/m, yj pg = 1.06 J/m in good agreement with the APB... 

Using the constructed potentials the y-surface for the (111) plane was calculated. (For more details see Girshick and Vitek 1995). T e lowest energy minimum on this surface corresponds to the ideal Llo structure. However, there are three different metastable stacking fault type defects on (111) the antiphase boundary (APB), the complex stacking fault (CSF) and the superlattice intrinsic stacking fault (SISF). The displacements... 

The recommended Mg-Au phase diagram is that of ref. 3 amended by the addition of several new compounds close to MgAu3 they are a family of close-packed structures modulated both by antiphase boundaries of the superstructure and by a... 

Also known for some time is a phase transition at low temperature (111K), observed in studies with various methods (NQR, elasticity measurement by ultrasound, Raman spectrometry) 112 temperature-dependent neutron diffraction showed the phase transition to be caused by an antiphase rotation of adjacent anions around the threefold axis ([111] in the cubic cell) and to lower the symmetry from cubic to rhombohedral (Ric). As shown by inelastic neutron scattering, this phase transition is driven by a low-frequency rotatory soft mode (0.288 THz 9.61 cm / 298 K) 113 a more recent NQR study revealed a small hysteresis and hence first-order character of this transition.114 This rhombohedral structure is adopted by Rb2Hg(CN)4 already at room temperature (rav(Hg—C) 218.6, rav(C—N) 114.0 pm for two independent cyano groups), and the analogous phase transition to the cubic structure occurs at 398 K.115... 

Figure 1.1 Defects in crystalline solids (a) point defects (interstitials) (b) a linear defect (edge dislocation) (c) a planar defect (antiphase boundary) (d) a volume defect (precipitate) (e) unit cell (filled) of a structure containing point defects (vacancies) and (/) unit cell (filled) of a defect-free structure containing ordered vacancies. ...

Similar antiphase boundaries form in metals with structures based upon a hexagonal close-packed array of metal atoms, such as magnesium. Condensation of vacancies upon one of the close-packed metal atom planes to form a vacancy loop, followed by subsequent collapse, will result in a hypothetical sequence. . ABABBABAB This arrangement will be unstable because of the juxtaposition... 

As in the case of twin planes, the antiphase relationship may affect only one part of the structure, for example, the cation substructure, while leaving the anion substructure unchanged. This is particularly common when the anion array can be considered to consist of a close-packed array of ions, which remains unchanged by the antiphase boundary (Fig. 3.28). 

A special type of modulated superstructure (long-period superstructure) is known for AuCu (AuCul) and results in the oI40-AuCu (II) type. This antiphase-domain structure has been discussed in 3.11.1 (Fig. 3.41). 

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