Extinction directions

Fig. 17. The structure of the ketone (1) molecule as determined by r-ray crystallography shown in the (approximate) orientations used in the polarization measurements. In (a) the in-plano versus out-of-plane polarization directions are compared. The in-plane extinction direction shown is nearly parallel to the Cl—C2 double bond, being tipped just 5°out of plane. In (b) the two directions in plane arc compared. The long axis extinction direction shown is exactly in plane and makes an angle of 40° with the Cl—C2 double bond, (From Jones, Kearns, and Wing, Ref, 9))...

Observations of crystals between crossed polarizers are particularly valuable in the case of some of those twin combinations which in their external shape simulate a single crystal having a symmetry higher than that of one of the individuals. The observation of different extinction directions in different regions demonstrates at once that the crystal is... 

Needle-like crystals naturally lie on the microscope slide wi h their long axes parallel to the slide, and it may not be possible to find tilted crystals and even crushing may not yield fragments which lie in all possible orientations. However, even when the needle axis is invariably parallel to the slide, all orientations obtainable by rolling a needle are likely to be encountered, and observations of a number of crystals should be sufficient to give all the information required. The first thing to do is to observe the extinction direction if extinction is consistently parallel to the length, the crystals may be uniaxial, the direction of... 

Use of crossed polarizers. Extinction directions. Interference colours 69 The indicatrix 72... 

Fig. 13. Polarized absorption spectrum (low energy part) in two extinction directions (cf. Ref. [96]) of rrans-[Co(NH3)4(CN)2]Cl at 4.2 K...

The fact that the crystals used in this study are primitive triclinic proved to be an advantage rather than a disadvantage. All the dinuclear molecules are lined up parallel in the crystal, and it was found that the crystal extinction directions in the relevant wavelength range more or less coincide with the molecular symmetry axes (6). In higher symmetry crystal systems one often encounters the problem that it is difficult or impossible to extract molecular po-... 

Fig. 6.19 Projections of the crystal structures of 6-XXXV (R = Cl) onto the faces studied spectroscopically (a) the (001) face of the trichnic form (b) the (110) face of the orthorhombic form, /niin and /max are the extinction directions that were oriented parallel to the electric vector of the incident light for the measurement of the reflection spectra. (From Bernstein et al. 1979, with permission.)...

If the point group symmetry is equivalent to orthorhombic or lower, i.e. if the optical properties are biaxial, the local extinction directions are not simply related to the director as projected onto the specimen plane. It may be possible to use light microscopy to establish whether or not a region of the specimen is optically biaxial ... 

If the microstructure is too fine, and the material is pleochroic, one can attempt to identify the azimuths for maximum and minimum absorption of linearly polarized light (171 in optically biaxial material, these need not coincide with the local extinction directions. The use of optical pleochroism as just described is one example of how studies limited to observations between crossed polars do not exhaust the information available from light microscopy. 

The possibility of optical biaxiality should be considered whenever molecules (or side chains) have a lath-like or "sanidic" conformation that might support correlated rotations about their chain axes. If the suggested tests involving optical microscopy are impractical or inconclusive, one must resort to characterization by complementary techniques. For example, optical biaxiality can be inferred if there is no preferred orientation of optical extinction directions in a specimen that consists of globally aligned molecules as demonstrated by x-ray or electron diffraction (181-... 

When a thin sheet of an anisotropic material is observed normally between crossed polarisers and rotated about the observation direction, the intensity of the transmitted light is found to pass through four equispaced minima (ideally zeros) separated by four equal maxima. The minima of intensity arise when the principal directions of refractive index in the plane of the sheet are parallel to the polariser or the analyser directions. It is convenient to label the direction of maximum refractive index the extinction direction and note that a minimum of transmitted intensity arises whenever the extinction direction is parallel to either the polariser or the analyser directions. 

In Fig. 4 we have seen typical deformation bands in PET. The observed extinction directions in the matrix (IDD) and in the bands (EDB) are marked showing the angle oc between them. From Fig. 9 it can be seen that a homogeneous shear could not explain the observed reorientation if all the chains were aligned parallel to the IDD, as the direction of maximum refractive index would then rotate in the opposite sense to that observed. 

Fy. 9. This illustrates that in a slippy band a line initially parallel to the initial drcnving direction, IDD, and therefore parallel to the average chain direction in the matrix, rotates in the opposite sense to the observed rotation of extinction direction. 

Brown et al. made the intuitive step of relating the direction of maximum elongation to the direction of maximum refractive index, regardless of strain-history. Thus the angle at calculated above should relate exactly to the angle a between the extinction directions for material inside and outside the deformation bands. 

Fig. 12. A simple alternating crystalline amorphous block model could not in general predict that the extinction direction coincides with the direction of maximim...

Optical microscopy has therefore shown (a) that the direction of maximum refractive index in the band (EDB) is parallel to the direction of maximum extension, (b) A pseudo-fibrillar texture is observed within the band which is not apparently related to the EDB nor to the direction a fibril in the drawn material would have if it followed an affine deformation in the band. WAXS measurements reveal, further, that the preferred crystallographic c-axis direction is neither parallel to the pseudo-fibrils nor to the extinction direction in the band (EDB). The WAXS photographs from material within the bands provide a clue to the explanation why the crystalline c-axes are not on average parallel to the maximum refractive index direction (EDB). For all materials examined (PET, PP... 

Both of these suggested mechanisms would be selective in reducing crystallinity mainly in those crystals oriented such as to experience large compressive strains during the deformation, whilst the majority of the structure must deform in a near afiine manner to reproduce the predicted extinction directions. The WAXS is dominated by the regions of higher... 

Figure 11.4. Typical inclined extinction directions in columnar flower-like domains between horizontal and vertical crossed polarizers and its interpretation assuming tilted disks in curled columns extinction occurs in diagonal directions where the disks are parallel to a polarizer. (Reprinted with permission from Ferroelectrics 179, 203 1996, Gordon and Breach [6].)...

An evolutionary precursor of the neural crest may be defined as a cell type that possesses some, but not all, of the characters of neural crest cells, and that has the potential to evolve into true neural crest. Clearly, this definition would only apply to cells within extinct, direct ancestors of vertebrates. Cephalochordates and urochordates are not ancestors of vertebrates. Since they diverged from the chordate lineage before vertebrate origins, however, they may possess cells that are homologous to neural crest precursors. Such cells should be recognisable as possessing some, but not all, of the characters of neural crest cells. 

Figure 7.4 Low-energy region of the Mg polarized absorption band of trafis-[Co(CN)2(tn)2]Cl3H20 at 4.2 K (next to the broad-band B, A C2h) C2h) absorption), at extinction directions in the...

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