Pseudo-hexagonal

Pyrophillite, talc, muscovite, and biotite have the following sequence of atom-planes along the pseudo-hexagonal axes ... 

Yet another type of canal structure has been reported for the urea inclusion compound of 1,4-dichlorobutane 51). Even though the canals are pseudo-hexagonal in dimension, there is a significant difference in their symmetry. The host lattice is orthorhombic, space group Pbcn. The difference lies in the directions of the six pseudo-3j helices of host molecules around the walls of each canal the sequence is cyc/o-RRRLLL, as opposed to cyc/o-RRRRRR in the hexagonal inclusion compounds and cvc/o-RLRLRL in the rhombohedral. This orthorhombic host structure probably occurs also in the urea inclusion compounds with 1,5-dichloropentane and 1,6-dibromohexane 51). 

Many aliphatic poly(amides), more commonly known as nylons, exhibit an unusual phase transition below their melting point. First noted by Brill [122], the phenomenon has been studied extensively [128-131]. It is observed for instance in nylon (6,6) at a temperature of about 210°C, when the stable low-temperature triclinic oc phase of the crystalline polymer changes to a pseudo-hexagonal phase. 

These phases very often share, aside from a relatively high disorder, another important feature with the bundle-like pre-crystalline entities that we discussed up to this point. This feature is hexagonal or pseudo-hexagonal packing of the polymer chains [61]. In this respect molecular order at a local level is similar, although in the thermotropic mesophases we are now discussing disorder is even more dynamic than in previous cases. Indeed they are essentially characterized by a high entropy. 

Fig. 9 Two arrangements of helical chains, represented as continuous screws, packing in a hexagonal or a pseudo-hexagonal mode. In a the three screws are isomorphous in b the screw B is enantiomorphous with respect to both A and C and the overall packing is less satisfactory. Reproduced with permission from [13]. Copyright 1998 Am Chem Soc...

Group 2 Each chain has cylindrical or nearly cylindrical symmetry (A unit cell must be made up of only one chain, and the symmetry is restricted to be hexagonal or pseudo-hexagonal). 

Fig. 16.9 Electron micrographs of soil lepidocro-cite. a) Large multidomainic lath-like crystal viewed perpendicularto [001] with laminar pores from a re-doximorphic soil, Natal, South Africa, b) Poorly crystalline grassy lepidocrocite crystals mixed with tiny ferrihydrite particles and pseudo-hexagonal kaolinite platelets. Origin as before (a. b courtesy P. Self), c) Small lepidocrocite crystal from a hydromorphic soil (with ferrihydrite) viewed perpendicularto [001] and showing (020) lattice fringes (see also Schwert-mann. Taylor, 1989,with permission).

Fig. 4.8 Theoretical model of the (1 x 5) reconstructed surface of Ir (001). Six [110] atomic rows squeeze into 5 row-spacings of the (1 x 1) structure, resulting in a quasi-hexagonal, or pseudo-hexagonal, atomic arrangement with buckled [110]...

The arrangement of helices in the solid and liquid crystalline states of poly(a-phenylethyl isocyanide) were determined by X-ray and electron diffraction. Well-defined diffraction patterns were obtained from oriented films using selected area electron diffraction. Intermolecular and intramolecular patterns were calculated from the five Debye-Scherrer rings. All the reflections were indexed in terms of a pseudo-hexagonal triclinic unit cell, with... 

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