Trigonal cell

Figure 4. Diffraction from a screw-disordered" arrangement of poly U poly A poly U molecules. The Bragg pattern from the fiber containing these triple-stranded polynucleotide molecules corresponds to a trigonal cell with a = b = 2.71 nm c = 3.65 nm. The molecules are 12, helices.

The structure of ilmenite, FeTiCh, is essentially the same as that of corundum (01-AI2O3). The symmetry of the trigonal cell is lower... 

The crystal polymorphism of the chiral but racemic P5MH1 is, to some extent, very reminiscent of that of isotactic polypropylene. It exists in two crystal modifications. One crystal modification is stable at high temperature, and was observed early on by Corradini et al [39]. Its structure has been redefined as a chiral, frustrated one based on a trigonal cell with three threefold helices per cell. We have also discovered a second crystal modification produced from solution. It has an orthorhombic unit cell that contains four chains in - again - three-fold helical conformation, for which one must assume coexistence of two right- and two left-handed helices. Contrary to the a and ft phases of iPP, the frustrated structure of poly( 5-methyl-hexene-1) is the more stable one [40]. 

This equals the trigonal cell a parameter. The rhombohedral a gives the next nearest like-ion neighbor distances. 

The ability to nucleate p crystals can be explained by the surface geometry of phthalocyanine crystals. For a modification of the phthalocyanine the distance between grooves on the lateral surfaces equals 1.19 nm (Honigmann et al. 1%5). This dimension is comparable with the spacing of 1.1 nm between helices of the same hand in the trigonal cell of the P form of polypropylene. 

The crystal structiue of the p form of iPP was only recently described despite the fact that this phase has been known to exist for 40 years. The structure consists of sections of three isochiral helices packed in a trigonal cell. One of the three helices is embedded in a crystallographic environment which is different from the other two, i.e. the structure is frustrated [5]. AFM studies of the (110) plane confirmed this frustrated character. In addition, it seems that in epitaxy the chains in the contact plane of polymer crystals (grown on dicyclohexylterephthalamide) expose rows of 2, 2 and 1 methyl groups. 

P phase rich or even pure p-phase with spherulitic structure can be prepared by using a selective p-nucleating agent [1]. The experimental data indicated a possible upper (TO ) = 140°C) and lower temperature (T(ap) = 100-110 C) limit of the formation of pure 3-iPP [1, 2, 9]. The p-modification of iPP (p-iPP) has a trigonal cell with parameters a = b = 1.101 nm, c = 0.65nm containing three isochiral helices. This cell is frustrated the three helices do not have similar orientation [7]. 

For centered trigonal cells, the value of Z applies the rhombohedral setting. - Fiir zentrierte trigonale Zellen gilt der Z-Wert fiir die rhomboedrische Aufstellung. 

Fig. 31.13. Structures of the trigonal cells of Cu2Er2/3S2 and CuScS2, along the 100 planes.

For centered trigonal cells, the value of Z applies to the rhombohedral setting. 

Within the approximate temperature ranges 850 to 750°C, 750 to 550°C and 550 to 450°C three dilTerent types of superstructures were foimd with trigonal cells... 

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