Pseudo-orthorhombic cell

In the paper by Jozefowicz et al. [320], a pseudo-orthorhombic cell similar to that of ES-1 is proposed for... 

The crystal stmcture of PPT is pseudo-orthorhombic (essentially monoclinic) with a = 0.785/nm b = 0.515/nm c (fiber axis) = 1.28/nm and d = 90°. The molecules are arranged in parallel hydrogen-bonded sheets. There are two chains in a unit cell and the theoretical crystal density is 1.48 g/cm. The observed fiber density is 1.45 g/cm. An interesting property of the dry jet-wet spun fibers is the lateral crystalline order. Based on electron microscopy studies of peeled sections of Kevlar-49, the supramolecular stmcture consists of radially oriented crystaUites. The fiber contains a pleated stmcture along the fiber axis, with a periodicity of 500—600 nm. 

To clarify the tacticity problem, trans-l,4-hexadiene and 5-methyl-l,4-hexadiene polymers were examined by X-ray diffraction. Fiber diagrams were obtained from samples stretched to four times their original lengths. Eight reflections from the poly(trans-1,4-hexadiene) fiber pattern may be interpreted on the0basis of a pseudo-orthorhombic unit cell with a = 20.81 + 0.05 A b =... 

Almost all unit cells shown in Table I are either orthorhombic or pseudo-orthorhombic, with a majority of space groups P2] 2] 2] and P2. Only a few structures exhibit higher symmetry and none shows lower symmetry. All structures have an antiparallel packing of chains (however, see A- and B-amyloses). On the other hand, a large variety of helix characteristics are evident, in addition to the variability in the unit cell dimensions. Some of the features useful for classifying amylose structures are shown in Table II. The distance between the two nearest antiparallel-... 

The structure of Th2(P04)2P030H(H20) has recently been determined in space group P2 [31], This compound has also been reported to have orthorhombic s mimetry [32] and it is likely that it is actually pseudosymmetric. The matrix 102/100/010] transforms the monoclinic P cell reported in the stmcture refinement to a pseudo-orthorhombic C setting, a 21.36, b 6.70, c 7.02 A, p 89.8°, which is extremely similar to the cell reported solely from powder data a 21.37, b 6.70, c 7.02 A, p 90° [32]. In contrast, the uranium analog U2(P04)2P030H(H20) has cell dimensions similar to its Th analog in the orthorhombic setting, but monoclinic symmetry a 21.15 b 6.61 c 6.99 A, p 91.7° its stmcture has not been refined [35]. 

Several metaphosphates of the tetravalent actinides have been described (Table 6), and the stmcture of U(P03)4 has been refined in the orthorhombic space group Pbcn [92]. However, in this model, five of the nine symmetrically independent oxygen positions are disordered and partially occupied. It is likely that the actinide metaphosphates with stoichiometry An(P03)4 actually adopt the ordered monoclinic Zr(P03)4 stmcture [96]. The unit cell of Zr(P03)4 is pseudo-orthorhombic and nearly four times the volume of the actinide metaphosphates a 13.50, b 28.80, c 8.66 A, (3 90.04(2)°. The matrix [200/0-20/00-1] transforms the primitive orthorhombic cell of U(P03)4 to a larger C-centered cell comparable to that of Zr(P03)4. It is probable that the unit cell data reported for the actinide metaphosphates with stoichiometry An(P03)4 in Table 6 correspond to subcells, and that their tme cells are four times larger and have C-centered monoclinic symmetry. 

Table 19 Crystal and molecular structure parameters of fibrous sulfur, [8, 54, 128-130]. The pseudo-orthorhombic unit cell proposed by Tuinstra has been overcome in favor of a monoclinic structure. The unit cell contains eight segments of ten atoms of the helices ...

From the coincidence of the X-ray layer lines of Sa,i and Sa,2 with those of Sf Tuinstra concluded that these polymers have the same molecular conformation. Since the diffraction patterns of Sa,i and Sa,2 were much poorer than that of the exact positions of the atoms of the helices remained uncertain [54, 115]. However, the gross arrangement of the helices in the units of Sa,i and S b2 could be determined. Accordingly, the structure of Sa,i is an orthorhombic lattice which has nearly the same size as the unit cell of on a pseudo-orthorhombic setting (Table 20). In these structures the helices are arranged parallel to each other. Since the densities of both were found to be the same the unit cell was expected to be build by four helices as in the case of Sf. A maximum of interlocking of the helices was estimated for the direction of the h-axis which consists of alternating helices with opposite turns (see Fig. 18). Because of the similarity with the fibrous sulfur allotrope S, ... 

The precipitation process is assumed to lead either from the lyotropic, nematic liquid crystalline state via predpitation with water under maintenance of ordered water-polymer layers to the crystal form II, or with other solvents (and also from low polymer concentrations with water) through disordered solvent-polymer structures to crystal form I In both crystal structures the ultimate polymer crystal layers are H-bonded in the aystallographic bc-plane (100), as shown in Fig. 6.2. In crystal form 1 the second molecular chain goes through the center of the unit cell (Pn or P2j/n space group, 2 chains per unit cell, monoclinic, pseudo-orthorhombic)... 

The a-form is the most common form of PLLA. It expands under normal crystallization conditions, such as crystallization from molten, glassy state, or in solution [54]. The a-form is characterized by two antiparallel chains in a left-handed IO3 helix conformation [54, 55]. As shown in Table 8.3, the chains are stacked in an orthorhombic or pseudo-orthorhombic crystalline unit cell. A notable structural feature of the a-form is the distortion of the helix conformation and the crystal structure change. According to the authors named in Table 8.3 the interactions between closest neighbour chains, that is to say the electrostatic dipole-dipole interactions, are the cause of these changing data. 

Fig. 16. Epistilbite. (a) Partial stmcture projected onto (010) plane. Pseudomirror plane (pm) is vertical in the pseudo-orthorhombic stmcture. (b) Projection along the a axis. The mirror plane (m) is vertical. The side face of growth step, which is inclined to the b axis, moves in the direction of the arrow. In (a) and (b) T(A1/Si) sites (solid circles), oxygen atoms (empty circles), and Ca atoms (stars) are illustrated. The numbers give the heights of the atoms in thousandths of the cell edge. Water molecules are omitted [88A1],...

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