Unit cell angles

System Characteristics Essential Symmetry Axes in Unit Cell Angles in Unit Cell... 

HT high temperature modification P unit cell angle (in degrees)... 

Figure 2.7 provides a schematic of a general structure for a unit cell. It is convenient to describe these units as having three vectors (o, b, and c) that may or may not be aligned along the Cartesian axes, depending on the values of unit cell angles. 

In the solid phase, fatty acids and related compounds pack with the hydrocarbon chains aligned and, usually, the polar groups together. The details of the packing, such as the unit cell angles and head-to-tail or head-to-head arrangement depend on the fatty acid structure (Fig. 2). 

In Table 7.1, where real and reciprocal cell dimensions, or other distances are related, an orthogonal system is assumed for the sake of simplicity. For nonorthogonal systems, the relationships are somewhat more complicated and contain trigonometric terms (as we saw in Chapter 3), since the unit cell angles must be taken into account. Rotational symmetry is preserved in going from real to reciprocal space, and translation operations create systematic absences of certain reflections in the diffraction pattern that makes them easily recognized. As already noted, because of Friedel s law a center of symmetry is always present in diffraction space even if it is absent in the crystal. This along with the absence of... 

Space group Unit cell lengths (A) Unit cell angles (deg) ... 

FIGURE 11.15 The seven types of unit cells. Angle a is defined by edges b and c, angle fi by edges a and c, and angle y by edges a and b. 

Unit cell angles degrees Monoclinic, modification I 13 60 (19)... 

Unit cell angles Degrees Isotactic poljmier a = 0 = 90 7 = 94.5 (8)... 

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