Y

Figure 2 Scalars of lattice structure.

Table 1 Seven Crystal Systems

Crystal system

Axial lengths and angles

Cubic a = b = c a = ß = g = 90o a = b = c a = ß = g = 90o a = b = c a = ß = g = 90o a = b = c a = ß = g = 90o a = b = c a = ß = 90o, g = 120o a = b = c a = g = 90o = ß

Tetragonal

Orthorhombic

Rhombohedral

Trigonal

Hexagonal

Monoclinic

Triclinic

A crystalline particle is characterized by definite external and internal structures. Habit describes the external shape of a crystal, whereas polymorphic state refers to the definite arrangement of molecules inside the crystal lattice. Crystallization is invariably employed as the final step for the purification of a solid. The use of different solvents and processing conditions may alter the habit of recrystal-lized particles, besides modifying the polymorphic state of the solid. Subtle changes in crystal habit at this stage can lead to significant variation in raw-material characteristics. Furthermore, various indices of dosage form performance, such as particle orientation, flowability, packing, compaction, suspension stability, and dissolution can be altered even in the absence of significantly altered polymorphic state. These effects are a result of the physical effect of different crystal habits. In addition, changes in crystal habit either accompanied or not by polymorphic transformation during processing or storage, can lead to serious implications of physical stability in dosage forms. Therefore, in order to minimize the variations in raw-material characteristics, to ensure the reproducibility of results during preformulation, and to correctly judge the cause of instability and poor performance of a dosage form, it is essential to recognize the importance of changes in crystal surface appearance and habit of pharmaceutical powders.

The crystal habit is also affected by impurities present in the crystallizing solution; often these impurities provide the earliest nucleation of crystal growth, and become an integral part of the crystal. In some instances, the presence of impurities inhibit crystal growth, as shown, when certain dyes or heavy metals are mixed with solutions. If an impurity can adsorb at the growing face, it can significantly alter the course of crystal growth and geometry. The habits bound by plane faces are termed euhedral and those with irregularly shaped ones are called anhedral. The symmetry of a crystal is generally studied by using optical goniometer that allows the measurement of the angles between the crystal faces. This technique is of use only when good crystals of size >0.05 mm in each direction can be obtained, which is generally not the case.

Chemical crystallography provides accurate and precise measurements of molecular dimensions in a way that no other science can begin to approach. Historically, single crystal X-ray diffraction was used to determine the structure of what

Hexagonal Figure 3 Bravais lattice system.

was thought of as "small molecules." Twenty years ago, it was possible to solve structures with an average of only 100 nonhydrogen atoms. However, with developments in hardware and software, the upper limit has risen to about 500 and recently, even a 1000-atom structure was solved. The APEX II line of Chemical Crystallography Solutions (1) allow single crystal structure determination. The APEX II detector is suitable for fast processing. The Brucker SHellXTL software system

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