INTRODUCTION—THE CONCEPT OF CRYSTAL

INTRODUCTION—THE CONCEPT OF CRYSTALLINITYThe perfectly ordered crystal lattice with every molecule in its expected lattice position is an ideal that is seldom, if ever,achieved. The other extreme is the amorphous state, in which a solid contains the maximum possible density of imperfections(defects of various dimensionalities), such that all long-range order is lost while only the short-range order, imposed by itsnearest neighbors, remains. Real crystals lie somewhere between these two extremes. A crystal’s position on a scale boundedby these two extremes is termed crystallinity.All real crystals, even in the pure state, possess some lattice imperfections or defects, which increase both the energy (en-thalpy under conditions of constant atmospheric pressure) and the disorder (expressed as the entropy) of the crystal lattice. Acrystal with a relatively low density of imperfections is said to be highly crystalline and to possess a high crystallinity. By con-trast, a particle with a relatively high density of imperfections is said to be partially amorphous and to possess a low crystallini-ty. In ideal terms, a totally amorphous particle corresponds to zero crystallinity. Amorphous particles may contain somewhatordered domains that can act as nuclei for crystallization; such so-called amorphous particles are said to possess a low-level,but finite, crystallinity.The ability to detect and quantify the amount of amorphous material within a highly crystalline substance is of great impor-tance during the development and subsequent manufacture of a pharmaceutical preparation.