Crystal Unit Cells and Lattices

Unit Cell

• A unit cell is a representative part of a lattice that contains all the necessary information to define how the pattern is repeated
• All unit cells in a specific crystal are the same size and contain the same number of molecules or ions, arranged in the same way
• Unit cells can be divided into two classes: primitive unit cells and centered unit cells

Primitive and Centered Unit Cells

• Primitive unit cells: constituent particles (atoms or molecules) are present only at the corner positions of the unit cell
• Centered unit cells: one or more constituent particles are present at positions other than corners in addition to those at corners
• There are seven possible primitive unit cells with atoms or molecules only at each corner of the unit cell
• Centered unit cells can have atoms or molecules at the center of the top or bottom faces (end-centered), at the center of every face (face-centered), or with a single atom in the center of the cell (body-centered)

Crystal Form and Crystallization

• Most APIs are crystalline in nature and are produced by crystallization
• The physical properties of the solid state in crystals and powders of both drugs and pharmaceutical excipients are important as they can affect production of dosage forms and performance of the finished product
• Crystals of a given substance may vary in size, shape, relative development of faces, and number and kind of faces present
• The overall shape of the crystal is termed the crystal habit, which depends on the conditions of crystallization, such as solvent used, temperature, and concentration and presence of impurities

Crystallization Process

• Crystallization is mostly accomplished by creating a state of supersaturation of solute in a solution
• Supersaturation of solution can be achieved by cooling, evaporation, or a chemical reaction that changes the nature of the solute
• Crystallization is achieved through spontaneous formation or extraneous addition (seeding) of crystal nuclei
• Crystal growth occurs round the nuclei when solute is transferred from the solution to the crystal state (a reverse dissolution process)

Crystal Structure

• A crystal or crystalline solid is a solid material whose constituents (atoms, molecules, or ions) are arranged in a highly ordered array forming a three-dimensional network that extends in all directions
• The three-dimensional arrangement is called a crystal lattice
• The positions occupied by particles in a crystal lattice are called lattice sites or lattice points
• The crystal lattice is a collection of lattice points, with molecules and atoms held together by noncovalent interactions