11.5: Imperfections in Crystal Structure: Point, Line and Plane Defects

A perfect crystal, in theory, has a uniform structure with the same unit cell and lattice points throughout. However, any deviation from this periodic arrangement is known as an imperfection or defect. These defects can be categorized into three types: point, line, and plane defects.

Point defects occur when there is a deviation from the ideal due to missing atoms, displaced atoms, or additional atoms. These imperfections might occur due to imperfect packing during crystallization or because of thermal vibrations at high temperatures. The increased thermal energy can cause atoms to leap out of their low-energy positions. The most commonly observed point defects are the Schottky defect, the Frenkel defect, the metal excess, and the metal deficiency defects.

Line defects, also known as dislocations, are another type of imperfection that disrupts the periodicity of the atomic lattice array in certain directions, reducing its long-range order. The presence of dislocations makes a crystal more susceptible to deformation under shear stress. If long-range order is disrupted to a certain degree, the structure can become amorphous, which alters the physical, electronic, and optical properties of the material. Edge and screw dislocations are the most common types of line defects.

Edge dislocations occur when there is a slight mismatch in the orientation of adjacent parts of a crystal, resulting in an extra row of atoms that extends only partway through the crystal, making the crystal mechanically weak. This misalignment causes atoms near the dislocation to be pushed together above the edge and pulled apart below it. As a consequence, impurities with larger diameters tend to concentrate below the edge, and those with smaller diameters concentrate above it. This makes it more difficult to move a dislocation in an impure material, explaining why alloys require a greater shear force for permanent deformation than pure metals.

Screw dislocations, on the other hand, can be visualized like a rubber stopper cut parallel to its axis and then twisted at one end to create a jog. This results in a spiral ramp-like structure of atoms known as a screw dislocation. These types of dislocations facilitate easy crystal growth as atoms can be added at the step. Real dislocations are usually a mix of edge and screw dislocations.

Dislocations also serve as preferred sites within a crystal for chemical reactions and physical changes, such as phase transformation, precipitation, or etching. The number of dislocations per unit area can be measured by counting the etch pits formed on the surface.

A stacking error is a type of plane defect. Most crystalline solids are not single crystals but consist of many tiny crystals held together. Neighboring crystals have random orientations, and the boundaries between their faces are also considered plane defects.