In chemistry, the Terrace Ledge Kink model (TLK), which is also referred to as the Terrace Step Kink model (TSK), describes the thermodynamics of crystal surface formation and transformation, as well as the energetics of surface defect formation. It is based upon the idea that the energy of an atom’s position on a crystal surface is determined by its bonding to neighboring atoms and that transitions simply involve the counting of broken and formed bonds. The TLK model can be applied to surface science topics such as crystal growth, surface diffusion, roughening, and vaporization. History The TLK model is credited as having originated from papers published in the 1920s by the German chemist W. Kossel and the Bulgarian chemist I. N. Stranski wherein the thermodynamic stability of step edges were discussed. Definitions Depending on the position of an atom on a surface, it can be referred to by one of several names. Figure 1 illustrates the names for the atomic positions and point defects on a surface for a simple cubic lattice. Figure 2 shows a scanning tunneling microscopy topographic image of a step edge that shows many of the features in Figure 1.Figure 3 shows a crystal surface with steps, kinks, adatoms, and vacancies in a closely packed crystalline material, which resembles the surface featured in Figure 2. Thermodynamics The energy required to remove an atom from the surface depends on the number of bonds to other surface atoms which must be broken. For a simple cubic lattice in this model, each atom is treated as a cube and bonding occurs at each face, giving a coordination number of 6 nearest neighbors. Second-nearest neighbors in this cubic model are those that share an edge and third-nearest neighbors are those that share corners. The number of neighbors, second-nearest neighbors, and third-nearest neighbors for each of the different atom positions are given in Table 1. Most crystals, however, are not arranged in a simple cubic lattice. The same ideas apply for other types of lattices where the coordination number is not six, but these are not as easy to visualize and work with in theory, so the remainder of the discussion will focus on simple cubic lattices. Table 2 indicates the number of neighboring atoms for a bulk atom in some other crystal lattices. The kink site is of special importance when evaluating the thermodynamics of a variety of phenomena. This