Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 83

The unit cell of a crystal is defined in terms of the length of the cell edges from a given lattice point, taken as the origin, and the angle between the cell edges (Figure 3.5). The general convention in orienting a crystal is to use a coordinate system in which the x‐axis points toward us as we look directly at the page, the y‐axis points to the right, the z‐axis points upward, and the origin is the point of intersection of the three axes. Then, the unit cell dimensions are taken as the edge lengths a, b, and c, respectively, in the x, y, and z directions. The angle between the edges b and c, that is, opposite the edge a, is taken as α, the angle between the edges a and c, opposite the edge b, is β, and the angle between the edges a and b, opposite the edge c, is γ. The lattice constants are the edge lengths a, b, and c, and the angles α, β, and γ, which define the crystal structure. The cubic structure, for example, has a = b = c and α = β = γ = 90°.

Figure 3.5 Geometry and parameters of a unit cell.