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As we discussed in lecture, we can apply selection rules to determine what kind of crystal structure a sample has. For each of the Bravais lattices, selection rules tell us which planes have coherent scattering and which do not:
24 de nov. de 2022 · The Body-Centered Cubic (BCC) crystal structure is important because it is extremely common in metals, and results in interesting properties like the ductile-to-brittle transformation temperature (DBTT) and high melting points.
When h+k+l is even F = non-zero → reflection. When h+k+l is odd F = 0 → no reflection. Consider the previous body-centered unit cell containing two atoms of the same kind located at origin, uvw=000 and uvw=1⁄2, 1⁄2, 1⁄2 basis. (S = h2+k2+l2) (001) planes are out of phase and cancel whereas (002) planes are allowed.
Table 1.1 summarizes the selection rules (or extinction conditions as they are also known) for cubic lattices. According to these selection rules, the h2 + J<2 + [2 values for the different cubic lattices follow the sequence Primitive I, 2, 3,4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 16, ... Body-centered 2,4, 6, 8, 10, 12, 14, 16, ...
Because of the incomplete lattice planes the extinction rules of fcc and bcc lattice are not fulfilled completely resulting in additional small peaks at forbidden peak positions below the...
Learning Objectives for Types of Unit Cells: Body-Centered Cubic and Face-Centered Cubic Cells Identify what defines a unit cell; distinguish between the three common cubic unit cell types and their characteristics.
