Crystal Structures

1. Substitutional Impurity Defect: A substitutional impurity defect occurs when an atom in the crystal lattice is replaced by an atom of a different element. This replacement causes a disturbance in the lattice structure. The impurity atom may be of a different size or charge, leading to changes in the properties of the material. For example, when sodium ions replace some of the potassium ions in a potassium chloride crystal, a substitutional impurity defect is created. 2. Interstitial Impurity Defect: An interstitial impurity defect happens when an atom occupies a space (interstice) between the regular atoms in the crystal lattice. The impurity atom is smaller than the host atoms and fits into the voids in the lattice. This defect can cause distortions in the lattice and affect the material's properties. For example, when small atoms like hydrogen or carbon are introduced into the interstitial sites of a metal lattice, an interstitial impurity defect is formed.

Step 1: Understanding the body centered cubic lattice.
In a body centered cubic (BCC) lattice, the relationship between the edge length and the radius of the atom is given by the formula: This comes from the geometry of the lattice, where the diagonal of the cube is equal to , and the body diagonal is related to the edge length by . Step 2: Solving for the radius.
Rearranging the formula: Thus, the correct answer is (B).