3.29 The unit cell for Al₂O3 has hexagonal symme- try with lattice parameters a = 0.4759 nm and c = 1.2989 nm. If the density of this material is 3.99 g/cm³, calculate its atomic packing factor. For this computation, use ionic radii listed in Table 3.4. Cation AP. Ba²+ Fot Fo K+ Mg Ma² Na' N₁² Si Ti Jonic Radius (am) 0.053 0.136 0.100 0.170 0.077 0.069 0.138 0.072 0.067 0.102 0.069 0.040 0.061 Anion Br a I™ 0² Jonic Radius (am) 0.196 0.181 0.133 0.220 0.140 0.184

The crystalline imperfections are described as the geometrical defects in the crystal due to incorrect arrangement of atoms, impurity, vacancy, and many more. It is not possible to achieve a perfect or ideal crystal, practically.