Point Defects in a Crystal:
Almost all the crystals encountered in practice suffer from imperfections or defects of various kinds. An ideally perfect crystal is one which has the same unit cell and contains the same lattice points throughout the crystal.
If the deviation occurs due to missing atoms, displaced atoms or extra atoms, the imperfection is named as a point defect. Such defects arise due to imperfect packing during the original crystallization or they may arise from thermal vibrations of atoms at elevated temperatures. The most common point defects are the Schottky defect and Frenkel defect. Comparatively less common point defects are the metal excess defect and the metal deficiency defect.