Question

Inspite of long range order in the arrangement of particles why are the crystals usually not perfect ?

Answer 1

Crystals have a long-range repeated pattern of arrangement of constituent particles but in the process of crystallisation, some deviations from the ideal arrangement (i.e., defects) may be introduced. Because when crystallisation occurs at a fast rate or moderate rate, the constituent particles may not get sufficient time to arrange themselves in a perfect order. Therefore, crystals are usually not perfect.