Ionic crystals are composed of alternating positively and negatively charged ions. The strong electrostatic forces between these ions hold the crystal together in a rigid structure. When a force is applied to an ionic crystal, the ions can move slightly, but they are not able to move past each other without breaking the crystal. This means that ionic crystals are brittle and cannot be deformed without breaking.

However, if the layers of an ionic crystal are shifted, the crystal can undergo plastic deformation. This is because the layers can slide past each other without breaking the crystal. This type of deformation is possible because the ions are not directly bonded to each other within the layers. Instead, they are held together by the electrostatic forces between the layers.

Plastic deformation of ionic crystals is important in a number of applications, such as the production of ceramics and the processing of metals.