Ionic compounds are brittle. This is because the ions in the compound are held rigidly in place in a lattice due to the strong electrostatic forces of attraction in between the cations and the anions. However, if a large enough force is applied along a certain plane, this causes the ions to shift along that layer, displacing that layer with respect to the next, and there will hence be repulsion between like charges within the ionic compound. This repulsion will shatter the crystal lattice of the ionic compound.