Ionic compounds are brittle due to the strong bond between the positive and negative ions that make up the molecules. These positive and negative bonds create crystals in rigid, lattice structures. Applying pressure shifts the alignment of the ions and results in brittleness. It takes a lot of energy to break them apart from each other.
Ions form rigid crystalline structures. This is due primarily to the strength of the bonds between both the positive and negative ions. The positive and negative ions form patterns so that no two molecules with the same charge are too close together. When pressure is applied, this pattern of ions shifts so that two ions with the same charge are brought into alignment. When this happens the ions in the compound undergo electrostatic repulsion, splitting the compound.
This strong attraction and bond between positive and negative ions also results in high boiling and melting points because it takes a great deal of heat and pressure to break those bonds. Ionic bonds are also highly conductive. They conduct electricity in liquid form or when they are dissolved in water. However, in solid form ionic solids are excellent insulators because the tight bonds between the positive and negative ions make it impossible to move electrons.