In ferroelectrics, upon lowering the temperature below T c , a spontaneous dipole moment is induced along an axis of the unit cell. Although individual dipole moments can sometimes be small, the effect of 10 24 unit cells gives rise to an electric field that over the bulk substance that is not insignificant. An important point about ferroelectrics is that they cannot exist in a centrosymmetric crystal. A centrosymmetric crystal is one where a lattice point ( x , y , z ) can be mapped onto a lattice point ( − − x , − − y , − − z ) .