Domain, hysteresis switching curves

In general a ferroelectric perovskite single crystal will be composed of a roughly equal number of domains oriented in all the equivalent directions allowed by the crystal symmetry. The overall polarisation of the crystal will be zero. The application of an electric field will cause a polarisation switch and lead to a classical hysteresis loop in which the important values are P, (the remanent or residual polarisation when the electric field is reduced to zero), and E, (the coercive field, which is the reverse field required to reduce the polarisation to zero). Extrapolation of the high-field portion of the curve to =0 gives the value of the spontaneous polarisation P (Figure 6.9a). 

---