28.9: Order of Phase Transitions
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Two common types of phase transition may be identified. These are named depending on how the order changes during the transition. In ferroelectrics this is the polarisation, which is called the order parameter. A first order transition is one which has a discontinuity in the order parameter itself, while a second order transition is one which has a discontinuity in the first derivative of the order parameter. Plots of the spontaneous polarisation vs. temperature appear as follows.
In a first order transition the polarisation varies continuously, until the Curie temperature at which there is a discontinuity. In a second order transition, the order parameter itself is a continuous function of temperature, but there is a discontinuity in its first derivative at TC.