Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Teaching & Learning Packages Ferroelectric Materials Order of phase transitions
Ferroelectric Materials AimsBefore you startIntroductionThe dipole momentPolarisationSwitching polarisation (1)Switching polarisation (2)Measurement of polarisationFabrication of a KNO3 ferroelectric capacitorTemperature dependence of the Hysteresis loopBarium titanateBarium titanate and phase changesOrder of phase transitionsFerroelectrics - why?SummaryQuestionsGoing further
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Order of phase transitions

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.

First order
Second order
Plot of the spontaneous polarisation vs. temperature Plot of the spontaneous polarisation vs. temperature

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.

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