2.1.7 Ferroelectric Crystals and Critical Phenomena

The transition from a ferroelectric ordered crystal state to a disordered paraelectric state is accompanied by the thermodynamic anomalies of a second order critical point. This is easily seen in measurements of the dielectric constant versus temperature in which the dielectric constant diverges with an exponent at the transition (critical) temperature. The order parameter of the transition, the electric polarization, can also be measured versus temperature to determine the ordering exponent .

The lab has some pieces of the crystal triglycine sulfate (TGS) which can be cleaved to make thin flat samples for capacitance measurements. Our supply of TGS is limited and requires special care to assure its continued usability. Electrodes can be made u sing silver paint or evaporation. The transition is at 49.25°C and can be easily reached in a heated oil bath. The lab has a large dewar, temperature control and a special 0.01°C mercury thermometer for this temperature region. The capacitance can be measured using a capacitance bridge. Since the dielectric constant becomes quite large and temperature depende nt near T_c, achieving good temperature control and sensitive temperature measurements is a major challenge in the measurements.

Extensions of the experiment might include a thermistor bridge for improved temperature resolution, automated recording of 1/ versus T using an X-Y recorder with an AC voltmete r and temperature bridge having recorder outputs. One might also use the X-Y recorder or PC equivalent with an electrometer to measure the field and temperature dependence of spontaneous polarization. The hysteresis loops can be directly recorded on the recorder.

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