Effect of Hydrostatic Pressure on the Ferroelectric Properties of Tri-Glycine Sulfate and Selenate

The transition temperatures $T_c$ of tri-glycine sulfate and isomorphous tri-glycine selenate are raised by the application of hydrostatic pressure. Up to 2700 atmos, the dependence of $T_c$ on pressure is linear with a slope of 2.6×${10}^{-3\mathrm{}}$ degrees/atmos for the sulfate and 3.8×${10}^{-3\mathrm{}}$ degrees/atmos for the selenate. The Curie-Weiss law, ${\epsilon }_b\cong \frac{C}{(T-T_c)}$, is unaffected by pressure except for the shift in transition temperature. The curve of the spontaneous polarization is shifted accordingly along the temperature axis; in the vicinity of the transition point, the square of the spontaneous polarization depends linearly on temperature and pressure. The results are discussed in terms of the thermodynamic theory of ferroelectrics.