Critical Region in Ferroelectric Triglycine Sulfate

Careful measurements have been made for triglycine sulfate (TGS), in the vicinity of the critical point $T_c$, on the temperature dependence of the spontaneous polarization and the field dependence of the polarization at $T_c$. It is found that deviations from the mean-field theory occur only within $T_c-T<\mathrm{}\Delta T_c\approx 5\times {10}^{-3\mathrm{}}°$C and $E<\mathrm{}{E}_c\approx 0.5$ V/cm. The coherence length ${\xi }_0$ at 0°K is calculated on the basis of Menyhárd's theory, which gives a condition for the breakdown of the mean-field theory. The minimum values of ${\xi }_0$ obtained by $\Delta T_c$ and $E_c$ are 7 and 6 Å, respectively, which are very close to the average distance, 6.8 Å, between statistical units of TGS.