Critical Region in Ferroelectric Triglycine Sulfate

Careful measurements have been made for triglycine sulfate (TGS), in the vicinity of the critical point Tc, on the temperature dependence of the spontaneous polarization and the field dependence of the polarization at Tc. It is found that deviations from the mean-field theory occur only within Tc-T<ΔTc≈5×10^-3^∘C and E<Ec≈0.5 V/cm. The coherence length ξ₀ at 0^∘{}K is calculated on the basis of Menyh\'ard's theory, which gives a condition for the breakdown of the mean-field theory. The minimum values of ξ₀ obtained by ΔTc and Ec are 7 and 6 {}, respectively, which are very close to the average distance, 6.8 {}, between statistical units of TGS.