Equation of State for the Cooperative Transition of Triglycine Sulfate nearTc

Measurements of polarization versus field in the vicinity of the Curie temperature from triglycine sulfate, both below and above $T_c$, allow first, the determination of a number of critical exponents and second, the characterization of the ferroelectric equation of state. The relationship, below and above $T_c$, between the "scaled" variables $\frac{\hat{P}}{{[1-(\frac{T}{T_c}\left)\right]}^{\beta }}$ and $\frac{\hat{E}}{{[1-(\frac{T}{T_c}\left)\right]}^{\beta \delta }}$ was determined from a log-log plot which showed clearly a well-defined asymptotic behavior for the small and large "scaled" field. Comparison of the scaled data with the results from the mean-field theory showed good agreement. A phenomenological expression for the equation of state which matches all the empirical and homogeneity requirements has been formulated. Evidence for the validity of this equation of state for other transitions for which accurate data are available is discussed.