Dipolar theory of ferroelectrics revisited

An analysis of experimental data has been done for various ferroelectrics within the framework of the dipolar theory, along the same lines of the recent work on triglycine sulfate (TGS) and triglycine fluoberillate (TGFB). A simple method is applied which, using solely dielectric data near $T_C$, leads to the determination of the main parameters of the theory. The resulting numbers $N$ of elementary dipoles per unit volume appear to be close to the numbers of unit cells per unit volume as determined from crystallographic data. The elementary dipole moments $\mu$ are reasonably consistent with obervations of low-temperature spontaneous polarization. The mean-field coefficients $\beta$ appear to be consistent with an independent evaluation of the "reaction" field in cases for which quasi-point-dipoles in the lattice can be assumed. The relative contribution of the dipolar polarization to the total (dipolar plus atomic) polarization appears to be larger in the more ionic ferroelectrics. The transition entropy due to the order-disorder process can account for the observed values in most (but not all) cases. No adjustable parameters have been used.