The Rhodes-Wohlfarth parameter as assessment of the displacive degree in ferroelectrics: The case of the Phi(4) model

The Rhodes-Wohlfarth parameter extended to ferroelectrics by Tokunaga [J. P hys. Sec. Jap. 57, 4275 (1988)] is here analyaed within the Phi(4) model. I t is shown that it carl be directly related with the displacive degree of t he transition as described Ly the ratio C/E-0, between the non-local coupli ng, C, driving the transition and the depth of the energy well, Eo, associa ted with the distorted structure. However, the Rhodes-Wohlfarth parameter b ecomes asymptotically constant as C/E-0 decreases, i.e. for systems closer to the order disorder limit. Under this viewpoint, the very limited range o f values observed for this experimental parameter is explained and is shown that, in general, it can only assess quantitatively the character of the t ransition in rather displacive cases. The argument can be generalized to mo re complex systems, and when applied to well known materials, a rough estim ation of the displacive degree and the relevant microscopic energetic param eters in rather displacive ferroelectrics is possible.