Displacive vs. order-disorder in structural phase transitions

The quantitative assessment of the displacive degree of a structural phase transition is discussed both from a microscopical and an empirical viewpoin t. Using Monte Carlo results for the Phi(4) model as a qualitative guide, t he relation between both perspectives is shown. Based on the general trends observed, an empirical parameter for describing the displacive degree of a ny structural phase transition is proposed. The Rhodes-Wolfarth factor exte nded to ferroelectrics by Tokunaga [J. Phys. Sec. Jpn (1988) 4275] is a rea lization of this general parameter for the particular case of proper ferroe lectrics. The power and limitations of such approach are discussed.