SOME ASPECTS OF THE THERMODYNAMIC BEHAVIOR OF FERROELASTIC AND CO-ELASTIC PHASE-TRANSITIONS

Ferroelastic phase transitions and phase transitions involving coupling between the driving order parameter and elastic degrees of freedom (i.e, co-elastic phase transitions) follow empirically a Landau-type mean field behaviour over large temperature intervals. At temperatures close to the transition point the upper critical dimension is reduced to three if fluctuations are sufficiently constrained in reciprocal space. At low temperatures, the observed temperature independence of the order parameter in the saturation regime (S-regime) is correlated with the quantum limit of the dissipation-fluctuation theorem. The good agreement between experimental observations and the predictions of a Landau-type treatment of the excess Gibbs free energy even for large values of the order parameter leads to the conclusion that the convergency of the polynomial form of the Gibbs free energy is due to small energy coefficients rather than due to the order parameter being small. Fluctuations are also constrained by wall energies in kinetic experiments where walls are described in terms of solitary waves with positive wall energies. Crystals in the precursor regime T > Tcand kinetically quenched or shock-heated material show typical microstructures with modulations and tweed structure on a mesoscopic scale (i.e. some 100 Å). The kinetic rate laws are shown to follow from a non-conserved relaxation of the mesoscopic order parameter using the same excess Gibbs free energies as in case of the equilibrium behaviour. © 1991, Taylor & Francis Group, LLC. All rights reserved.