The role of elastic strains at non-ferroelastic displacive phase transitions

The role of elastic strains at structural phase transitions is illustrated within the Landau theory and its first-order corrections due to critical fl uctuations and defects are described. The Landau theory is sufficient to de monstrate the impossibility of bulk nucleation in a supercooled symmetrical phase and the absence of heterophase fluctuations in solids. The critical fluctuations are known to convert a second-order transition in an Ising-lik e system in a solid to a first-order one. Close to the mean-field tricritic al point the effect can be described, for displacive systems, within a firs t-order perturbation theory and takes place for the Heisenberg systems as w ell. The influence of defects on these transitions is mediated essentially by the elastic strains. Defects smear the transition. For the "random local field" defects and an incommensurate (Heisenberg-like) transition this eff ect is so strong that first-order perturbation theory leads to a divergence .