Strain effects, particularly in phase transitions

Recent progress in understanding kinetics of texturing during structural phase transitions with long-range strain interactions is reviewed. We analyse systems with elastically mediated interaction which due to its anisotropy and infinite range has a pronounced effect on kinetics and textures occurring during the phase transition. As a generic example the textures ('stripe' and 'tweed' phases) due to oxygen ordering in layered high-T-c cuprates (YBCO) are discussed. An atomic ordering process in cell i generates a local stress field, which is propagated elastically to a distant cell j. The effective elastic interaction J(R(ij)) at large distances can be broken into a highly anisotropic spatial part, defining a few 'soft' directions coinciding with the orientation of twin boundaries, and the so-called Zener interaction J(Z) of infinite range. The spatial part of the interaction falls off proportional to 1/R(ij)(3) in ferroelastic and proportional to 1/R(ij)(5) in anti-ferroelastic materials, with corresponding differences of the properties of domain walls in these two cases. We analyse the origin of tweed texture usually seen when a ferroelastic material is quenched through the transition temperature T-c, and under other conditions. It is argued that a dense mass of embryos of the ordered phase is present as thermodynamic fluctuations at temperatures well above T-c. These fluctuations control the length and width of the tweed microdomains observed after quenching. Further coarsening of the tweed towards a stripe texture proceeds via the creation of needle domains governed by the strains around right-angled domain walls. Unusual texture formation under sudden heating is discussed.