Time evolution of tetragonal-orthorhombic ferroelastics - art. no. 064101

We study numerically the time evolution of two-dimensional (2D) domain patt erns in proper tetragonal-orthorhombic (T-O) ferroelastics. Our equations o f motion are derived from classical elasticity theory, augmented by nonline ar and strain-gradient terms. Our results differ from those found by other dynamical methods. We study first the growth of the 2D nucleus resulting fr om homogeneous nucleation events. The later shape of the nucleus is largely independent of how it was nucleated. In soft systems, the nucleus forms a flowerlike pattern. In stiff systems, which seem to be more realistic, it f orms an X shape with twinned arms in the 110 and (1) over bar 10 directions . Second, we study the relaxation that follows completion of the phase tran sition; at these times, the T phase has disappeared and both O variants are present, separated by walls preferentially in 110-type planes. We observe a variety of coarsening mechanisms, most of them counterintuitive. Our patt erns are strikingly similar to those observed in transmission electron micr oscopy of the improper T-O ferroelastic YBa2Cu3O7.