Statics and dynamics of domain patterns in hexagonal-orthorhombic ferroelastics - art. no. 094110

We study the statics and the dynamics of domain patterns in proper hexagona l-orthorhombic ferroelastics; these patterns are of particular interest bec ause they provide a rare physical realization of disclinations in crystals. Both our static and dynamical theories are based entirely on classical, no nlinear elasticity theory; we use the minimal theory consistent with stabil ity, symmetry, and ability to explain qualitatively the observed patterns. After scaling, the only parameters of the static theory are a temperature v ariable and a stiffness variable. For moderate to large stiffness, our stat ic results show nested stars, unnested stars, fans and other nodes, triangu lar and trapezoidal regions of trapped hexagonal phase, etc, observed in el ectron microscopy of Ta4N and Mg-Cd alloys, and also in lead orthovanadate (trigonal-monoclinic) and samarium sesquioxide (hexagonal-monoclinic); we e ven find imperfections in some nodes, like those observed. For small stiffn ess, we find patterns like those observed in the mineral Mg-cordierite. Our dynamical studies of growth and relaxation show the formation of these sta tic patterns, and also transitory structures such as 12-armed bursts, strea mers, and striations which are also seen experimentally. The major aspects of the growth-relaxation process are quite unlike those in systems with con ventional order parameters, for it is inherently nonlocal; for example, the changes from one snapshot to the next are not predictable by inspection of the strain patterns alone.