Sandwich domain walls in cordierite: a computer simulation study

Below T-c,, cordierite shows an unusual domain pattern consisting of walls which are not well oriented along elastic soft directions. This is due to a low spontaneous strain and low anisotropy energies. As a result of this, short-ranging interactions normally negligible compared to strain energies become important and there is a competition between two types of wall: those due to strain and those due to local interactions. This leads to the formations of 'sandwich walls', which represent a compromise between these two wall types, becoming energetically favourable. Here we simulate the formation of sandwich walls using an atomistic computer model. The sandwich walls are shown to be chiral in nature with a vector order parameter rotating as we move through the wall. The chirality direction is fixed since the two chirality types are not degenerate. The sandwich wall trajectories are similar to what would be expected from symmetry-adapted Ginzburg-Landau theory. The sandwich 'filling' appears in two shapes: a thick, well defined domain at low temperatures and a thin layer of wetting at higher temperatures. The latter is reminiscent of surface segregation phenomena.