Tweed microstructures: Experimental observations and some theoretical models

Experimental observations, mainly by transmission electron microscopy, of tweed microstructures in minerals (K-feldspar and cordierite), high-T _c superconductors (Co-doped YBCO) and metals are reviewed. Structural models of the local structure within the tweed are compared and evaluated with respect to spectroscopic measurements in a number of systems. It is concluded that in tweed microstructures which result from symmetry changes driven by cation-ordering, the local degree of order is not itself modulated. A model based on twin-related domains of the low symmetry form is consistent with experimental observations. Mechanisms of coarsening of the tweed microstructure to form the lamellar twinning observed in the fully ordered structure are briefly reviewed.