Morphological aspects of the shear transformation in YBa₂Cu₃O_7-δ to twin or not to twin?

The fine structure of YBaCu3 07 is examined: Twin organization, planar defects, and structural phase co-existence are discussed in the context of the tetragonal to orthorhombic transformation. Heterogeneous nucleation of an insulating phase at the twin boundary and sub-grain boundaries is demonstrated. On the basis of the morphological observations, the transformation is classified as martensitic. Considering the role of twin in relieving the martensitic transformation strain, by producing a low strain energy habit plane, the feasibility of the formation of a twinless structure is discussed.