TWINS AND DOMAIN-STRUCTURE OF YBACUO

Orthorhombic YBa2Cu3O7-delta (YBaCuO) compounds usually exhibit a (110)[1(1) over bar0$] twinned microstructure. The twinning phenomenon starts with the transformation of the tetragonal high temperature form into the orthorhombic low temperature phase and leads to the existence of four orientation states (i.e. four cristallographically related types of domains). The resulting domain structure is endowed with special properties related to the mobility of domain walls, and complex interactions between contiguous domains may be observed. After an account of the basic knowledge (crystallography and ferroelastic character of YBaCuO) essential to a comprehensive understanding of the subject, we will examine, in order of increasing complexity, the microstructures currently observed in YBaCuO single crystals, that is the basic domain microstructure, and local microstructures resulting from interacting domains. Moreover, as the ferroelastic character of the orthorhombic phase is largely responsible for the rich variety of microstructures, a tentative effort has been carried out to describe microstructure features (coherent twin walls, mobility of the walls, canvas contrast) within the framework of the defect theory.