A modal of twin domains in YBa2(Cu1-xCox)(3)O-y

The existence of twin domains in YBa2(Cu1-xCox)(3)O-y samples was confirmed by many experiments. However, the physical picture of the twin domains has not been reported up to now. By analyzing oxygen content, Cu and Co valenc es, and Co coordination in those samples: we propose a domain model, which consists of two pairs of parallel zigzag Co-O chains surrounding orthorhomb ic cells, one along (110) direction and the other along (1(1) over bar 0) d irection. The zigzag Co-O chains are the twin walls, with neighboring walls perpendicular and the opposite ones parallel. The distance between paralle l walls (i.e., zigzag Co-O chains) depends upon Co concentration of sample. The twin domain model predicts the geometry, size, and absence of systemat ic symmetry of the domains. The model also predicts the modulation periodic ity of electron diffraction patterns, which depends on Co concentration. Al l the predictions by the model agree well with the experimental results rep orted in literatures. Furthermore, the model indicates the possible existen ce of two domains, which have incommensurable modulation periodicity.