Nucleation and growth of the orthorhombic phase in melt textured YBa2Cu3O6+δ

The nucleation and growth of the orthorhombic phase by transformation through oxygen ordering from the tetragonal phase in melt textured YBa2Cu3O6+δ (123) are investigated by polarized light microscopy. The samples for observation were collected from the interior of partially oxygenated bulk melt textured material. The orthorhombic phase is observed to nucleate basically at three different sites, viz. along domain boundaries, at platelet boundaries and around macroscopic cracks. The nuclei at domain boundaries are observed to grow along the platelet boundary gaps with a cellular morphology with sharp tips, whereas the growth front of the phase growing from the boundaries of platelets into their interior is planar. The growth of the phase from the domain boundaries into the domains differs depending on the orientation of the platelets at the point of nucleation. The origin of inter-platelet gaps in the melt processed material is discussed in the light of the observations made during this work. The ratio of growth rates of the orthorhombic phase along the gaps and into the platelets is estimated.