“Incipient Orthorhombic” Phase in Ba2YCu3O7-x Ctystals

Twinning in a wide oxygen-deficiency range of Ba2YCu3O7-x crystals (0.15<x<0.78) prepared by a quenching method was examined by electron optical methods. Prior to the complete tetragonal-orthorhombic phase transformation, an “incipient orthorhombic” phase was observed in the x=0.78 specimen, whose crystal was not a superconductor. The incipient orthorhombic phase is considered as having finely dispersed domains in a tetragonal crystal. The incipient phase was also observed in a 5% Co-doped Ba2YCu3O7-x crystal, which is a superconductor with a T c value of 60 K. Lowering of T c values due to Co-dopants seems to be attributed to development of the incipient orthorhombic phase.