Direct evidence was found, for local O non-stoichiometry in T’-type samples, by using a high-resolution analytical scanning electron microscopic technique. A correlation was established between the O non-stoichiometry, and superconductivity. It was noted that the O content increased in going from the grain boundary to the interior of a superconducting grain. This suggested that there was a gradual decrease, in the superconducting transition temperature, when advancing towards the core. The results revealed that critical levels of carriers and O vacancies were essential for T’-type cuprates which exhibited superconductivity.

O.G.Singh, C.S.Harendranath, O.Prakash, B.D.Padalia: Solid State Communications, 1996, 100[10], 721-5