Single crystals which contained ferro-elastic twin walls were deoxygenated via gas transport reaction with Na vapor. The reaction product consisted of a non-superconducting matrix of tetragonal oxide, with superconducting twin boundaries. The superconducting transition temperature was 3K. The upper critical field temperature dependence exhibited BCS-type behavior below 2.5K, and a weak tail near to the critical temperature.

A.Aird, E.K.H.Salje: Journal of Physics - Condensed Matter, 1998, 10[22], L377-80