Abstract
X-ray powder diffraction studies show that an oxygen deficient perovskite structure is preserved in the whole range of x for the Y1-x Eu x Ba2Cu3O7-δ and Yb1-x Sm x Ba2Cu3O7-δ system. Resistivity measurements do not show a strong x-dependence of T c for either system which has the large difference of ionic radius between Y and Eu or Yb and Sm. In contrast, T c is more strongly depressed in the Eu(Ba1-x Sr x )2Cu3O7-δ system than in the Y(Ba1-x Sr x )2Cu3O7-δ system. This result gives suggestion that the decrease of T c is due to local distortions of the CuO2 planes induced by Sr substitutions on the Ba sites.