Neutron diffraction studies of isotope-substituted tetragonal superconductors RBa2Cu2.76Fe0.24O7+o (R=Sm, Y)

We report results of neutron diffraction measurements on tetragonal (P4/mmm) Sm zero matrix superconductors (152,154)Sm(0)Ba2Cu2.76Fe0.24O7+δ with Tc in the range 59 to 70 K, and similar YBa2Cu2.76Fe0.24O7+δ samples with Fe and Cu isotopes. The investigations were performed in the temperature range 8 to 300 K using three different high resolution diffractometers. Analysis of the measurements shows that Fe is distributed with approximately equal probability between the Cu(1) and Cu(2) positions. The temperature dependences of the lattice parameters of the Sm and Y systems showing similar nonlinear variations and no abrupt changes, which might be characteristic of intermediate valence instabilities in Sm, were observed.