STRUCTURAL ANOMALIES IN RBA(2)CU(3-X)CO(X)O(7+/-DELTA) (R EQUALS RARE-EARTH) DEFECT PEROVSKITES

A series of oxides RBa(2)Cu(2)CoO(7+delta) (R = La, Pr, Nd, Sm, Gd and Er) has been synthesized and found to crystallize in an orthorhombic YBa2Cu3O7-delta(123)-like structure. X-ray electron and neutron diffraction studies of LaBa2Cu3-xCoxO7+delta showed that for smaller concentrations of Co (0.05 less than or equal to x less than or equal to 0.3) the oxides crystallize in a tetragonal structure, and for higher concentrations, (0.3 less than or equal to x less than or equal to 1), in an orthorhombic structure. LaBa2Cu2CoO7+delta showed an orthorhombic to tetragonal structural transition with decreasing oxygen content, and thermal desorption of oxygen from LaBa2Cu2CoO7.35 supported the presence of two types of labile oxygens, O(1) at (0 1/2 0) and O(6) at (001/2). All members of RBa(2)Cu(2)CoO(7+delta) were semiconducting and the degree of orthorhombicity decreased with decreasing size of the central rare earth ion, Superconductivity was quenched faster in LaBa2Cu3-xCoxO7+/-delta for a lower value of x = 0.1, unlike in the case of YBa2Cu3-xCoxO7+/-delta where superconductivity persists up to x = 0.35. The compounds with x greater than or equal to 0.2 were all semiconducting. Magnetic susceptibility studies on LaBa2Cu3-xCoxO7+/-delta suggest a mixed valent state for cobalt.