Structural anomalies in RBa<SUB>2</SUB>Cu<SUB>3-x</SUB>Co<SUB>x</SUB>O<SUB>7&#177;&#948; </SUB> (R= rare earth) defect perovskites

A series of oxides RBa2Cu2CoO7+&#948;(R = La, Pr, Nd, Sm, Gd and Er) has been synthesized and found to crystallize in an orthorhombic YBa2Cu3O7-&#948;(123)-like structure. X-ray electron and neutron diffraction studies of LaBa2Cu3-xCoxO7&#177;&#948; showed that for smaller concentrations of Co (0.05&#8804;x&#8804;0.3) the oxides crystallize in a tetragonal structure, and for higher concentrations, (0.3&#8804;x&#8804;1), in an orthorhombic structure. LaBa2Cu2CoO7+&#948; 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&#189;0) and O(6) at ( 00&#189;). All members of RBa2Cu2CoO7+&#948; were semiconducting and the degree of orthorhombicity decreased with decreasing size of the central rare earth ion. Superconductivity was quenched faster in LaBa2Cu3-xCoxO7&#177;&#948; for a lower value of x = 0.1, unlike in the case of YBa2Cu3-xCoxO7&#177;&#948; where superconductivity persists up to x = 0.35. The compounds with x &#8805;0.2 were all semiconducting. Magnetic susceptibility studies on LaBa2Cu3-xCoxO7&#177;&#948; suggest a mixed valent state for cobalt