Structural anomalies in $RBa_2Cu_{3-x}CO_xO_{7 \pm \delta}$ ( R = rare earth) defect perovskites

A series of oxides $RBa_2Cu_2CoO_{7+\delta}$ (R = La, Pr, Nd, Sm, Gd and Er) has been synthesized and found to crystallize in an orthorhombic $YBa_2Cu_30_{7-\delta}$(123)-like structure. X-ray electron and neutron diffraction studies of $LaBa_2Cu_{3-x}CO_xO_{7 \pm \delta}$ showed that for smaller concentrations of Co $(0.05 \leq x \leq 0.3)$ the oxides crystallize in a tetragonal structure, and for higher concentrations, $(0.3 \leqx\leq I),$ in an orthorhombic structure. $LaBa_2CU_2CoO_{7+ \delta}$ showed an orthorhombic to tetragonal structural transition with decreasing oxygen content, and thermal desorption of oxygen from $LaBa_2Cu_2CoO_{7.35}$ supported the presence of two types of labile oxygens, O(1) at (0 ½ 0) and 0(6) at (00 ½). All members of $RBa_2Cu_2CoO_{7+ \delta}$ were semiconducting and the degree of orthorhombicity decreased with decreasing size of the central rare earth ion. Superconduc- tivity was quenched faster in $LaBa_2CU _{3- x}CO_xO_{7 \pm \delta}$ for a lower value of x = 0.1, unlike in the case of $YBa_2Cu_{3-x}Co_xO_{7\pm \delta}$ where superconductivity persists up to x= 0.35. The compounds with $x \geq 0.2$ were all semiconducting. Magnetic susceptibility studies o n $LaBa_2CU_{3-x}CO_xO _{7 \pm \delta}$ suggest a mixed valent state for cobalt