Structural anomalies in RBa₂Cu_3-xCo_xO_7±δ (R= rare earth) defect perovskites

Abstract

A series of oxides RBa₂Cu₂CoO_7+δ(R = La, Pr, Nd, Sm, Gd and Er) has been synthesized and found to crystallize in an orthorhombic YBa₂Cu₃O_7-δ(123)-like structure. X-ray electron and neutron diffraction studies of LaBa₂Cu_3-xCo_xO_7±δ showed that for smaller concentrations of Co (0.05≤x≤0.3) the oxides crystallize in a tetragonal structure, and for higher concentrations, (0.3≤x≤1), in an orthorhombic structure. LaBa₂Cu₂CoO_7+δ showed an orthorhombic to tetragonal structural transition with decreasing oxygen content, and thermal desorption of oxygen from LaBa₂Cu₂CoO_7.35 supported the presence of two types of labile oxygens, O(1) at (0½0) and O(6) at ( 00½). All members of RBa₂Cu₂CoO_7+δ were semiconducting and the degree of orthorhombicity decreased with decreasing size of the central rare earth ion. Superconductivity was quenched faster in LaBa₂Cu_3-xCo_xO_7±δ for a lower value of x = 0.1, unlike in the case of YBa₂Cu_3-xCo_xO_7±δ where superconductivity persists up to x = 0.35. The compounds with x ≥0.2 were all semiconducting. Magnetic susceptibility studies on LaBa₂Cu_3-xCo_xO_7±δ suggest a mixed valent state for cobalt.