NEUTRON STRUCTURAL INVESTIGATIONS OF Y1-XCAXBA2CU3-YCOYO7+ -DELTA/

The results of a systematic powder neutron study on Y1-xCaxBa2Cu3-yCoy O7+/-delta for A (x = y = 0), B (x = 0; y = 0.2), C (x = 0; y = 0.4), D (x = y = 0.2), and E (x = y = 0.4) are investigated with a view to u nderstanding the relation between the structural parameters and superc onductivity. Rietveld refinements of the structures show that: (a) Co substitutes at the chain Cu(1) sites only, except for sample E, where the presence of a minor amount of Co at the planar Cu(2) site cannot b e ruled out; (b) Co substitution reduced the c-parameter, which is red uced even further upon substitution of Ca at the Y-sites; (c) the occu pancy factors of the chain O(1) site indicate an average coordination of 4.8 and 5.1 for Co for samples B and C, but only 4 for D and E; (d) Cu/Co(1) atoms for B and C display large thermal parameters, suggesti ng a displacement from their ideal centrosymmetric location; (e) the a pical Cu(1)-O(4) bond lengthens upon substitution of Co in samples B a nd C but undergoes shortening upon substitution of Ca in the case of s amples D and E. The apical Cu(2)-O(4) bond, on the other hand, shows j ust the opposite trend; (f) samples D and E show a reduction in the se paration of CuO2 layers and their oxygen content; and (e) the bond val ence of the Cu(2) ion shows the lowest value of 2.127 for the nonsuper conducting sample C.