Local structure around F in the fluorine-doped Nd2CuO4 superconductor

Soft-x-ray-absorption fine-structure (XAFS) measurements were performed at the F K edge on Nd2CuO4-xFx compounds with x=0.15 and 0.30. It is found that a majority of F atoms substitute for the O(2) sites in fluorite-type NdO layers between CuO2 sheets. For the compositions x=0.15 and 0.30, a fraction of 0.330.15 and 0.390.14 of F dopants, respectively, substitute for the O(1) site in the CuO2 planes. Fluorine anions placed at the O(2) sites in the NdO layers form a reservoir of extra electrons for the superconducting CuO2 sheets in Nd2CuO4-xFx superconductors, analogous to the substitution of Ce or Th cations for Nd. This observation thus provides evidence for a symmetry of anionic (F) and cationic (Ce and Th) substitution in the NdO layers in connection with superconductivity.