Soft-x-ray-absorption fine-structure (XAFS) measurements were performed at the F K edge on ${\mathrm{Nd}}_2$${\mathrm{CuO}}_{4\mathrm{-}\mathit{x}}$${\mathrm{F}}_{\mathit{x}}$ 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 ${\mathrm{CuO}}_2$ sheets. For the compositions x=0.15 and 0.30, a fraction of 0.33±0.15 and 0.39±0.14 of F dopants, respectively, substitute for the O(1) site in the ${\mathrm{CuO}}_2$ planes. Fluorine anions placed at the O(2) sites in the NdO layers form a reservoir of extra electrons for the superconducting ${\mathrm{CuO}}_2$ sheets in ${\mathrm{Nd}}_2$${\mathrm{CuO}}_{4\mathrm{-}\mathit{x}}$${\mathrm{F}}_{\mathit{x}}$ 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.

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