Why does Ni suppress superconductivity in La_1.85Sr_0.15Cu_1-yNi_yO₄?

The role played by Ni impurities in destroying the superconducting state of La_2-xSr_xCuO₄ is still unclear. This is partly due to the lack of clear experimental evidence on the effect of Ni substitution upon the atomic and electronic structure of this cuprate. To provide this information, we performed Ni K-edge polarized XAFS measurements on oriented powder. A special magnetic alignment geometry allowed us to measure pure ĉ and âb-polarized XAFS in identical experimental conditions. We found that the NiO₆ octahedra are contracted along the ĉ-axis by 0.16(Å) relative to the CuO₆ octahedra in La_1.85Sr_0.15CuO₄, dragging La atoms and causing the observed macroscopic ĉ-axis contraction. This contraction reflects the non-Jahn-Teller 3d₈ state of Ni in this compound (similar to that in La₂NiO₄) and indicates that Ni does not "trap" hole carriers but rather, probably, "repels" them. We suggest that this repulsion is the mechanism behind T_c suppression.