Why does Ni suppress superconductivity in La1.85Sr0.15Cu1-yNiyO4?

The role played by Ni impurities in destroying the superconducting state of La2-xSrxCuO4 is still unclear. This is partly due to the lack of clear exp erimental evidence on the effect of Ni substitution upon the atomic and ele ctronic structure of this cuprate. To provide this information, we performe d Ni It-edge polarized XAFS measurements on oriented powder. A special magn etic alignment geometry allowed us to measure pure (c) over cap and (a) ove r cap b-polarized XAFS in identical experimental conditions. We found that the NiO6 octahedra are contracted along the (c) over cap-axis by 0.16(Angstrom) relative to the CuO6 octahedra in La1.85Sr0.15CuO4, drag ging La atoms and causing the observed macroscopic e-axis contraction. This contraction reflects the non-Jahn-Teller 3d(8) state of Ni in this compoun d (similar to that in La2NiO4) and indicates that Ni does not "trap" hole c arriers but rather, probably, "repels" them. We suggest that this repulsion is the mechanism behind T-c suppression.