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

The role played by Ni impurities in destroying the superconducting
state of La2_~Sr~CuO4 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 /(-edge polarized XAFS measurements
on oriented powder. A special magnetic alignment geometry
allowed us to measure pure ~ and aAb-polarized XAFS in identical
experimental conditions.
We found that the NiO6 octahedra are contracted along the t-axis
by 0.16(A) relative to the CuO6 octahedra in Lal.~sSro.~sCuO4,
dragging La atoms and causing the observed macroscopic t-axis
contraction. This contraction reflects the non-Jahn-Teller 3d a state
of Ni in this compound (similar to that in La2NiO4) and indicates
that Ni does not "trap" hole carders but rather, probably, "repels"
them. We suggest that this repulsion is the mechanism behind T~
suppression.