Ni-induced Local Distortions in La₁.₈₅Sr₀.₁₅Cu\u3csub\u3e1-y\u3c/sub\u3eNi\u3csub\u3ey\u3c/sub\u3eO₄ and their Relevance to Tc Suppression

We present results from angular-resolved x-ray-absorption fine-structure (XAFS) measurements at the Ni, La, and Sr K edges of oriented powders of La1.85Sr0.15Cu1-yNiyO4, with y=0.01, 0.03, 0.06. A special magnetic alignment procedure allowed us to measure pure ĉ- and ab-oriented XAFS at the Ni K edge in identical fluorescence geometries. Both the x-ray-absorption near-edge structure and the XAFS unequivocally show that the NiO6 octahedra are contracted along the c axis by ≈0.32Å relative to CuO6 octahedra while the in-plane distances of NiO6 and CuO6 octahedra are the same within 0.01Å. The NiO6 octahedral contraction drives the average ĉ axis contraction measured by diffraction with increasing content of Ni. The local ĉ axis shows strong spatial fluctuations, due to the different NiO6 and CuO6 octahedral configurations and the stronger bonding of a La3+ ion than a Sr2+ ion to the O(2) apical oxygens of such octahedra. We discuss the relevance of these findings to the mechanisms of loss of superconductivity at y≈0.03 and hole localization above y≈0.05 by Ni dopants