Ni-induced local distortions in La1.85Sr0.15Cu1-yNiyO4 and their relevanceto T-c suppression: An angular-resolved XAFS study - art. no. 104510

We present results from angular-resolved x-ray-absorption fine-structure (X AFS) 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 pr ocedure allowed us to measure pure (c) over cap- and ab-oriented XAFS at th e Ni K edge in identical fluorescence geometries. Both the x-ray-absorption near-ed.-e structure and the XAFS unequivocally show that the NiO6 octahed ra are contracted along the c axis by approximate to0.32 Angstrom relative to CuO6 octahedra while the in-plane distances of NiO6 and CuO6 octahedra a re the same within 0.01 Angstrom. The NiO6 octahedral contraction drives th e averaged axis contraction measured by diffraction with increasing content of Ni. The local axis shows strong spatial fluctuations, due to the differ ent NiO6 and CuO6 octahedral configurations and the stronger bonding of a L a3+ ion than a Sr2+ ion to the O(2) apical oxygens of such octahedra. We di scuss the relevance of these findings to the mechanisms of loss of supercon ductivity at y approximate to0.03 and hole localization above y approximate to0.05 by Ni dopants.