Ni-induced local distortions in (formula presented) and their relevance to (formula presented) suppression: An angular-resolved XAFS study

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 (formula presented) with (formula presented) 0.03, 0.06. A special magnetic alignment procedure allowed us to measure pure (formula presented)- and (formula presented)-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 (formula presented) octahedra are contracted along the c axis by (formula presented) relative to (formula presented) octahedra while the in-plane distances of (formula presented) and (formula presented) octahedra are the same within (formula presented) The (formula presented) octahedral contraction drives the average (formula presented) axis contraction measured by diffraction with increasing content of Ni. The local (formula presented) axis shows strong spatial fluctuations, due to the different (formula presented) and (formula presented) octahedral configurations and the stronger bonding of a (formula presented) ion than a (formula presented) 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 (formula presented) and hole localization above (formula presented) by Ni dopants.