Local structure in the stripe phase of La_1.6-xSr_xNd_0.4CuO₄

We describe the local structure of crystalline La_1.6-xSr_xNd_0.4CuO₄ (x = 0.12, 0.16) in the temperature range 10-300 K as determined from orientation-dependent La K-edge x-ray absorption fine structure (XAFS) measurements. Such XAFS measurements of c-axis-aligned powders permit distinguishing like atoms at similar bonding distances because the measurement determines the angle of the bonds relative to the c axis. The local structure of x=0.12 about La atoms up to their fourth nearest neighbors, the distance which can reliably be probed by XAFS, does not correspond to any of the average periodic structures determined for this material by diffraction. The Cu-O₆ octahedra for x = 0.12 are found to tilt 4.6°±0.4° from the c axis along an axis in the a-b plane 20.4°±5° from the a axis. This suggests the Pccn (LTO2) structure, though the octahedra become somewhat distorted and the more distant atoms do not fit the LTO2 structure. In contrast, the local structure of x=0.16 does fit the LTO2 structure and has corresponding tilt values of 3.8°±0.5° and 19.5°±7.5°. The local structures for both concentrations are found to be independent of temperature, indicating that the various crystal phase transitions found in diffraction are due to long-range averaging of local structure regions with orientational disorder. The local structure correlation length appears to be longer than the high-T_c coherence length, indicating that the local structure is the relevant one when considering the pairing mechanism.