Determination of Local Atomic Structure by 2p X-Ray Absorption Spectra of Rare-Earth Elements in T- and T'-Phase Materials

Abstract
We have found a genaral rule that links the presence of a particular feature at about 20 eV from the rising edge in the X-ray absorption near edge structure (XANES) spectra at the La, Nd, Gd, Y, and Sr L₃-edge in T- and T'-phase materials with the geometrical arrangement around the absorbing site. The absorption spectra of La_2-xSr_xCuO₄, Nd₂CuO₄, Gd₂CuO₄, and YBa₂Cu₃O₇ compounds are analysed by full multiple-scattering theory with complex potential of Hedin-Lundqvist type obtaining very good agreement between theory and experiments. As a result of this analysis we show that this feature appears everytime the photoabsorber has an eightfold hexahedron-type coordination with its nearest neighbors. This finding can be used to determine the rare-earth ordering in T*-phase superconductors.