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X-ray absorption calculations of Ca L23-edge spectra of calcium carbonate in its two main crystal phases, calcite and aragonite, are reported. The multichannel multiple scattering theory with a correlated particle-hole wave function and a partially screened core-hole potential is used. Very good agreement with experiment for both CaCO3 phases is obtained, while the independent particle approximation completely fails. For aragonite, appreciable linear dichroism is predicted in agreement with recent observations.

Trabajo presentado a la 15th International Conference on X-ray Absorption Fine Structure (XAFS15), celebrada del 22 al 28 de julio de 2012 en Pekín (China).-- Licencia Creative Commons Reconocimiento 3.0, We have performed a deep spectroscopic study of incommensurate satellite reflections (h, k, l) ± (0,0.278, 0) on a TbMnO3 single crystal in order to shed light on its physical origin. Resonant x-ray scattering experiments at the Mn K-edge and Tb L3-edge have been performed at different temperatures and azimuthal angles, analyzing the outgoing light polarization. Our experimental results agree with a magnetic origin for A and C-type reflections, while F and G-type have the hallmarks of anisotropic tensor susceptibility (ATS) reflections, so that they come purely from the structural modulation., The authors thank ESRF for granting beam time, especially ID20 staff. Financial support from Spanish MICINN (project FIS08-03951 and MAT2007-61621) and DGA (CAMRADS) is acknowledged.

International audience; We explore the convergence of the multiple scattering series by studying the spectral radius of the corresponding multiple scattering matrix. The energy variations of this spectral radius exhibit strong oscillations. These oscillations are shown to depend strongly on the electronic and crystallographic structure in the low energy regime. As the calculation of the eigenvalues of the multiple scattering matrix is very long, we devise a fast algorithm to approximate this spectral radius.