Origin of the resonant x-ray scattering in LaMn O3

We have performed a detailed resonant x-ray scattering (RXS) study at the Mn K edge of LaMn O3 by measuring the (h,0,0), (0,k,0) and (0,0,l) forbidden reflections (h,k,l=3,5) between 10 and 300 K in the σ-π channel. For all three types of reflections we observed strong resonant peaks presenting the sin2 intensity modulation as a function of the azimuth scanning angle, characteristic of dipolar transitions. Their origin was ascribed to anisotropic tensor scattering, since no change either in energy line shape or in intensity was observed when crossing the Néel temperature at ∼140 K. Integrated intensities were roughly constant up to 300 K, contrary to previous reports found in the literature. The energy dependence of the scattered intensity for the (h,0,0) and (0,k,0) reflections was identical but different from the (0,0,l) reflection. All reflections have been explained within multiple scattering theory in terms of long-range structural distortions around Mn atoms, without invoking any kind of 3d orbital ordering. We also studied the energy dependence of the principal axes of the anomalous-scattering tensor in a case, like the present one, where the scattering atoms do not possess any point-symmetry axes. We found that they are not constant with the photon energy, implying the absence of a direct correlation between RXS and quadrupolar charge distribution in the ground state.