Structural origin for the dipole x-ray resonant scattering in the low temperature phase of Nd0.5Sr0.5MnO3 manganite

We have investigated the low temperature phase of a Nd0.5Sr0.5MnO3 single crystal by x-ray resonant scattering at the Mn K-edge of the (3 0 0), (0 3 0) and (0 5/2 0) reflections. Strong resonances were observed for the s-s' channel in the (3 0 0) and (0 3 0) reflections and for the s-p' channel in the (0 5/2 0) reflection. These resonances show a p-periodicity on the azimuthal angle, having the intensity at the minimum position almost zero. The intensity dependence on the photon energy, azimuthal angle and polarisation dependencies has been analysed using a semi-empirical structural model. Contrary to previous claims of charge (Mn3+-Mn4+) and orbital ordering in this compound, our results show that the dipole resonant superlattice reflections can be explained by the presence of two types of Mn sites with different local geometric structure. One of the Mn sites is surrounded by a tetragonal-distorted oxygen octahedron whereas the other site has a symmetric octahedral environment. This model also establishes that no real space charge ordering is needed to explain the experimental data. Intermediate valence states according to a fractional charge-segregation Mn+3.42-Mn+3.58 were deduced.