  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.
