Analysis of the x-ray resonant scattering at the Mn K edge in half-doped mixed valence manganites

We reconsider the interpretation of x-ray resonant scattering experiments performed on the so-called charge-ordered manganites with Mn3+/Mn4+ = 1. The comparison between these experiments and our,x-ray absorption data shows that no real Mn3+/Mn4+ charge ordering occurs in these compounds. However, these experiments demonstrate the presence of two different types of manganese atom with a different local geometrical structure. We propose a structural model which accounts for these resonant scattering experiments. Within this model, the charge-ordering phase transition can be explained as a structural phase transition driven by the softening of a phonon mode. This produces a periodic arrangement of local distortions, which is responsible for the observed resonances. Furthermore, electronic localization in these materials would occur on a length scale larger than the atomic one.