Resonant x-ray diffraction study of the magnetoresistant perovskite Pr0.6Ca0.4MnO3

We report a x-ray resonant diffraction study of the perovskite Pr0.6Ca0.4MnO3. At the Mn K-edge, this technique is sensitive to details of the electronic structure of the Mn atoms. We discuss the resonant x-ray spectra measured above and below the charge and orbital ordering phase transition temperature (TCOO = 232 K), and present a detailed analysis of the energy and polarization dependence of the resonant scattering. The analysis confirms that the structural transition is a transition to an orbitally ordered phase in which inequivalent Mn atoms are ordered in a CE-type pattern. The Mn atoms differ mostly by their 3d orbital occupation. We find that the charge disproportionation is incomplete, 3d^{3.5-\delta} and 3d^{3.5+\delta} with \delta\ll0.5 . A revised CE-type model is considered in which there are two Mn sublattices, each with partial e_{g} occupancy. One sublattice consists of Mn atoms with the 3x^{2}-r^{2} or 3y^{2}-r^{2} orbitals partially occupied, the other sublattice with the x^{2}-y^{2} orbital partially occupied.