Magnetism and charge ordering in Pr0.5CaxSr0.5-xMnO3 (x=0.09 and 0.5)

The structural and magnetic transitions in two Mn3+/Mn4+(1:1) systems have been investigated using neutron diffraction and election microscopy. In Pr0.5Ca0.5MnO3, the charge and orbital order develops in the paramagnetic phase of Pnma symmetry below T-co = 250 K through a sequence of incommensurate configurations, The ordering process is accomplished at T-N = 170 K where the charge-exchange-(CE) type antifer- romagnetism sets in. Despite the superstructure, the tilt pattern of Pnma type is preserved down to the lowest temperatures. The second compound, Pr0.5Ca0.09Sr0.41MnO3, is ferromagnetic below T-C = 240 K. The charge ordering occurs at T-N = 180 K and is accompanied by the onset of the CE antiferromagnetism. The transition is associated with a change of the tilt pattern from I4/mcm to Imma type. The low-temperature phase is commensurate both in the structural and magnetic sense. It is argued that the main energy gain of the charge ordering in Mn3+/Mn4+ (1:1) perovskites results from electron-exchange interactions favored by the e(g) orbital polarization. The onset of the incommensurate configurations can be understood supposing the existence of a soft lattice mode in which the electron transfer (charge-density wave) is correlated with dynamics of e(g) orbitals.