CHARGE ORDERING IN MIXED-VALENT MANGANITES

Z.Jirák¹, F.Damay², M.Hervieu² and C. Martin²

¹Institute of Physics of ASCR, Cukrovarnická 10, 16200 Prague 6, Czech Republic
²Laboratoire CRISMAT, URA CNRS 1318, ISMRa, Bd du Maréchal Juin, 14050 CAEN Cedex, France

The structural and magnetic transitions in two Mn³⁺/Mn⁴⁺ (1:1) perovskites have been investigated by the neutron diffraction and electron microscopy. In Pr_0.5Ca_0.5MnO₃ the charge order originates in the paramagnetic phase of the Pnma symmetry and develops below T_co= 250 K through a sequence of incommensurate configurations. The ordering process is accomplished at T_N=170 K where the CE type antiferromagnetism sets in. The second compound Pr_0.5Ca_0.09Sr_0.41MnO₃ becomes ferromagnetic below T_C=240 K. The charge ordering occurs at T_N=180 K and it is accompanied by an onset of the CE antiferromagnetism. The transition is abrupt and it is associated with a structural transition in which tilt pattern is changed from the I4/mcm to Imma type. The low temperature phase is completely long-range ordered in the structural and magnetic sense and it is strictly commensurate. An unexpected feature of the structural refinement is the off-centre location of the Mn⁴⁺ ions and small difference between the calculated bond valence sums for the formally Mn³⁺ and Mn⁴⁺ sites.

It is discussed that the charge and magnetic orderings in the Mn³⁺/Mn⁴⁺ (1:1) perovskites are driven by interactions which originate in the same, electron exchange mechanism and are primarily dependent on 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 the dynamics of e_g orbitals.