Double “Mn³⁺” Stripes in Bi_1-xSr_xMnO₃: New Type of Charge Ordering at Room Temperature

The electron microscopy study of the manganites Bi 1 - x Sr x MnO 3, especially for x = 1 / 2 and x = 2 / 3, has allowed a new type of modulated structure, correlated to charge ordering (CO) phenomena, to be evidenced. The Bi 1 - x Sr x MnO 3 manganites are characterized by a modulated structure, the modulation vector being qa* ( q ≈ 1 − x ), and the I-type symmetry of the perovskite subcell. A model is proposed: the structure can be described by double Mn 3+ stripes alternating with double ( x = 1 / 2 ) or with quadruple ( x = 2 / 3 ) Mn 4+ stripes. This type of ordering, observed at room temperature (RT), is compared to that observed in the manganites Ln 1 - x Ca x MnO 3 at low temperature and in Bi 1 - x Ca x MnO 3 at RT. The absence of magnetoresistance up to 7 T suggests that the CO state of Bi 0.5 Sr 0.5 MnO 3 is exceptionally stable, with a very high T CO close to 500 K. The electron diffraction study carried out versus T shows that the superstructure disappears at 500 K and that the phenomenon is reversible.