Double "Mn3+" stripes in Bi1-xSrxMnO3: New type of charge ordering at roomtemperature

The electron microscopy study of the manganites Bi1-xSrxMnO3, especially fo r x 1/2 and x = 2/3, has allowed a new type of modulated structure, correla ted to charge ordering (CO) phenomena, to be evidenced. The Bi1-xSrxMnO3 ma nganites are characterized by a modulated structure, the modulation vector being qa* (q approximate to 1 - x), and the I-type symmetry of the perovski te subcell. A model is proposed: the structure call be described by double Mn3+ stripes alternating with double (x = 1/2) or with quadruple (x = 2/3) Mn4+ 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 tempe rature and in Bi1-xCaxMnO3 at RT. The absence of magnetoresistance up to 7T suggests that the CO state of Bi0.5Sr0.5MnO3 is exceptionally stable, with a very high T-CO close to 500 K. The electron diffraction study carried ou t versus T shows that the superstructure disappears at, 500 It and that the phenomenon is reversible.