We have characterized the RT structure of ${\mathrm{Bi}}_{0.75}{\mathrm{Sr}}_{0.25}{\mathrm{MnO}}_3$ by means of electron diffraction, synchrotron x-ray, and neutron powder diffraction. These data evidence that $a\sim b\sim \sqrt{2}{a}_p$ and $c\sim {2a}_p$ average structure presents a modulation that doubles the a and c lattice parameters. The temperature evolution of this compound above RT has been investigated using magnetization, calorimetry measurements, and neutron powder diffraction. These data reveal that the superstructure disappears above $\sim 575\mathrm{K}$ where a structural transition takes place. Susceptibility data show that, concomitant to the formation of the superstructure (on cooling), there is an enhancement of the effective paramagnetic moment as has also been found in other $\mathrm{Bi}-(\mathrm{Sr},\mathrm{Ca}){\mathrm{MnO}}_3$ bismuth manganites. The observed features and cell evolution signals that a new type of charge and polaron ordering occurs at this composition.

• Received 6 February 2003

DOI:https://doi.org/10.1103/PhysRevB.68.134408