Structural properties and charge-ordering transition inLaSr2Mn2O7

Transmission-electron-microscopy measurements characterizing the structure and charge-ordered states in ${\mathrm{LaSr}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ are presented. The crystal structure of this phase has been identified as a well-defined 327 layered structure by high-resolution electron microscopy and image simulation. Electron diffraction reveals at low temperature the presence of additional superstructure spots along the [110] direction. This superstructure is interpreted in terms of the charge ordering of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ associated with the $d_{z^2}$ orbital ordering of ${\mathrm{Mn}}^{3+}.$ Possible ordering models are proposed.