Crystal structure and charge density analysis of the charge- and orbital-ordered phase of La2-2xSr1+2xMn2O7 (x=0.525) using CBED

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University* Institute for Solid State Physics, University of Tokyo, Japan** Department of Physics, Graduate School of Science, Tohoku University, Japan***
â—‹Yoichiro Ogata* Kenji Tsuda* Kazuma Hirota** Youichi Murakami***

We developed a method to refine crystal structural parameters and charge density using convergent-beam electron diffraction (CBED) and applied the method to some materials. We have applied the method to La2-2xSr1+2xMn2O7 (LSMO327) (x=0.525) which is a charge-ordering material and shows a phase separation phenomenon.
LSMO327 has a bi-layered perovskite structure with space group I4/mmm at room temperature. Recently, the phase separation between the A-type antiferromagnetic phase (A-phase) and the CE-type charge- and orbital-ordered phase (CE-phase) was found in the range of x=0.5-0.6 of LSMO327 at temperature 150-210 K by using neutron diffraction and resonant X-ray scattering experiments. In the present study, we have observed the domain structure of the phase separation in LSMO327 (x=0.525) using dark-field electron microscopy, and have analyzed the crystal structure and charge density using CBED.
The dark-field images were taken at 160 K with accelerating voltage 200 kV using an electron microscope JEM-2000FX. The domain size of CE-phase was about 2 μm. The CBED experiments were carried out using an energy-filter electron microscope JEM-2010FEF at 160 K at an accelerating voltage 100 kV, the space groups of the A-phase and CE-phase being determined to be I4/mmm and Bbmm, respectively. The lattice constants of the CE-phase were determined to be a=5.4993 Å, b=10.999 Å and c=20.090 Å from higher-order Laue zone (HOLZ) lines in the CBED patterns. An initial structure model for the refinement was constructed from simulations of CBED intensities with systematically varying atom positions and Debye-Waller factors.