Distinguishing between the bi-stripe and Wigner-crystal model:: A crystallographic study of charge-ordered La0.33Ca0.67MnO3

In order to resolve discrepancies in the charge-ordered structure between the Wigner-crystal and bi-stripe models, La0.33Ca0.67MnO3 was studied using quantitative electron diffraction and high-resolution imaging. Image simulations based on dynamic electron-diffraction theory suggest that the apparent difference in spacing between the Mn3+-Mn3+ and Mn3+-Mn4+ stripes, which is the basis of the bi-stripe model, can vary significantly with imaging conditions and may not directly represent the difference in actual spacing of atomic planes. Electron-diffraction study of crystal regions far away from defects, using parallel and convergent beams, reveal the existence of a[0 0 1] glide planes and n[1 0 0] diagonal glide planes that are incompatible with the large longitudinal displacement of the bi-stripe model. Although our study supports the Wigner-crystal model, detailed analysis suggests that, in our samples, the incommensurate charge modulation in the material has an average wave vector q=(0.284,0, xi) with \xi\=0.010. The symmetry breaking associated with the small component xi along the c axis has not been previously observed by high-resolution x-ray or neutron powder diffraction.