Wigner-crystal and bi-stripe models for the magnetic and crystallographic superstructures ofLa0.333Ca0.667MnO3

The crystallographic (charge-ordered) and magnetic superstructures of ${\mathrm{La}}_{0.333}{\mathrm{Ca}}_{0.667}{\mathrm{MnO}}_3$ were studied by high-resolution synchrotron x-ray and neutron powder diffraction. In the antiferromagnetic structure, which was refined using a noncollinear model, the a lattice parameter is tripled and the c lattice parameter is doubled with respect to the average crystallographic unit cell (Pnma setting). The crystallographic structure below the charge-ordering temperature ${(T}_{\mathrm{CO}}\sim 260\mathrm{K})$ is characterized by ordering of the $d_{z^2}$ orbitals of the Jahn-Teller-distorted ${\mathrm{Mn}}^{3+}{\mathrm{O}}_6$ octahedra in the orthorhombic ac plane, and the appearance of superlattice peaks in the x-ray patterns corresponding to a tripling of the a axis lattice parameter. The intensities of the superlattice peaks can be accounted for satisfactorily in terms of ordering of the ${\mathrm{Mn}}^{3+}$ cations in sites as far apart as possible in the ac plane (“Wigner-crystal” model) and transverse displacements of the ${\mathrm{Mn}}^{4+}{\mathrm{O}}_6$ octahedra in the c direction. These results are not consistent with a recently proposed model based on transmission electron microscopy (TEM) data in which the ${\mathrm{Mn}}^{3+}{\mathrm{O}}_6$ octahedra are arranged in a bi-stripe pattern (“bi-stripe” model). In particular, the large longitudinal stripe modulation revealed by TEM is not observed, suggesting that the TEM data may not be representative of the bulk sample. Within the framework of the Wigner-crystal model, the magnetic structure at 1.5 K and the charge-ordered superstructures at 160 and 1.5 K were refined from the neutron data.