Neutron diffraction study of average and local structure in La _0.5Ca _0.5MnO ₃

We used neutron powder diffraction to obtain the local and long-range structure of La _0.5Ca _0.5MnO ₃ at room temperature and 20 K. By combining Rietveld and pair distribution function analysis of the total neutron scattering data, we have analyzed the structure of the compound using two competing models describing the low temperature phase: first the charge-ordered-orbital-ordered model and second the Mn - Mn dimer model. These structural models fit the 20 K neutron powder diffraction pattern equally well using a Rietveld analysis. Therefore, pair distribution function analysis is used to probe the local and medium-range structure revealing a system with two distinctly distorted Mn octahedra and Mn ions with nonintegral valence states. The distorted octahedra differ with the structural model for the Zener polaron-type Mn - Mn dimer picture proposed for Pr _0.60Ca _0.40MnO ₃ and order in a similar checkerboard configuration associated with the CE-type antiferromagnetic structure. Therefore, locally the charge difference and structural ordering between the two Mn is appreciable enough to describe the system at 20 K as "partially charge ordered."