Zener Polaron Ordering in Half-Doped Manganites

We have refined the crystal structures of a $\mathrm{Pr}{}_{0.60}{\mathrm{C}\mathrm{a}}_{0.40}{\mathrm{M}\mathrm{n}\mathrm{O}}_3$ single crystal from neutron diffraction data. The result at low temperature gives a superstructure that cannot be interpreted as ${\mathrm{M}\mathrm{n}}^{3+}/{\mathrm{M}\mathrm{n}}^{4+}$ charge ordering. The pattern of atom displacements suggests the trapping of electrons within pairs of Mn sites, involving both a local double exchange and a polaronic-like distortion. The two mechanisms act together to form vibronic localized electronic states: Zener polarons. We have confirmed this picture by showing how it elucidates the unconventional paramagnetic behavior of half-doped manganites.