Microstructure related to charge and orbital ordering inPr0.5Ca0.5MnO3

We have investigated the microstructure related to the charge and orbital ordering in the manganites ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ by transmission electron microscopy. Electron diffraction clearly shows the presence of the incommensurate structure in the paramagnetic insulator phase of ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ between 180 and 260 K. Our experimental results clearly show that the incommensurate-to-commensurate (IC-to-C) charge-ordering transition was correspondent with the paramagnetic to antiferromagnetic transition around 180 K. Dark field images also revealed the motion of the discommensurations during the IC-to-C transition. A model of the IC structure which involves the presence of the partial orbital disordering and the complete charge ordering is proposed.