Ordered structures of Pb(Mg1/3Nb2/3)O3 ceramics investigated by high-resolution transmission electron microscopy

In this work, high-resolution transmission electron microscopy (HRTEM) and computer image simulation were performed to study the ordered structures of undoped and La-doped Pb(Mg1/3Nb2/3)O3. The [110] selected area electron diffraction pattern consisted of (strong) allowed reflections from the cubic perovskite structure and extra (weak) superlattice reflections. The long-range order parameters (P) of ordered regions were in the range of ∼0.2 to ∼0.7. Most Mg and Nb cations in the ordered regions with high P values were located at their specific B-site sublattices, thus suppressing the atomic displacement. On the other hand, Mg and Nb cations in the ordered regions with small P values were displaced from their original positions along the directions, thus forming the strain waves along the directions. An anti-phase boundary between two ordered regions was often observed, indicating a change in the sequence of the (111) planes of Mg and Nb cations. It was also found that the boundary between ordered and disordered regions was coherent. In the current study, detailed ordered structures of Pb(Mg1/3Nb2/3)O3 will be systematically discussed, depending on the occupancy of the cations on the B-site sublattice, long-range order parameter, microscope imaging conditions, etc.