Superlattice reflections in hot-pressed Pb(Mg1/3Nb2/3)O3 ceramics

Pb(Mg1/3Nb2/3)O3 (PMN) ceramics have been hot-pressed and post-hot-pressing annealed at 1000 °C in a conventional tube furnace under 1 atm of oxygen and argon, respectively; their microstructures investigated via transmission electron microscopy (TEM) and crystalline phases analyzed via X-ray diffractometry (XRD). Three types of the superlattice reflections from the fundamental spots of the perovskite structure were detected in both the as-hot-pressed and annealed samples by selected-area electron diffraction (SADP). The {1/2 1/2 1/2}-type of the superlattice F-spots is observed in as-hot-pressed samples. The nanoscale ordered domains of approximately 5 nm size giving rise to F-spots are observed by dark field imaging. α-Spots of the {1/2 1/2 0}-type have appeared in post-hot-pressing-annealed samples after heat-treatment at 1000 °C in an oxygen as well as argon atmosphere. Superlattice reflections of the {1/2 1/2 1}-type, termed α′-spots, are also observed from hot-pressed samples annealed at the same temperature for 500 h in an oxygen atmosphere. The occurrence of α-spots associated with lead vacancies (VPb″) is discussed by considering that Pb(Mg1/3Nb2/3)O3 grains is consisted of the acceptor-type nanoscale ordered domains in a donor-type disordered matrix.