Nanoscale global mixed ordering of B-site cations in Pb2M′M"O6 complex perovskite relaxors

We have observed compositionally heterogeneous, uniformly distributed and closely interconnected nanoscale and sub-nanoscale domains in complex perovskite-structured relaxors. These domains are interpreted as originating from global mixed ordering, formed by numerous non-stoichiometric combinations of B-site cations across the entire bulk of the relaxor crystals and ceramics. The association between global mixed ordering and the origin of the forbidden reflections has been explained theoretically and experimentally. Various models of global mixed order-disorder are proposed, simulated and subsequently compared with dark-field and high-resolution TEM images, showing unambiguous agreement between the models and the observed TEM images and the diffraction patterns. Careful and extensive case studies of Pb-2(ScTa)O-6 crystals and (0.4)Pb-2(InNb)O-6:(0.6)Pb(Mg1/3Nb2/3)O-3 ceramics show that mixed order - disorder originates from a global B-site chemical heterogeneity. The findings are analyzed using a normal statistical distribution approach, which is supported by the length-scale of the experimentally observed domains.