Phase Diagram and Phase Transitions in the Relaxor Ferroelectric Pb(Fe2/3W1/3)O3–PbTiO3 System

Microstructures of three Bi–W–Nb–O phases have been examined by using highresolution transmission electron microscopy. Bi17W2Nb3O39 and Bi17WNb3O36 have incommensurate superstructures derived from the defect fluorite-type delta-Bi2O3 and can be regarded as intermediate phases between the type II solid solutions in the Bi–Nb–O and Bi–W–O systems. Bi8W2Nb2O23 has a Bi2WO6-like subunit cell with a stepped superstructure. Formation mechanisms of various superstructures are discussed.