This book systematizes data on the heterophase states and their evolution in perovskite-type ferroelectric solid solutions. It also provides a general interpretation of heterophase and domain structures on changing temperature, composition or electric field, as well as the complete analysis of interconnections domain structures, unit-cell parameters changes, heterophase structures and stress relief. The description of numerous examples of heterophase states in lead-free ferroelectric solid solutions is also included. Domain state–interface diagrams contribute to the interpretation of heterophase states in perovskite-type ferroelectric solid solutions and describe the stress relief in the presence of polydomain phases, the behavior of unit-cell parameters of coexisting phases, the effect of external electric field etc. This 2 edition generalizes the results on the heterophase ferroelectric solid solutions and the stress relief and presents new results on heterophase/domain structures and phase contents in lead-free ferroelectric solid solutions.