Understanding the structural mechanisms in functional ceramics is key for developing new functionalities and sustainable materials. However, especially in complex materials the local structural responses in bulk ceramics are not accessible with experimental methods. With advanced phase-field simulations this gap can be bridged to combine simulations and experiments for a broader understanding of the general mechanisms of ferroelectric ceramics. A novel multiphase-field approach is introduced to analyse ferroelectrics with phase coex-istence and electric field induced phase transformation in multigrain systems [1]. The new approach allowed to prove the hypothesis that grain orientation determines the induced phase in lead zirconate titanate (PZT) at the morphotropic phase boundary (MPB). The phase-field simulations will be presented in the context of previous in situ investigations on the structural mechanisms during the application of electric fields [2]. With our initial model we aimed to study the microstructural evolution when multiple fer-roelectric phases coexist. This model was modified, so that the polarization states of each ferroelectric phase variant are predefined. In this case, the domain structure is obtained by evolving only the multiphase-field order parameter φ, which represents these predeter-mined polarization variants. This efficiently allowed 3D simulations and simulations of realistic microstructures, based on the model system barium titanate (BT) [3]. With simu-lation of complex microstructures, we could show that during the response to an applied electric field grains and their orientation influence each other. Stress localizes not only along ferroelastic domain walls within grains but also at grain boundaries, with particularly strong concentrations at boundary junctions. With a modified version we were able to investigate the growth of lead titanate films on a potassium titanate substrate and could analyze how substrate misfit strain and short-circuited boundary conditions control domain evolution in thin films [4]. By varying the strain misfit between substrate and film we could change the resulting domain configura-tions. This allows predictions on the formation of domain morphologies depending on the misfit strain. REFERENCES [1] L. Fan, W. Werner, S. Subotić, D. Schneider, M. Hinterstein, and B. Nestler, Multigrain phase-field simulation in ferroelectrics with phase coexistences: An improved phase-field model, Comput. Mater. S