Phase-field simulation (PFS) have revolutionized the understanding of domainstructure and switching behavior in ferroelectric thin films and ceramics.Generally, PFS is based on solution of a (set) of Ginzburg-Landau equations fora defined order parameter field(s) under physical boundary conditions (BCs) offixed potential or charge. While well-matched to the interfaces in bulkmaterials and devices, these BCs are generally not applicable to freeferroelectric surfaces. Here, we developed a self-consistent phase-field modelwith boundary conditions based on electrochemical equilibria. We chosePb(Zr0.2Ti0.8)O3 ultrathin film consisting of (001) oriented single tetragonaldomain (Pz) as a model system, and systematically studied the effects of oxygenpartial pressure, temperature and surface ion on the ferroelectric state, andcompared it with the case of complete screening. We have further explored thepolarization switching induced by the oxygen partial pressure and observedpronounced size effect induced by chemical screening. Our work thus helps tounderstand the emergent phenomena in ferroelectric thin films brought about bythe electrochemical ionic surface compensations.
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