Self‐Regulated Chemical Substitution in a Highly Strained Perovskite Oxide

Abstract Deterministic control of substitution sites is the holy grail of chemical engineering in materials science. The substitution of the dopants in the host crystal lattice usually obeys the empirical Hume‐Rothery rules, in which the mismatch of atomic size must be small and thus site‐specific. Herein, we report an unconventional case of mixed‐site substitution in highly‐strained BiFeO 3 thin films. Driven by the delicate balance between chemical strain and epitaxial strain energies, the solid‐solution system demonstrates a self‐regulation behavior in dopant site occupancy to minimize the total free energy. This proposed mechanism offers valuable insight into the chemical substitution pathway in epitaxial thin films and provides a potential route to selectively control the substitution sites.