Quantitative Control of Oxygen Non‐Stoichiometry and Negative Raman Chemical Shift During Topotactic Phase Transition in a Ca‐Doped BiFeO ₃ Thin‐Film

Controlling the non-stoichiometry is an effective way to tune physicochemical properties of functional oxides and explore novel physical phenomena in complex oxides. Therefore, quantitative control of oxygen non-stoichiometry in perovskite oxides plays an important role in understanding the mechanism of topotactic phase transition and improving the applicability of electrochemical devices. Here, an electrochemical titration cell is fabricated to control the oxygen non-stoichiometry of a Bi_0.4Ca_0.6FeO_3- _δ thin film grown on yttria-stabilized zirconia substrate. The amount of oxygen pumped into the thin film depending on the applied voltage is quantitatively determined through chronoamperometry measurements. Non-monotonic changes in lattice parameters are observed using X-ray diffraction. Considerable negative Raman chemical shifts have occurred even in the range of small applied voltages that result in little changes in the non-stoichiometry or average lattice parameters. These observations on chemo-mechanical coupling can be interpreted as characteristics of vacancy-ordered perovskite oxides. This work offers useful insights into the mechanism of topotactic phase transition in perovskite oxides.