Oxygen vacancy order-disorder transition at high temperature in Bi-Sr-Fe-based perovskite-type oxides

${\mathrm{Bi}}_{1\ensuremath{-}\mathit{x}}{\mathrm{Sr}}_{\mathit{x}}{\mathrm{FeO}}_{3\ensuremath{-}\ensuremath{\delta}}$ (BSFs) are well known not only as multiferroic materials but also as mixed oxide-ion and electronic conductors. BSFs show remarkable ionic conductivity despite their limited electrochemical activity due to their low electronic conductivity. In this study, the discontinuous change in the electrical conductivity of ${\mathrm{Bi}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{FeO}}_{3\ensuremath{-}\ensuremath{\delta}}$ (BSF30) at around 770 ${}^{\ensuremath{\circ}}\mathrm{C}$ was investigated in the context of their unique defect order-disorder transition. Based on the electrical conductivity and the Seebeck coefficient of BSF30, both functions of temperature and oxygen partial pressure, the discontinuous change in both hole and oxide-ion conductivities was the result of a reduction in their mobility but unrelated to their concentration term. High-temperature x-ray diffraction and scanning transmission electron microscopy revealed that periodic oxygen-deficient planes with fivefold $d$-spacing of ${001}$ emerge in an ordered low-temperature phase while the ordered defect structure disappears in a high-temperature phase. The discontinuous change of electrical conductivity of BSF30 was then found to be the result of the order-disorder transition of its oxygen sublattice, i.e., oxygen vacancies. Because there was no such transition in the cation sublattice, the cation did not affect the electrical conductivity of BSF30. This study suggests that BSFs with the desired electrochemical and multiferroic functionalities can be designed by controlling their order-disorder transition.