Ordered oxygen vacancies in tetragonal BiFeO3

Bismuth ferrite (BiFeO 3 , BFO) is a prototypical multiferroic material, whose crystal structures and physical properties are sensitively dependent on the strain state, forming an exotic super tetragonal phase with compressive strain. Although the compressive strain is associated with dramatically reduced oxygen vacancy formation energy, it is intuitively considered that the crystal structures of BFO are all stoichiometric. However, through multislice electron ptychography with adaptive propagator, we found that the tetragonal phase of bismuth ferrite has approximately 1/4 oxygen vacancies at the apical oxygen positions, challenging the conventional wisdom of stoichiometric BFO. Since oxygen vacancies significantly influence the physical properties of transition metal oxides, this finding offers new insights into the physicochemical behavior of bismuth ferrite and opens opportunities for the development of novel functionalities.