Origin of a Tetragonal BiFeO 3 Phase with a Giant c / a Ratio on SrTiO 3 Substrates

A tetragonal BiFeO 3 phase with giant c / a of approximately 1.25 has been of great interest recently as it potentially possesses a giant polarization and much enhanced electromechanical response. This super‐tetragonal phase is known to be a stable phase only under high compressive strains of above approximately 4.5%, according to first principle calculations. However, in previous work, this super‐tetragonal BiFeO 3 phase was obtained in films deposited at high growth rate on SrTiO 3 substrates with compressive strain of only around 1.5%. By detailed structure analysis using high resolution synchrotron X‐ray diffraction, atomic force microscopy, and transmission electron microscopy, the parasitic β‐Bi 2 O 3 phase is identified as the origin inducing the formation of super‐tetragonal BiFeO 3 phase on SrTiO 3 substrates. In addition, ab initio calculations also confirm that this super‐tetragonal phase is more stable than monoclinic phase when Bi 2 O 3 is present. Using Bi 2 O 3 as a buffer layer, an alternative route, not involving strain engineering, is proposed to stabilize this promising super‐tetragonal BiFeO 3 phase at low growth rates.