Origin of a Tetragonal BiFeO3 Phase with a Giant c/a Ratio on SrTiO3 Substrates

A tetragonal BiFeO3 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 BiFeO3 phase was obtained in films deposited at high growth rate on SrTiO3 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‐Bi2O3 phase is identified as the origin inducing the formation of super‐tetragonal BiFeO3 phase on SrTiO3 substrates. In addition, ab initio calculations also confirm that this super‐tetragonal phase is more stable than monoclinic phase when Bi2O3 is present. Using Bi2O3 as a buffer layer, an alternative route, not involving strain engineering, is proposed to stabilize this promising super‐tetragonal BiFeO3 phase at low growth rates.