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 beta-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.