Anisotropic epitaxial stabilization of a low-symmetry ferroelectric with enhanced electromechanical response

Abstract

Piezoelectrics interconvert mechanical energy and electric charge and are widely used in actuators and sensors. The best performing materials are ferroelectrics at a morphotropic phase boundary (MPB), where several phases can intimately coexist. Switching between these phases by electric field produces a large electromechanical response. In the ferroelectric BiFeO3, strain can be used to create an MPB-like phase mixture and thus to generate large electric field dependent strains. However, this enhanced response occurs at localized, randomly positioned regions of the film, which potentially complicates nanodevice design. Here, we use epitaxial strain and orientation engineering in tandem -anisotropic epitaxy -to craft a hitherto unavailable low-symmetry phase of BiFeO3 which acts as a structural bridge between the rhombohedral-like and tetragonal-like polymorphs. Interferometric displacement sensor measurements and first-principle calculations reveal that under external electric bias, this phase undergoes a transition to the tetragonal-like polymorph, generating a piezoelectric response enhanced by over 200%, and associated giant field-induced reversible strain. These results offer a new route to engineer giant electromechanical properties in thin films, with broader perspectives for other functional oxide systems.

Ultrahigh Ferroelectric and Piezoelectric Properties in BiFeO3–BaTiO3 Epitaxial Films Near Morphotropic Phase Boundary

“…Given the fact that the spontaneous polarization is along the (001) direction for the T-type phase but in the (111) direction for the R-type phase, the polarization in the measured out-of-plane direction is much superior in the tetragonal-structured film. 35 This is revealed experimentally with TEM, the displacements of Fe ions are enhanced in the T-phase compared to that in the R-phase. 36 Thus, the BF−BT film prepared under PO 2 of 8 Pa has a huge c/a ratio of 1.09 with enhanced ferroelectricity in the out-ofplane direction.…”

“…It can be clearly seen that as the proportion of the tetragonal phase increases, the ferroelectric properties of the film are enhanced, manifested by a higher remanent polarization. Given the fact that the spontaneous polarization is along the (001) direction for the T-type phase but in the (111) direction for the R-type phase, the polarization in the measured out-of-plane direction is much superior in the tetragonal-structured film . This is revealed experimentally with TEM, the displacements of Fe ions are enhanced in the T-phase compared to that in the R-phase .…”

“…All of the samples show good linearity with a fixed slope in the electric field range of 0–1.3 MV/cm. According to the reported literature, if there exists a phase change induced by the electric field, the slope increases sharply at the corresponding electric field . Thus, the contribution of phase change to the large strain of three samples can be excluded.…”

“…According to the reported literature, if there exists a phase change induced by the electric field, the slope increases sharply at the corresponding electric field. 57 Thus, the contribution of phase change to the large strain of three samples can be excluded. The mechanism of the large strain is further judged by analyzing its hysteresis characteristics.…”

Epitaxial stabilization of an orthorhombic Mg-Ti-O superconductor

“…Nevertheless, extensive study on TOS is hampered by the lack of high-qualified single crystal sample owning to the thermodynamic or chemical instability of their crystal lattice [28,37]. Fortunately, many metastable or unstable phases can be stabilized in the form of single-crystalline films by the means of epitaxial stabilization [38][39][40][41][42]. Exotic behaviors comparable to high-temperature superconductors have been exhibited in the films of TOS [16-20, 29, 30].…”