Ultrahigh electromechanical response from competing ferroic orders

Abstract

Materials with electromechanical coupling are essential for transducers and acoustic devices as reversible converters between mechanical and electrical energy^1-6. High electromechanical responses are typically found in materials with strong structural instabilities, conventionally achieved by two strategies-morphotropic phase boundaries⁷ and nanoscale structural heterogeneity⁸. Here we demonstrate a different strategy to accomplish ultrahigh electromechanical response by inducing extreme structural instability from competing antiferroelectric and ferroelectric orders. Guided by the phase diagram and theoretical calculations, we designed the coexistence of antiferroelectric orthorhombic and ferroelectric rhombohedral phases in sodium niobate thin films. These films show effective piezoelectric coefficients above 5,000 pm V^-1 because of electric-field-induced antiferroelectric-ferroelectric phase transitions. Our results provide a general approach to design and exploit antiferroelectric materials for electromechanical devices.

Fig. 1. Strategies to enhance the electromechanical response in thin films.

a, Effective piezoelectric coefficients $d_{33,f}^{*}$ of representative thin films from each design strategy (Extended Data Table 1). PTO, PbTiO₃; BFO, BiFeO₃; BTO, BaTiO₃; KNN, K_0.5Na_0.5NbO₃; PLZT, (Pb_0.94La_0.04)(Zr_0.6Ti_0.4)O₃; Sm-PMN-PT, Sm-PbMg_1/3Nb_2/3O₃-PbTiO₃ (71/29); PZT, PbZr_0.52Ti_0.48O₃; PZT (001), (001)-oriented PbZr_0.52Ti_0.48O₃; NPR-NNO, NaNbO₃ with nanopillar regions; PF-KNN, (K,Na)NbO₃ with planar faults. AFE, antiferroelectric; and FE, ferroelectric. b, Phase transition of NNO from FE N phase (rhombohedral R3c) to AFE P phase (orthorhombic Pbcm) as temperature increases. FE and AFE phases coexist in bulk NNO between 12 K and 173 K. The purple and blue arrows, respectively, represent the polarization directions of the N and P phases. The top panel shows the schematics of Landau energy versus polarization in FE (left), FE and AFE coexistence (middle) and AFE (right) phases. c, Calculated free energy as a function of lattice constants (in pseudocubic lattice) for N and P phase NNO. Note that the intersection is located at about 3.9 Å, which guides us to choose SrTiO₃ (STO) as the substrate for epitaxial growth. The insets show the schematics of crystal structures and octahedral rotations (Glazer’s notations) of NNO R3c and Pbcm phases. f.u., formula unit.

Fig. 2. Crystal structure and surface morphology of the 200-nm-thick NNO films.

a–c, Synchrotron X-ray based (113) reciprocal space mapping (a), phi scan (b) and characterization of 7/4 diffraction peaks by H/K/L scan (c) of the NNO/Nb-STO (111) heterostructure. The pseudocubic coordinate system is used for convenience. d, Surface morphology of the NNO film using an atomic force microscope. The RMS roughness is about 300 pm. The white dashed lines show the angle between two domains from the top view. The inset shows the fine terrace features. The colour bar represents the height. e, Low-magnification cross-sectional bright-field TEM image (top) and schematic of phase coexistence (bottom) of NNO/Nb-STO (111) heterostructure along the zone axis $\left[1\stackrel{\circledR }{1}0\right]$. The arrows represent the directions of polarization. f,g, High-resolution TEM images of the P phase (f) and N phase (g) along the zone axis $\left[1\stackrel{\circledR }{1}0\right]$. The insets show their respective indexed FFT patterns, demonstrating different crystal symmetries. a.u., arbitrary units. Scale bars, 2 μm (d); 200 nm (d, inset); 100 nm (e); 5 nm (f,g).

Fig. 3. Antiferroelectric and ferroelectric behaviour of the 200-nm-thick NNO films.

a, Electric-field-dependent polarization curves of the NNO film. The inset shows the remnant polarization as a function of the applied electric field. b, Dynamic ferroelectric hysteresis loop and corresponding switching current curve of the NNO film. c, Simulation of the ferroelectric hysteresis loop of FE and AFE coexistence with a ratio of 1:3 based on Landau theory. d–f, PFM images of topography (d), amplitude (e) and phase (f) measured after writing the FE domain. a.u., arbitrary units. Scale bar, 2 μm (d,e,f).

Fig. 4. Electromechanical response of the 200-nm-thick NNO films and simulation of domain dynamics during AFE–FE phase transition.

a, Three-dimensional mapping of surface displacement of the NNO film measured under a.c. drive amplitude of 1 V at 1 kHz. b, Electric-field-dependent effective piezoelectric coefficient and strain of the NNO film at 1 KHz. c,d, Evolution of strain change (c) and domain switching and interphase boundary motion (d) in the NNO film under various electric fields based on the phase-field simulation. The dynamics of dipoles (black arrows in d) responsive to an electric field manifests itself as a motion of FE/AFE interfaces followed by a domain switching (red arrows) and finally a transformation into a pure FE phase.

Extended Data Fig. 1. The role of the interaction between octahedral tilt and B-site ionic displacements in the phase diagram of sodium niobate.

(a) The schematic of perovskite structure and the preferable behaviour of ionic displacements (t > 1) or oxygen octahedral rotation (t < 1) in terms of the tolerance factor t. The tilt will be considered equal when the tilting angle is equal, while the tilt is independent when the tilt angle is different. (b) Phase diagram of sodium niobate as a function of temperature, where D represents the displacement and T represents the tilts. The notation eq and in represent the equal and independent respectively. The independent tilting promotes the stabilization of the antiferroelectric phase. The ionic displacements and octahedral rotations induce the competition of ferroelectric and antiferroelectric orders in sodium niobate.

Extended Data Fig. 2. Structural characterization of the 200 nm-thick NNO films.

(a) Reciprocal space mapping of the NNO film along the (222) and (312) reflections. (b) Structure information of bulk NNO P and N phase^,. (c) Calculated structure parameters of the NNO film based on the RSV method.

Extended Data Fig. 3. Oxygen octahedral rotation behaviour in the 200 nm-thick NNO films.

(a) Half order diffraction characterization of the NNO film. (b) Summarized oxygen octahedral rotation behaviors in the NNO film based on the combination of the half order diffraction peaks.

Extended Data Fig. 4. RSM of quarter order diffractions of the 200 nm-thick NNO films.

(a) HK mapping (left) and ($\left[1\stackrel{\circledR }{1}0\right]$)L mapping (right) around (111) diffraction peaks of substrate, showing quarter-order diffraction peaks along all H, K, L directions. (b) A three-fold symmetry in RSM of (1 1 3/4), (1 3/4 1) and (3/4 1 1) diffractions as a result of the arrangement of the P phase domain along H, K, and L directions.

Extended Data Fig. 5. TEM images and SAED patterns of the 200 nm-thick NNO films.

(a) Low magnification cross-sectional TEM images were taken along ZA $\left[\stackrel{\circledR }{1}\stackrel{\circledR }{1}2\right]$. (b) N phase and P phase boundary image. The P phase shows clear dark stripes that are perpendicular to the [001] direction compared to the N phase. These regular stripes indicate the AFE modulation of the NNO P phase. Indexed selected area electron diffraction (SAED) patterns taken along (c) ZA $\left[1\stackrel{\circledR }{1}0\right]$ and (d) ZA $\left[\stackrel{\circledR }{1}\stackrel{\circledR }{1}2\right]$. Only one set of main diffraction spots was demonstrated, suggesting that the N and P phases have very similar symmetry and lattice. The superlattice indices including half and quarter reflections indicate that there may be some modulation like oxygen octahedral rotation or atomic anti-ferroelectric arrangement in the NNO film. (e) Extracted quarter diffraction peaks from the SAED pattern. (f) Intensity profile image of figure e.

Extended Data Fig. 6. HR-TEM images and diffractograms from the 200 nm-thick NNO films.

(a, b) HR-TEM images of (a) P phase and (b) N phase taken along ZA$\left[1\stackrel{\circledR }{1}0\right]$. (c, d) HR-TEM images of (c) P phase and (d) N phase taken along ZA$\left[\stackrel{\circledR }{1}\stackrel{\circledR }{1}2\right]$ and (e, f) their corresponding indexed FFT patterns. The red square shows the respective enlargement images of P and N phases.

Extended Data Fig. 7. Schematics of domain arrangement of the N and P phase in the 200 nm-thick NNO films.

(a) Schematic drawing of the domain structures of twin 1 in the NNO film. The AFE phases are separated by the FE phase and the FE phase has polarization along the out-of-the-plane direction (up or down), while the AFE phase shows pairs of spontaneous polarizations with an inclined polarization direction. (b) Schematic drawing of three-fold symmetric domain arrangement from the [111] projection view. The P phase are formed in the directions of [001], [010], and [100], resulting in three-fold symmetry, agreeing with the symmetry modulation from the (111) oriented substrate.

Extended Data Fig. 8. Ferroelectric characterization of the 200 nm-thick NNO films.

(a-c) P-E loops of (a) 125–500 kV/cm, (b) 625–1125 kV/cm, (c) 1250–1750 kV/cm, and (d-f) corresponding switching current curves: (d) 125–500 kV/cm, (e) 625–1125 kV/cm, (f) 1250–1750 kV/cm.

Extended Data Fig. 9. Electromechanical response of the 200 nm-thick NNO films.

(a-f) External electric field-dependent surface displacement measured by laser Doppler vibrometer with an a.c. voltage of 1 V at a frequency of 1 kHz. (g) Inducing external electric field by applying d.c. voltage during laser Doppler vibrometer test (only show 10 ms data).