Direct observation of the recovery of an antiferroelectric phase during polarization reversal of an induced ferroelectric phase

Electric fields are generally known to favor the ferroelectric polar state over the antiferroelectric nonpolar state for their Coulomb interactions with dipoles in the crystal. In this paper, we directly image an electricfieldassisted ferroelectric-to-antiferroelectric phase transition during polarization reversal of the ferroelectric phase in polycrystalline Pb0.99{Nb0.02[(Zr0.57Sn0.43)0.92Ti0.08]0.98}O3.With the electric-field in situ transmission electron microscopy technique, such an unlikely phenomenon is verified to occur by both domain morphology change and electron-diffraction analysis. The slower kinetics of the phase transition, compared with ferroelectric polarization reversal, is suggested to contribute to this unusual behavior. Disciplines Ceramic Materials | Other Materials Science and Engineering | Semiconductor and Optical Materials Comments This article is from Physical Review B 91 (2015): 1, doi:10.1103/PhysRevB.91.144104. Posted with permission. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/mse_pubs/201 PHYSICAL REVIEW B 91, 144104 (2015) Direct observation of the recovery of an antiferroelectric phase during polarization reversal of an induced ferroelectric phase Hanzheng Guo and Xiaoli Tan* Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA (Received 12 January 2015; revised manuscript received 30 March 2015; published 10 April 2015) Electric fields are generally known to favor the ferroelectric polar state over the antiferroelectric nonpolar state for their Coulomb interactions with dipoles in the crystal. In this paper, we directly image an electric-fieldassisted ferroelectric-to-antiferroelectric phase transition during polarization reversal of the ferroelectric phase in polycrystalline Pb0.99{Nb0.02[(Zr0.57Sn0.43)0.92Ti0.08]0.98}O3. With the electric-field in situ transmission electron microscopy technique, such an unlikely phenomenon is verified to occur by both domain morphology change and electron-diffraction analysis. The slower kinetics of the phase transition, compared with ferroelectric polarization reversal, is suggested to contribute to this unusual behavior. DOI: 10.1103/PhysRevB.91.144104 PACS number(s): 77.80.−e, 81.30.Hd, 61.05.jm, 77.84.Cg It is generally believed that electric dipoles will be aligned to the applied electric-field direction either in a liquid or a gaseous phase. A similar phenomenon has been observed in the ferroelectric (FE) and antiferroelectric (AFE) solid-state crystals where electric dipoles are parallel to each other in a FE phase and antiparallel in an AFE phase [1,2]. It has been experimentally shown that an electric field favors the FE phase over the AFE phase by forcing the antiparallel dipoles being switched to along the external field direction, leading to a first-order AFE-to-FE phase transition [3,4]. In the most studied AFE compositions that are chemically modified from the prototype PbZrO3, the electric-field-triggered AFE-to-FE phase transition is generally manifested by the development of a large polarization as well as a significant volume expansion when the applied field reaches a critical value [4–6]. Microscopically, the nanoscale feature of incommensurate modulations in the AFE state transforms into large ferroelectric domains, accompanied with the disappearance of the characteristic satellite diffraction spots [7,8]. Other external stimuli, such as mechanical stresses, are also known to influence or even trigger the transition between the AFE and the FE phases [9–14]. It has been generalized that a symmetric external stimulus (i.e., hydrostatic pressure) stabilizes the AFE phase, whereas an asymmetric stimulus (i.e., electric field) only favors the FE phase in Snand Timodified PbZrO3-based ceramics [15]. However, our recent investigation based on in situ x-ray-diffraction and macroscopic strain measurements has unambiguously demonstrated that it is possible for an electric field to induce an AFE phase out of a FE phase by appropriately choosing the chemical composition and manipulating the electric-field application process [6]. Such a FE-to-AFE phase transition is manifested by the volume stain reduction as well as the reappearance of AFE x-ray-diffraction peaks when the polarity of the applied field is reversed [6]. Further verification of such an unusual phenomenon has been revealed in a NaNbO3-based lead-free ceramic with a finely tuned composition at the AFE/FE phase boundary where a comparable free-energy profile of both phases is present, presumably the prerequisite condition for the recovery of the AFE phase upon electric reversal [16]. *Corresponding author: [email protected] In the present paper, the unlikely electric field-assisted FE-to-AFE transition is directly visualized via microstructure imaging. A PbZrO3-based ceramic is employed for the demonstration due to its finely tuned composition located at the AFE/FE phase boundary as well as the distinct microstructural features between the AFE and the FE states. Employing the electric-field in situ transmission electron microscopy (TEM) technique [17–21], the domain morphologies and their corresponding electron-diffraction patterns are simultaneously monitored during the electric loading and reversal process. The polycrystalline Pb0.99{Nb0.02[(Zr0.57Sn0.43)0.92 Ti0.08]0.98}O3 (PNZST 43/8.0/2) ceramic was synthesized using the solid-state reaction method. Details can be found in our previous report [22]. X-ray diffraction was used to ensure phase purity of the sintered pellet. Dielectric properties were measured at 1 kHz with an LCZ meter (Keithley 3322) during heating and cooling at a rate of 3 °C/min. The first two cycles of polarization vs electric-field hysteresis loops were recorded with a standardized ferroelectric test system (RT66A, Radiant Technologies) at room temperature and 4 Hz. For the electric-field in situ TEM experiments, disk specimens (3 mm in diameter) were prepared from as-processed pellets through standard procedures including grinding, cutting, dimpling, and ion milling. The dimpled disks were annealed at 200 °C for 2 h to minimize the residual stresses before Ar-ion milling to the point of electron transparency. In situ TEM experiments were carried out on a Phillips CM30 microscope operated at 200 kV. Experimental details are similar to those reported in Refs. [17–21]. The PNZST 43/8.0/2 ceramic is found to be at the AFE/FE phase boundary at room temperature [23,24]. Due to the thermal hysteresis of the first-order AFE-FE phase transition, either the AFE or the FE phase can be dominant in the ceramic at room temperature depending on thermal history [22]. This is experimentally verified by the dielectric properties measured during heating and successive cooling, shown in Fig. 1(a). The anomalies at ∼43 °C during heating and at ∼−8 °C during cooling define the thermal hysteresis of the first-order AFE-FE transition [22]. The result indicates that at room temperature (25 °C), the ceramic is in the AFE state when cooled from sintering or annealing and is in the FE state when warmed up from low temperatures (−100 °C). The AFE and FE phases are hence energetically comparable at room temperature in 1098-0121/2015/91(14)/144104(6) 144104-1 ©2015 American Physical Society HANZHENG GUO AND XIAOLI TAN PHYSICAL REVIEW B 91, 144104 (2015) FIG. 1. (Color online) (a) Dielectric properties of a bulk polycrystalline PNZST 43/8.0/2 specimen measured at 1 kHz and 3 °C/min during heating and cooling. The anomalies at −8 °C and 43 °C are indicated by the dashed vertical lines. (b) The first two cycles of the polarization (P ) vs electric field (E) hysteresis loops from a bulk polycrystalline PNZST 43/8.0/2 specimen measured at room temperature and 4 Hz. The data points in red are from the first quarter cycle of the applied field. this composition, which is believed to be essential for the electric-field-assisted FE-to-AFE phase transition to occur [6,16]. The behavior shown in Fig. 1(a) for the PNZST 43/8.0/2 ceramic is verified by an irreversible AFE-to-FE phase transition at room temperature when the electric field is applied to the virgin AFE ceramic [6,24]. The induced FE phase remains upon removal of the applied field as shown in Fig. 1(b). The critical electric field that triggers the AFE-to-FE phase transition is determined to be 1.2 kV/mm during the first quarter cycle of the electric field (highlighted in red). The large polarization developed in the induced ferroelectric phase is largely preserved in the second quarter cycle when the applied field is unloaded. A normal ferroelectriclike behavior is observed during the subsequent electric loading, manifested by the square-shaped loop with a large remanent polarization of 0.29 C/m2 at a frequency of 4 Hz. It should be noted that FIG. 2. Typical microstructures of the polycrystalline PNZST 43/8.0/2 recorded under the [001] zone axis. (a) TEM bright field micrograph of a grain of the AFE phase, (b) its corresponding electron-diffraction pattern, (c) a grain with mixed FE and AFE phases, and (d) a close examination of the AFE/FE interface. the polarization loop remains square shaped without apparent pinches even at a frequency as low as 10 mHz at room temperature. Microstructure analysis of the PNZST 43/8.0/2 ceramic supports the macroscopic property measurements depicted in Fig. 1. In the virgin state of PNZST 43/8.0/2, TEM examination reveals that most grains are predominantly in the AFE phase [Fig. 2(a)]. The characteristic incommensurate modulation is seen to occupy most of the grain. The corresponding satellite diffraction spots, aligning along the 〈110〉 direction [7,8], are present in the [001] zone-axis selected area electron-diffraction pattern [Fig. 2(b)]. A few large grains in the TEM specimen display mixed AFE and FE phases and grains occupied primarily by the FE phase are occasionally noticed. Figure 2(c) shows such a large grain with the coexisting AFE (in the circled region) and FE (the rest part of the grain) phases. In contrast to the AFE phase, the FE phase