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Investigation of switching behavior of acceptor-doped ferroelectric ceramics
Acta Materialia 170 (2019) 100e108
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Investigation of switching behavior of acceptor-doped ferroelectric ceramics Chenxi Wang a, b, Xiaoming Yang a, b, Zujian Wang a, Chao He a, *, Xifa Long a a
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China b University of Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 January 2019 Received in revised form 11 March 2019 Accepted 23 March 2019 Available online 26 March 2019
Switching behavior is a general feature in ferroelectrics. The related fatigue effects influenced by defect dipoles in ferroelectrics are still controversial that is focused on the positive and negative effects of oxygen vacancies. Here, we report the polarization switching behavior of acceptor-doped ceramics using the first-order reversal curve (FORC) approach, especially for the abnormal self-rejuvenation effect and the enhanced fatigue endurance in acceptor-doped ceramics. The reversible and irreversible components under electric field in the ceramics were distinguished by the FORC distribution of ideal “hysteron”. The abnormal self-rejuvenation behavior stemmed from dispersive response of hysteron for undoped samples while from the redistribution of defect dipoles for acceptor-doped samples. The self-rejuvenation was induced mainly by the irreversible component. For the fatigue effect, the pinning of domain walls was not the main reason. The re-annealing treatment for a fatigued sample weakened the interactions between the spontaneous polarizations and defect dipoles, but enhanced the dispersion of coercive field. The enhancement of fatigue endurance came from the phase stability of structure in acceptor-doped ceramics, while complex phase evolution existed in undoped ceramic with weak fatigue endurance. Our study shed new light on the interactions between spontaneous polarization and defect dipoles under repetitive AC electric field. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Ferroelectric FORC Self-rejuvenation Fatigue Defect dipole
1. Introduction Polarization switching is a general feature in ferroelectrics. It has been applied to describe the functional features of many devices, like random access memory and data storage [1]. Polarization switching is directly related to many important physical characteristics like hysteretic behavior, field-induced strain and dielectric permittivity, and underpins the application of ferroelectric devices [1e6]. The behavior of polarization switching can be affected by many factors, like temperature [7], light [8], point defects [9], and external electric field [10]. Studying the dynamics of polarization switching under various conditions helps to understand the mechanisms of polarization switching and enhance the reliability and life time of devices. The electric fatigue effect in ferroelectrics is the main hindrances to application and characterized by the detriment of
* Corresponding author. E-mail address: [email protected] (C. He). https://doi.org/10.1016/j.actamat.2019.03.033 1359-6454/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
electrical properties. For example, remnant polarization (Pr) and piezoelectric coefficients (d33) would decrease due to repetitive polarization switching under AC electric field [5,11], which is very harmful in applications, like reading or writing errors in memory devices [12]. It has been proved that fatigue effect is very sensitive to doping [10,13,14]. But it is still controversial whether oxygen vacancies in ferroelectrics have positive or negative effects on fatigue behavior [15e17]. The role of defect dipoles is complicated in fatigue process. Abnormal self-rejuvenation (revival of polarizations during fatigue process) and de-aging behavior related to the defect dipoles have been found in ferroelectric materials with abundant oxygen vacancies, like Pb-based ferroelectric materials [5,18,19] and hafnium oxide-based ferroelectric materials [1,20]. The total piezoelectric and dielectric response of ferroelectrics comprises of reversible and irreversible components that contribute differently to the material [21]. To be able to confirm the effect of defect dipoles, the reversible and irreversible components under electric field in ferroelectrics must be taken into account because they are notably influenced by defect dipoles. For example, both weakening and strengthening of the reversible components
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have been found in different PZT films [22,23]. While the reversible component didn't change during fatigue in PZT ceramics [24]. It's still desired to study the mechanisms behind such controversial behavior in the fatigue process [3,25,26]. The first-order reversal curve (FORC) approach, which was developed based on Preisach model [27], has turned out to be a powerful tool in characterizing hysteretic systems. The common polarization-electric field (P-E) loops are a statistical result of the polarization switching. The FORC is the deconvolution of the statistical result, revealing the information of polarization switching at local level, and is very sensitive to the domain dynamics. The FORC distribution diagrams provide information on the microscopic mechanisms of the domain switching [28]. This method characterizing hysteretic phenomena is universal and has been applied to various kinds of materials including spin transition materials [29], magnetic materials [30], ferromagnetic materials [31], ferroelectric materials [1,32e34] and superconductors [35]. Here, we report the switching behavior of acceptor-doped ceramics using FORC approach. As mentioned above, switching dynamics and fatigue are notably influenced by defect dipoles. As acceptor-doped ferroelectric ceramics with abundant defect dipoles, Sn-doped Pb(Lu1/2Nb1/2)O3ePbTiO3 (PLN-PT) ceramics exhibit abnormal self-rejuvenation effect and strong fatigue endurance [36]. The influence of the reversible and irreversible domain evolution on the self-rejuvenation effect in ferroelectric materials is still lacking in study, and the difference of structure evolution in the fatigue process needs to be further verified. Therefore, self-rejuvenation effect in the fatigue process and structure evolution was studied in acceptor-doped ferroelectric ceramics using the FORC approach in this work. This work may present a new idea for the study of the interaction between spontaneous polarization and defect dipoles, and switching behavior in acceptor-doped ferroelectric ceramics.
2. Experimental section 2.1. Measurements Acceptor-doped ferroelectric ceramics (0.51 Pb(Lu1/2Nb1/2)O3 0.49PbTi1-xSnxO3 (x ¼ 0e0.12)) were prepared as previously reported [36]. The green samples were annealed at 500 C (3 h), which was well above the Curie temperature. The composition x ¼ 0.1 was chosen to compare with x ¼ 0 for fatigue investigations
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because it showed the best fatigue endurance and strongest selfrejuvenation [36]. The samples for electrical measurements were pasted with high-purity silver paint (YFS-05002, SPI, England) as the electrodes. The electric field was set up to two times of the coercive field with 100 Hz for fatigue measurements. The crystalline phase of the samples at different fatigue stages was analyzed at room temperature with X-ray diffractormeter (XRD) (Miniflex 600, Rikagu, Japan) from 2q ¼ 10 to 80 with a speed of 2 deg/min and a step of 2q ¼ 0.02. The XRD was measured on the surface of the sample immediately after the fatigue treatment and removing the electrode. The electrode can be totally removed by submerging the sample in acetone without damaging the surfaces.
2.2. FORC approach FORC was measured using a standard ferroelectric system (aixACCT TF2000 Analyzer) with modulated triangle waves of electric field as shown in Fig. 1a. The maximum field (Em) was set to about 3 times of the coercive field (60 kV/cm in this work) to positively pole the sample. The reversal resided on the descending branch of the main hysteresis loop (MHL) depicted as dashed curve in the inset of Fig. 1a. The MHL results from the statistical superposition of local responses of polarization switching. The FORC measured with this specific waveform is shown in the inset of Fig. 1a. The Preisach density can be obtained from FORC using equation (1):
r FORC ðE; Er Þ ¼
1 v2 P FORC ðE; Er Þ 2 vEr vE
(1)
where P FORC ðE; Er Þ is the polarization of FORC. The minus sign indicates that the FORC started on the descending branch of the MHL. The Preisach density represents the distribution of density of ideal “hysteron” (switching units distributed statistically) and also gives information about local switching behavior. The FORC distribution diagram of Preisach density shown in Fig. 1b which describes the distribution of “hysteron” about the real electric field E and the reversal electric field Er, exhibiting a contour plot with E and Er as horizontal and vertical axes in the diagram. Every FORC is obtained in the range of E Er and every point of the FORC distribution diagram is assumed to be associated with the measured material. As a result, only the grey region in Fig. 1b has true physical meaning. The FORC distribution can also be interpreted in terms of ideal coercive
Fig. 1. (a) Wave form of the electric field for FORC measurement, the inset shows the measured FORC; (b) the sketch of the FORC distribution diagram.
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field E’C and internal field E'i via equations (2) and (3):
E0 C ¼ ðE Er Þ=2
(2)
E0 i ¼ ðE þ Er Þ=2
(3)
As axes to depict the FORC distribution, E’C and E'i are distinguished from EC and Ei in the MHL. Specifically, E’C and E'i are the coercive and internal fields of the switched hysteron. In the present work, the data analysis and processing were based on the mathematical procedure reported by Pike et.a. [28] using total 200*100 data points for each measurement. And FORC at different fatigue stages was measured. The frequency to measure the FORC was 5 Hz, in accordance with the frequency to measure the MHL. 3. Results 3.1. FORC distribution for different doping levels Fig. 2 shows the FORC distribution diagrams for different Sn doping levels with E and Er as the horizontal and vertical axes in the same color scale. The E’C and E'i axes are the diagonals of the diagrams as shown in Fig. 1b. The MHLs were shown in the upper left of the FORC distribution diagrams. Two regions were detected in the FORC distribution diagrams. One was the region with strip shape along the E'i axis. The other was a peak centered near E'i ¼ 0 along E’C axis with a minor ridge parallel with E'r axis. The ridge represented specific hysterons that can be switched at a relatively low E no matter how strong Er was. Fig. 2 also shows some negative sub-regions in navy blue adjacent to the ridge. The negative subregions are related to the system geometry [37] and finite time dependence of polarization switching [38,39]. The FORC distribution of Sn-PLN-PT resembled that of PZT ferroelectrics with similar shapes and locations of reversible and irreversible components [40]. The reversible (prev) and irreversible (pirre) components can be distinguished using FORC distribution diagrams. The reversible in the FORC distribution diagrams corresponding to the region along E'i axis is defined as equation (4):
1 vP FORC ðE; Er Þ fcfεr ; vEr Er /E 2
prev ðEÞ ¼ lim
(4)
where c is the dielectric susceptibility and is proportional to the dielectric constant εr . The irreversible contribution was located along E’C axis [23]. Fig. 2h shows the reversible contribution prev(E), which decreased and moved to low field with increase of Sn content, indicating weakened reversible contribution. The prev(E) of different doping level converged at high electric field. Like the intrinsic contributions (lattice), the extrinsic contributions exist in both reversible process (domain walls (DWs) bending and DWs planar movements between defects) and irreversible process (DWs move irreversibly across potentials or defects) [41]. The weakening of prev stems from the stabilization effect of defect dipoles on the domains, which will be discussed in section 4. To get an insight of the effect of Sn doping, the ratio (Rirre/re) of the maximum intensity of irreversible component over that of the reversible component at different doping levels was plotted in the inset of Fig. 2h. The reversible contribution was much stronger than irreversible contribution since the ratio Rirre/re was much smaller than 1. Rirre/re showed a decreasing trend with x increasing when the measurement noise and property fluctuation of the samples were ignored, suggesting the increase of the portion of the reversible component. It is concluded that the reversible component contributed more to the total polarization switching response as x increased. To see more details, the FORC distribution diagrams of the
composition x ¼ 0 and x ¼ 0.1 were extracted as shown in Fig. 3, showing the broadened distribution for x ¼ 0.1. To show the dispersion clearly, the white bars represented a decrease of 95% of the maximum of the irreversible component, indicating a dispersion of E'i about 10.7 kV/cm and 16.7 kV/cm for x ¼ 0 and 0.1, respectively. The peak stretched along E’C axis for both samples, showing a dispersion of E’C from about 4 kV/cm to 24 kV/cm (see Fig. 3). The dispersion of E’C is characteristic of normal Pb-based ferroelectrics [23,32,39], which is the result of the dispersive distribution of coercive field in the switching and back-switching processes. Therefore, the dispersion of E’C didn't change while the distribution along E'i axis became largely dispersed after doping. 3.2. FORC distribution of fatigued samples The FORC was measured at different fatigue stages, and the corresponding FORC distribution diagrams were plotted for x ¼ 0 (Fig. 4) and x ¼ 0.1 (Fig. 5). To better understand the fatigue process, four stages were selected: (A), the initial stage of fatigue process (n ¼ 0); (B), the most pronounced self-rejuvenation effect stage (n ¼ 3500 for x ¼ 0 and n ¼ 10000 for x ¼ 0.1); (C), the end stage (n ¼ 3 106 for x ¼ 0 and n ¼ 107 for x ¼ 0.1); (D), the re-anneal stage after fatigue process (500 C for 3 h, which was well above the Curie temperature between 300 and 330 C). Fig. 4 shows the FORC distribution diagrams of x ¼ 0. The irreversible component decreased and the distribution moved to high E’C and finally split up to multi-centers along E’C axis as the fatigue cycles increased (refer to the stage from A to C in Fig. 4). Multiple maxima were detected due to nanoscale inhomogeneity [42] stemming from the fatigue treatment. The reversible component became broad and dispersive as the fatigue cycles increased. The FORC distribution recovered slightly after re-annealing (Fig. 4d) in accordance with the partial recovery of the remnant polarization Pr as shown in the inset of Fig. 4d (P-E hystersis loops). The distribution of the irreversible component became more dispersive along E’C axis and sharper along E'i axis than that of stage C. The irreversible component still showed multi-center distribution at stage D and the maximum moved to lower E’C compared with stage C (41.5 kV/cm for stage C and 27.7 kV/cm for stage D) though the coercive field EC kept unchanged after the re-annealing (about 30 kV/cm) as shown in the insets of Fig. 4c and d. The reversible component was almost unaffected by the re-annealing. The variation of FORC distribution for x ¼ 0.1 at different fatigue stages (Fig. 5) was different from that of x ¼ 0. At stage B, the maximum of irreversible contribution enhanced drastically from 2 2 r FORC ¼ 0:116 mC=kV to rFORC ¼ 0:924 mC=kV (increased by 697%, see the color scale in Fig. 5a and b). The peak became sharper than stage A, indicating stronger self-rejuvenation than x ¼ 0. Like the case of x ¼ 0, the FORC distribution for x ¼ 0.1 exhibited a multicenter distribution of irreversible component with much weaker 2 intensity at stage C than that of stage A (r FORC ¼ 0:0032 mC=kV at stage C) (Fig. 5c), though the remnant polarization Pr decreased only slightly. Dispersive distribution along E’C axis can also be seen at stage C. The intensity of the maximum of the irreversible component was much higher than x ¼ 0 at stage C, indicating weaker inhomogeneity than the case of x ¼ 0. The reversible component became broad and dispersive at stage C (Fig. 5c). After re-annealing, the intensity of the FORC distribution didn't recover as the case of x ¼ 0. The irreversible component moved to low E’C values (maximum moved from E’C ¼ 53.2 kV/cm at stage C to E’C ¼ 28.6 kV/cm at stage D). Unlike x ¼ 0, the coercive field EC extracted from the MHL decreased from 34.8 kV/cm to 27.9 kV/cm for x ¼ 0.1 after re-annealing. To investigate the change of reversible component, the prev of x ¼ 0 and x ¼ 0.1 at different stages were plotted in Fig. 6. For x ¼ 0,
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Fig. 2. (aeg) FORC distribution diagrams for different doping concentration, the bottom right shows the color scale, the inset shows the MHL for each composition; (h) the variation of the reversible distribution prev and the inset shows the ratio of peak value of irreversible contribution over that of reversible contribution. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 3. FORC distribution diagrams for (a) x ¼ 0 and (b) x ¼ 0.1, white bars show the dispersion along E’C and E'i axes.
Fig. 4. FORC distribution diagrams for x ¼ 0 at different fatigue stages: (a) n ¼ 1, (b) n ¼ 3500, (c) n ¼ 3 106 and (d) re-annealed.
the reversible component broadened as the fatigue cycles increased (see prev shown in Fig. 6a). Detectable peak was not found for the fully fatigued sample at stage D. The prev partially recovered at high field at stage D. The prev of x ¼ 0.1 in different fatigue stages were plotted in Fig. 6b. Apart from a shift to high electric field and a broadened peak, a slight enhancement of prev was detected at stage
B. The peak value of the reversible component (prev) was 1.42 mC/ kVcm at stage A and 1.58 mC/kVcm at stage B, increasing only about 11.3%. prev of the fatigued sample shifted toward high field (Fig. 6b). After re-annealing, the peak moved to lower electric field than that of the fatigued sample, and no sign of recovery was found (Fig. 6b). It has been reported that a dispersive FORC distribution with large
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Fig. 5. FORC distribution diagrams for x ¼ 0.1 at different fatigue stages: (a) n ¼ 1, (b) n ¼ 3 104 , (c) n ¼ 107 and (d) re-annealed.
Fig. 6. Reversible component of (a) x ¼ 0, (b) x ¼ 0.1 at the four different fatigue stages.
areas of reversible component indicates an inhomogeneous structure [32]. This proved that the fatigue induced inhomogeneity. 4. Discussion 4.1. Polarization switching behavior Polarization switching is the key issue in ferroelectrics. As was mentioned above, the Preisach density in the FORC distribution represents the information of the polarization switching. For the 2 Sn-doped samples, the decreased intensity (r FORC ¼ 0:225 mC/kV
2 for x ¼ 0 and r FORC ¼ 0:116 mC=kV for x ¼ 0.1) and dispersive distribution (a range of E'i of 10.7 kV/cm for x ¼ 0 and a range of E'i of 16.7 kV/cm for x ¼ 0.1) along E'i axis of irreversible component were induced by the effect of defect dipoles (see white bar along E'i axes in Fig. 3). We previously revealed that the valence reduction of Sn4þ ions to Sn2þ ions leads to the generation of oxygen vacancies and 00 subsequent defect dipoles SnTi V$$ O in both polycrystalline ceramics and single crystals [36,43]. The defect dipoles in ferroelectrics abide a symmetry-confirming property [9]. The domain stability in local scale is enhanced and the polarization switching kinetic is weakened. Preisach density decreased accordingly. The
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random distribution of defect dipoles in the bulk [44e46] suggests random interaction between spontaneous polarization and the defect dipoles, increasing the inhomogeneity of the local internal bias. As a result, the range of electric field for domain response enlarges, showing strong dispersion of internal bias field E'i (see Figs. 2 and 3). The decreasing of both the peaks of prev and Rirre/re with the increasing of Sn-doping content (Fig. 2h) is in good agreement with acceptor-doped PZT [21,39,47], resulting from the stabilization effect of defect dipoles. The trend suggested that the irreversible component decreased faster than the reversible component. DW motions were hampered by defect dipoles. Subsequently, the irreversible component was weakened as a result of pinning or clamping effect by defect dipoles [1,23]. The reversible contribution contains both intrinsic component and the extrinsic component. The extrinsic component is enhanced due to the pinning or clamping effect, while the intrinsic component weakened. The 00 switching of defect dipoles formed as SnTi V$$ O is much harder than that of spontaneous polarization since the migration of oxygen vacancies between lattice sites requires very long time to complete [9]. The switching of spontaneous polarization was hampered by defect dipoles, hence the intrinsic component weakened in the reversible component. Considering the slight enhancement of the reversible extrinsic component, the irreversible component decreased faster than the reversible component. 4.2. Polarization switching during fatigue process Figs. 4e6 show that the compositions of x ¼ 0 and x ¼ 0.1 undergo distinct fatigue processes. During the self-rejuvenation process (stage A to stage B), both compositions experienced a selfrejuvenation at stage B (see the MHLs in the insets of Figs. 4b and 5b), but the effect of self-rejuvenation for x ¼ 0.1 was much stronger than that of x ¼ 0. The self-rejuvenation is caused by the redistribution of defect dipoles reported in the previous work [36]. The redistribution weakens the pinning effect of defect dipoles. The response of domains under the external field is more simultaneous and instantaneous at stage B than stage A, leading to a drastic increase of polarization (see MHL in the inset of Fig. 5a and b). With defects distributing more uniformly in the bulk, chances for DWs move irreversibly across defects increase a lot. As a result, the maximum r FORC of the irreversible component for x ¼ 0.1 increased drastically from 0.116 mC=kV2 to 0.924 mC=kV2 with a sharpened and centered distribution. We'll verify this via the reversible component later in this section. The increasing of Preisach density of the irreversible component at stage B was not found for x ¼ 0 even if Pr rejuvenates a little at stage B (Fig. 4). Instead the maximum r FORC of the irreversible component dropped from 0.225 mC=kV2 to 0.143 mC=kV2 (stage A to B) and the peak broadened, indicating more dispersive interaction between spontaneous polarizations and defect dipoles at stage B than stage A. The selfrejuvenations of polarization for x ¼ 0 and x ¼ 0.1 exhibit completely different mechanism as shown in Figs. 4 and 5. The slight rejuvenation of Pr for x ¼ 0 stems from the broad distribution of hysterons that respond at wider range of electric field at stage B than stage A, which was caused by fatigue effect. Note that in Fig. 5b, there's a mutual permeation of the negative sub-regions and the minor ridge of irreversible component, showing change of position of the negative sub-regions. It results from the change of system geometry due to the reorientation of defect dipoles verified by the enhanced internal bias during the fatigue process [36]. Next, we will discuss the evolution of the reversible component during the self-rejuvenation process from stage A to B. For x ¼ 0, the evolution of the reversible component mainly results from two aspects: one is the phase evolution that weakens the response as
we will verify in section 4.3; the other is the screening effect of degraded layer induced by fatigue effect (local degraded layer/ phase decomposition, see Supplementary Information), which weakens the sensitivity of reversible process and shifts the peak to high field [22]. For x ¼ 0.1, the reversible peak shifted to high field and broadened slightly from stage A to B. The slight increase of the peak value of the reversible component prev (11.3%) is negligible compared with the increase of the irreversible component (697%), indicating that the reorientation of defect dipoles enhances mainly the irreversible component. The slight increase of the reversible component stems from the enhanced reversible bending of DWs due to the random distribution of defect dipoles. The shift and broadening of the peak stem also from the redistribution of defect dipoles, which enhances the interaction between defect dipoles and spontaneous polarization in the domains. To conclude, the selfrejuvenation was induced mainly by the irreversible component instead of the reversible component. The FORC distribution of x ¼ 0 at stage C (Fig. 4c) is very similar to PZT film [2,23]. For x ¼ 0.1 (Fig. 5c), the irreversible component was stronger and sharper than that of x ¼ 0 (Fig. 4c), indicating weak fatigue effect. The irreversible component for both x ¼ 0 and x ¼ 0.1 shifted to high values along E’C axis, indicating that the switching of polarization becomes hard as the cycles increases. It's hard to further analyze the fatigue mechanism without looking into the reversible component. The reversible component for x ¼ 0 decreased drastically at stage C and no obvious peak was detected due to complex phase evolution induced by fatigue (Fig. 6a), while for x ¼ 0.1, the reversible component deteriorate slightly with a large shift toward high field (Fig. 6b). The change of FORC distribution for fully fatigued sample might be a result of DWs pinning, which weakens the DWs mobility. As a result, a subsequent increase of reversible component should have been detected. But prev for both compositions decreased during fatigue treatment despite a shift to high electric field (see Fig. 6). Hence DWs pinning is not mainly responsible for the fatigue behavior for both doped and undoped PLN-PT ceramics. Most of fully fatigued ferroelectric materials recover the ferroelectric properties after re-annealing [10,13,48,49]. Lou et al. proposed a universal model [50], in which fatigue is induced by local phase decomposition. Perovskite phase decomposes into pyrochlore-like phase to form “dead layers”, which would be restored to perovskite phase after re-annealing. The obvious degraded layer was not detected by SEM (see Fig. S1). But the local phase decomposition has been detected by Raman spectra (see Fig. S2) for PLN-PT-based ceramics without composition change (see Tables S1eS4). The discussion of phase decomposition was presented in Supplementary Information. The FORC distribution got sharper along E'i axis after re-annealing, indicating weaker interactions between spontaneous polarization and defect dipoles, but the distribution of the irreversible component stretched to low values and became dispersive along E’C axis in spite of the partial recovery of the ferroelectric properties as shown in Figs. 4d and 5d. It indicates that some switching units that were hard to be switched somehow recover from the fatigue effect, but some other don't recover and they are still hard to be switched. The interactions between defect dipoles and spontaneous polarization also weaken after the re-annealing. Such weakened dispersion along E'i axis and strengthened dispersion along E’C axis have not been reported to our knowledge. In the regime of local phase decomposition mechanism, the stretching of the irreversible component toward low E’C is essentially the increasing number of available nucleation sites due to the restoration of perovskite phase (supported by Raman spectra as shown in Fig. S2). The reversible component for both x ¼ 0 and x ¼ 0.1 also recovered a bit after re-annealing because of the increasing number of available nucleation sites
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(Fig. 6). It is noted that re-annealing helps re-active the nucleation sites that is frozen by the local phase decomposition induced by fatigue. But the restoration is not complete, which is verified by the FORC distribution at stage D (Fig. 5d) with the dispersed irreversible component. 4.3. Structure evolution The structure evolution of the sample surface was examined as shown in Fig. 7. Complex multi-phase was found for x ¼ 0, while no obvious extra phase was detected for x ¼ 0.1. The undoped sample (x ¼ 0) showed different symmetry at different stages (Fig. 7a and b). The profiles of (200) peaks were decomposed with three Gaussian peaks (Fig. 7b). At stage A, the XRD pattern shows symmetry near the MPB (morphotropic phase boundary) comprised of rhombohedral (R) and tetragonal (T) phases. The two peaks corresponding to (200)T and (002)T merged into one single peak at stage B, showing the main phase of R phase. It is known that the ratio of the intensities of (200)T and (002)T is about 1: 2 for T phase, but the ratio of intensities of (200)T and (002)T at stage B was about 1: 1. It can be speculated that another phase exists except for R and T phase. The third phase is the orthorhombic (O) phase elucidated hereafter. At stage C, the T phase disappeared confirmed by the change of the intensities of (200). Different from T phase, the
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intensities of (200) at low angle was larger than that of (200) peak at high angle (Fig. 7b), indicating O phase. Therefore, a mixture of O phase and R phase coexist at stage C. It can be deduced that the structure change from T phase to O phase results from the fatigue effect. The structure bears much resemblance at stage D and stage B dominated by R phase, except the difference of intensity. For x ¼ 0.1, the R phase kept unchanged and no obvious phase transition was detected all the time, showing excellent phase stabilization under AC-field cycling process. The structure evolution inside the bulk was similar to the surface (see Fig. S3 and analysis in Supplementary Information). Therefore the phase analysis is reliable and persuasive. The ferroelectric ceramics with perovskite structure have excellent piezoelectric properties near the MPB region. But the ferroelectric ceramics with single phase have excellent fatigue endurance. Therefore, the effective way of improvement of the fatigue endurance is to avoid the MPB, sacrificing the piezoelectric properties [10]. It is now very clear that the enhancement of fatigue endurance comes from the phase stability of rhombohedral throughout the whole fatigue process. The reason for the phase stabilization by acceptor doping is the domain stabilization effect of defect dipoles [9], while the complex phase evolution results in the drastic decreasing of reversible component as can be seen in Fig. 6a. 5. Conclusions In conclusion, the polarization switching properties of acceptordoped ferroelectric ceramics (Sn-doped Pb(Lu1/2Nb1/2)O3ePbTiO3) were studied using the first-order reversal curve (FORC) approach. For acceptor doping, the response of polarization switching weakened. The reversible and irreversible switching components both decreased. The abnormal self-redistribution effect for undoped ceramics stemmed from the dispersed responses of switching units to electric field, while that for Sn-doped ceramics resulted from the redistribution of defect dipoles. The abnormal self-rejuvenation was induced mainly by the irreversible contribution. Domain wall pinning was not responsible for the fatigue effect. The structure decomposition was the main reason for fatigue endurance. The enhancement of fatigue endurance comes from the phase stability of structure in acceptor-doped ceramics, while complex phase evolution exists in undoped ceramics with weak fatigue endurance. The present work provided a new aspect to investigate the interaction between spontaneous polarization and defect dipoles influenced by repetitive AC electric field by using the FORC approach, which allow scientists to better understand the behavior of polarization switching. Acknowledgements This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000); the National Natural Science Foundation of China (41874214); the Science and Technology Project of Fujian Province (2018H0044); the Youth Innovation Promotion Association CAS, the “Chunmiao” Talents Program for Young Scientists of Haixi Institute of the Chinese Academy of Sciences (CMZX-2016-006). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.actamat.2019.03.033.
Fig. 7. XRD patterns of (a) x ¼ 0 and (c) x ¼ 0.1 at different fatigue stages (the index (hkl) refers to cubic phase); (b, d) show the enlarged XRD patterns of (200) peak, hollow circles represent the experiment data, green lines and red lines are the fitted results. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Investigation of switching behavior of acceptor-doped ferroelectric ceramics Chenxi Wang a, b, Xiaoming Yang a, b, Zujian Wang a, Chao He a, *, Xifa Long a a
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China b University of Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 January 2019 Received in revised form 11 March 2019 Accepted 23 March 2019 Available online 26 March 2019
Switching behavior is a general feature in ferroelectrics. The related fatigue effects influenced by defect dipoles in ferroelectrics are still controversial that is focused on the positive and negative effects of oxygen vacancies. Here, we report the polarization switching behavior of acceptor-doped ceramics using the first-order reversal curve (FORC) approach, especially for the abnormal self-rejuvenation effect and the enhanced fatigue endurance in acceptor-doped ceramics. The reversible and irreversible components under electric field in the ceramics were distinguished by the FORC distribution of ideal “hysteron”. The abnormal self-rejuvenation behavior stemmed from dispersive response of hysteron for undoped samples while from the redistribution of defect dipoles for acceptor-doped samples. The self-rejuvenation was induced mainly by the irreversible component. For the fatigue effect, the pinning of domain walls was not the main reason. The re-annealing treatment for a fatigued sample weakened the interactions between the spontaneous polarizations and defect dipoles, but enhanced the dispersion of coercive field. The enhancement of fatigue endurance came from the phase stability of structure in acceptor-doped ceramics, while complex phase evolution existed in undoped ceramic with weak fatigue endurance. Our study shed new light on the interactions between spontaneous polarization and defect dipoles under repetitive AC electric field. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Ferroelectric FORC Self-rejuvenation Fatigue Defect dipole
1. Introduction Polarization switching is a general feature in ferroelectrics. It has been applied to describe the functional features of many devices, like random access memory and data storage [1]. Polarization switching is directly related to many important physical characteristics like hysteretic behavior, field-induced strain and dielectric permittivity, and underpins the application of ferroelectric devices [1e6]. The behavior of polarization switching can be affected by many factors, like temperature [7], light [8], point defects [9], and external electric field [10]. Studying the dynamics of polarization switching under various conditions helps to understand the mechanisms of polarization switching and enhance the reliability and life time of devices. The electric fatigue effect in ferroelectrics is the main hindrances to application and characterized by the detriment of
* Corresponding author. E-mail address: [email protected] (C. He). https://doi.org/10.1016/j.actamat.2019.03.033 1359-6454/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
electrical properties. For example, remnant polarization (Pr) and piezoelectric coefficients (d33) would decrease due to repetitive polarization switching under AC electric field [5,11], which is very harmful in applications, like reading or writing errors in memory devices [12]. It has been proved that fatigue effect is very sensitive to doping [10,13,14]. But it is still controversial whether oxygen vacancies in ferroelectrics have positive or negative effects on fatigue behavior [15e17]. The role of defect dipoles is complicated in fatigue process. Abnormal self-rejuvenation (revival of polarizations during fatigue process) and de-aging behavior related to the defect dipoles have been found in ferroelectric materials with abundant oxygen vacancies, like Pb-based ferroelectric materials [5,18,19] and hafnium oxide-based ferroelectric materials [1,20]. The total piezoelectric and dielectric response of ferroelectrics comprises of reversible and irreversible components that contribute differently to the material [21]. To be able to confirm the effect of defect dipoles, the reversible and irreversible components under electric field in ferroelectrics must be taken into account because they are notably influenced by defect dipoles. For example, both weakening and strengthening of the reversible components
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have been found in different PZT films [22,23]. While the reversible component didn't change during fatigue in PZT ceramics [24]. It's still desired to study the mechanisms behind such controversial behavior in the fatigue process [3,25,26]. The first-order reversal curve (FORC) approach, which was developed based on Preisach model [27], has turned out to be a powerful tool in characterizing hysteretic systems. The common polarization-electric field (P-E) loops are a statistical result of the polarization switching. The FORC is the deconvolution of the statistical result, revealing the information of polarization switching at local level, and is very sensitive to the domain dynamics. The FORC distribution diagrams provide information on the microscopic mechanisms of the domain switching [28]. This method characterizing hysteretic phenomena is universal and has been applied to various kinds of materials including spin transition materials [29], magnetic materials [30], ferromagnetic materials [31], ferroelectric materials [1,32e34] and superconductors [35]. Here, we report the switching behavior of acceptor-doped ceramics using FORC approach. As mentioned above, switching dynamics and fatigue are notably influenced by defect dipoles. As acceptor-doped ferroelectric ceramics with abundant defect dipoles, Sn-doped Pb(Lu1/2Nb1/2)O3ePbTiO3 (PLN-PT) ceramics exhibit abnormal self-rejuvenation effect and strong fatigue endurance [36]. The influence of the reversible and irreversible domain evolution on the self-rejuvenation effect in ferroelectric materials is still lacking in study, and the difference of structure evolution in the fatigue process needs to be further verified. Therefore, self-rejuvenation effect in the fatigue process and structure evolution was studied in acceptor-doped ferroelectric ceramics using the FORC approach in this work. This work may present a new idea for the study of the interaction between spontaneous polarization and defect dipoles, and switching behavior in acceptor-doped ferroelectric ceramics.
2. Experimental section 2.1. Measurements Acceptor-doped ferroelectric ceramics (0.51 Pb(Lu1/2Nb1/2)O3 0.49PbTi1-xSnxO3 (x ¼ 0e0.12)) were prepared as previously reported [36]. The green samples were annealed at 500 C (3 h), which was well above the Curie temperature. The composition x ¼ 0.1 was chosen to compare with x ¼ 0 for fatigue investigations
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because it showed the best fatigue endurance and strongest selfrejuvenation [36]. The samples for electrical measurements were pasted with high-purity silver paint (YFS-05002, SPI, England) as the electrodes. The electric field was set up to two times of the coercive field with 100 Hz for fatigue measurements. The crystalline phase of the samples at different fatigue stages was analyzed at room temperature with X-ray diffractormeter (XRD) (Miniflex 600, Rikagu, Japan) from 2q ¼ 10 to 80 with a speed of 2 deg/min and a step of 2q ¼ 0.02. The XRD was measured on the surface of the sample immediately after the fatigue treatment and removing the electrode. The electrode can be totally removed by submerging the sample in acetone without damaging the surfaces.
2.2. FORC approach FORC was measured using a standard ferroelectric system (aixACCT TF2000 Analyzer) with modulated triangle waves of electric field as shown in Fig. 1a. The maximum field (Em) was set to about 3 times of the coercive field (60 kV/cm in this work) to positively pole the sample. The reversal resided on the descending branch of the main hysteresis loop (MHL) depicted as dashed curve in the inset of Fig. 1a. The MHL results from the statistical superposition of local responses of polarization switching. The FORC measured with this specific waveform is shown in the inset of Fig. 1a. The Preisach density can be obtained from FORC using equation (1):
r FORC ðE; Er Þ ¼
1 v2 P FORC ðE; Er Þ 2 vEr vE
(1)
where P FORC ðE; Er Þ is the polarization of FORC. The minus sign indicates that the FORC started on the descending branch of the MHL. The Preisach density represents the distribution of density of ideal “hysteron” (switching units distributed statistically) and also gives information about local switching behavior. The FORC distribution diagram of Preisach density shown in Fig. 1b which describes the distribution of “hysteron” about the real electric field E and the reversal electric field Er, exhibiting a contour plot with E and Er as horizontal and vertical axes in the diagram. Every FORC is obtained in the range of E Er and every point of the FORC distribution diagram is assumed to be associated with the measured material. As a result, only the grey region in Fig. 1b has true physical meaning. The FORC distribution can also be interpreted in terms of ideal coercive
Fig. 1. (a) Wave form of the electric field for FORC measurement, the inset shows the measured FORC; (b) the sketch of the FORC distribution diagram.
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field E’C and internal field E'i via equations (2) and (3):
E0 C ¼ ðE Er Þ=2
(2)
E0 i ¼ ðE þ Er Þ=2
(3)
As axes to depict the FORC distribution, E’C and E'i are distinguished from EC and Ei in the MHL. Specifically, E’C and E'i are the coercive and internal fields of the switched hysteron. In the present work, the data analysis and processing were based on the mathematical procedure reported by Pike et.a. [28] using total 200*100 data points for each measurement. And FORC at different fatigue stages was measured. The frequency to measure the FORC was 5 Hz, in accordance with the frequency to measure the MHL. 3. Results 3.1. FORC distribution for different doping levels Fig. 2 shows the FORC distribution diagrams for different Sn doping levels with E and Er as the horizontal and vertical axes in the same color scale. The E’C and E'i axes are the diagonals of the diagrams as shown in Fig. 1b. The MHLs were shown in the upper left of the FORC distribution diagrams. Two regions were detected in the FORC distribution diagrams. One was the region with strip shape along the E'i axis. The other was a peak centered near E'i ¼ 0 along E’C axis with a minor ridge parallel with E'r axis. The ridge represented specific hysterons that can be switched at a relatively low E no matter how strong Er was. Fig. 2 also shows some negative sub-regions in navy blue adjacent to the ridge. The negative subregions are related to the system geometry [37] and finite time dependence of polarization switching [38,39]. The FORC distribution of Sn-PLN-PT resembled that of PZT ferroelectrics with similar shapes and locations of reversible and irreversible components [40]. The reversible (prev) and irreversible (pirre) components can be distinguished using FORC distribution diagrams. The reversible in the FORC distribution diagrams corresponding to the region along E'i axis is defined as equation (4):
1 vP FORC ðE; Er Þ fcfεr ; vEr Er /E 2
prev ðEÞ ¼ lim
(4)
where c is the dielectric susceptibility and is proportional to the dielectric constant εr . The irreversible contribution was located along E’C axis [23]. Fig. 2h shows the reversible contribution prev(E), which decreased and moved to low field with increase of Sn content, indicating weakened reversible contribution. The prev(E) of different doping level converged at high electric field. Like the intrinsic contributions (lattice), the extrinsic contributions exist in both reversible process (domain walls (DWs) bending and DWs planar movements between defects) and irreversible process (DWs move irreversibly across potentials or defects) [41]. The weakening of prev stems from the stabilization effect of defect dipoles on the domains, which will be discussed in section 4. To get an insight of the effect of Sn doping, the ratio (Rirre/re) of the maximum intensity of irreversible component over that of the reversible component at different doping levels was plotted in the inset of Fig. 2h. The reversible contribution was much stronger than irreversible contribution since the ratio Rirre/re was much smaller than 1. Rirre/re showed a decreasing trend with x increasing when the measurement noise and property fluctuation of the samples were ignored, suggesting the increase of the portion of the reversible component. It is concluded that the reversible component contributed more to the total polarization switching response as x increased. To see more details, the FORC distribution diagrams of the
composition x ¼ 0 and x ¼ 0.1 were extracted as shown in Fig. 3, showing the broadened distribution for x ¼ 0.1. To show the dispersion clearly, the white bars represented a decrease of 95% of the maximum of the irreversible component, indicating a dispersion of E'i about 10.7 kV/cm and 16.7 kV/cm for x ¼ 0 and 0.1, respectively. The peak stretched along E’C axis for both samples, showing a dispersion of E’C from about 4 kV/cm to 24 kV/cm (see Fig. 3). The dispersion of E’C is characteristic of normal Pb-based ferroelectrics [23,32,39], which is the result of the dispersive distribution of coercive field in the switching and back-switching processes. Therefore, the dispersion of E’C didn't change while the distribution along E'i axis became largely dispersed after doping. 3.2. FORC distribution of fatigued samples The FORC was measured at different fatigue stages, and the corresponding FORC distribution diagrams were plotted for x ¼ 0 (Fig. 4) and x ¼ 0.1 (Fig. 5). To better understand the fatigue process, four stages were selected: (A), the initial stage of fatigue process (n ¼ 0); (B), the most pronounced self-rejuvenation effect stage (n ¼ 3500 for x ¼ 0 and n ¼ 10000 for x ¼ 0.1); (C), the end stage (n ¼ 3 106 for x ¼ 0 and n ¼ 107 for x ¼ 0.1); (D), the re-anneal stage after fatigue process (500 C for 3 h, which was well above the Curie temperature between 300 and 330 C). Fig. 4 shows the FORC distribution diagrams of x ¼ 0. The irreversible component decreased and the distribution moved to high E’C and finally split up to multi-centers along E’C axis as the fatigue cycles increased (refer to the stage from A to C in Fig. 4). Multiple maxima were detected due to nanoscale inhomogeneity [42] stemming from the fatigue treatment. The reversible component became broad and dispersive as the fatigue cycles increased. The FORC distribution recovered slightly after re-annealing (Fig. 4d) in accordance with the partial recovery of the remnant polarization Pr as shown in the inset of Fig. 4d (P-E hystersis loops). The distribution of the irreversible component became more dispersive along E’C axis and sharper along E'i axis than that of stage C. The irreversible component still showed multi-center distribution at stage D and the maximum moved to lower E’C compared with stage C (41.5 kV/cm for stage C and 27.7 kV/cm for stage D) though the coercive field EC kept unchanged after the re-annealing (about 30 kV/cm) as shown in the insets of Fig. 4c and d. The reversible component was almost unaffected by the re-annealing. The variation of FORC distribution for x ¼ 0.1 at different fatigue stages (Fig. 5) was different from that of x ¼ 0. At stage B, the maximum of irreversible contribution enhanced drastically from 2 2 r FORC ¼ 0:116 mC=kV to rFORC ¼ 0:924 mC=kV (increased by 697%, see the color scale in Fig. 5a and b). The peak became sharper than stage A, indicating stronger self-rejuvenation than x ¼ 0. Like the case of x ¼ 0, the FORC distribution for x ¼ 0.1 exhibited a multicenter distribution of irreversible component with much weaker 2 intensity at stage C than that of stage A (r FORC ¼ 0:0032 mC=kV at stage C) (Fig. 5c), though the remnant polarization Pr decreased only slightly. Dispersive distribution along E’C axis can also be seen at stage C. The intensity of the maximum of the irreversible component was much higher than x ¼ 0 at stage C, indicating weaker inhomogeneity than the case of x ¼ 0. The reversible component became broad and dispersive at stage C (Fig. 5c). After re-annealing, the intensity of the FORC distribution didn't recover as the case of x ¼ 0. The irreversible component moved to low E’C values (maximum moved from E’C ¼ 53.2 kV/cm at stage C to E’C ¼ 28.6 kV/cm at stage D). Unlike x ¼ 0, the coercive field EC extracted from the MHL decreased from 34.8 kV/cm to 27.9 kV/cm for x ¼ 0.1 after re-annealing. To investigate the change of reversible component, the prev of x ¼ 0 and x ¼ 0.1 at different stages were plotted in Fig. 6. For x ¼ 0,
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Fig. 2. (aeg) FORC distribution diagrams for different doping concentration, the bottom right shows the color scale, the inset shows the MHL for each composition; (h) the variation of the reversible distribution prev and the inset shows the ratio of peak value of irreversible contribution over that of reversible contribution. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 3. FORC distribution diagrams for (a) x ¼ 0 and (b) x ¼ 0.1, white bars show the dispersion along E’C and E'i axes.
Fig. 4. FORC distribution diagrams for x ¼ 0 at different fatigue stages: (a) n ¼ 1, (b) n ¼ 3500, (c) n ¼ 3 106 and (d) re-annealed.
the reversible component broadened as the fatigue cycles increased (see prev shown in Fig. 6a). Detectable peak was not found for the fully fatigued sample at stage D. The prev partially recovered at high field at stage D. The prev of x ¼ 0.1 in different fatigue stages were plotted in Fig. 6b. Apart from a shift to high electric field and a broadened peak, a slight enhancement of prev was detected at stage
B. The peak value of the reversible component (prev) was 1.42 mC/ kVcm at stage A and 1.58 mC/kVcm at stage B, increasing only about 11.3%. prev of the fatigued sample shifted toward high field (Fig. 6b). After re-annealing, the peak moved to lower electric field than that of the fatigued sample, and no sign of recovery was found (Fig. 6b). It has been reported that a dispersive FORC distribution with large
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Fig. 5. FORC distribution diagrams for x ¼ 0.1 at different fatigue stages: (a) n ¼ 1, (b) n ¼ 3 104 , (c) n ¼ 107 and (d) re-annealed.
Fig. 6. Reversible component of (a) x ¼ 0, (b) x ¼ 0.1 at the four different fatigue stages.
areas of reversible component indicates an inhomogeneous structure [32]. This proved that the fatigue induced inhomogeneity. 4. Discussion 4.1. Polarization switching behavior Polarization switching is the key issue in ferroelectrics. As was mentioned above, the Preisach density in the FORC distribution represents the information of the polarization switching. For the 2 Sn-doped samples, the decreased intensity (r FORC ¼ 0:225 mC/kV
2 for x ¼ 0 and r FORC ¼ 0:116 mC=kV for x ¼ 0.1) and dispersive distribution (a range of E'i of 10.7 kV/cm for x ¼ 0 and a range of E'i of 16.7 kV/cm for x ¼ 0.1) along E'i axis of irreversible component were induced by the effect of defect dipoles (see white bar along E'i axes in Fig. 3). We previously revealed that the valence reduction of Sn4þ ions to Sn2þ ions leads to the generation of oxygen vacancies and 00 subsequent defect dipoles SnTi V$$ O in both polycrystalline ceramics and single crystals [36,43]. The defect dipoles in ferroelectrics abide a symmetry-confirming property [9]. The domain stability in local scale is enhanced and the polarization switching kinetic is weakened. Preisach density decreased accordingly. The
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random distribution of defect dipoles in the bulk [44e46] suggests random interaction between spontaneous polarization and the defect dipoles, increasing the inhomogeneity of the local internal bias. As a result, the range of electric field for domain response enlarges, showing strong dispersion of internal bias field E'i (see Figs. 2 and 3). The decreasing of both the peaks of prev and Rirre/re with the increasing of Sn-doping content (Fig. 2h) is in good agreement with acceptor-doped PZT [21,39,47], resulting from the stabilization effect of defect dipoles. The trend suggested that the irreversible component decreased faster than the reversible component. DW motions were hampered by defect dipoles. Subsequently, the irreversible component was weakened as a result of pinning or clamping effect by defect dipoles [1,23]. The reversible contribution contains both intrinsic component and the extrinsic component. The extrinsic component is enhanced due to the pinning or clamping effect, while the intrinsic component weakened. The 00 switching of defect dipoles formed as SnTi V$$ O is much harder than that of spontaneous polarization since the migration of oxygen vacancies between lattice sites requires very long time to complete [9]. The switching of spontaneous polarization was hampered by defect dipoles, hence the intrinsic component weakened in the reversible component. Considering the slight enhancement of the reversible extrinsic component, the irreversible component decreased faster than the reversible component. 4.2. Polarization switching during fatigue process Figs. 4e6 show that the compositions of x ¼ 0 and x ¼ 0.1 undergo distinct fatigue processes. During the self-rejuvenation process (stage A to stage B), both compositions experienced a selfrejuvenation at stage B (see the MHLs in the insets of Figs. 4b and 5b), but the effect of self-rejuvenation for x ¼ 0.1 was much stronger than that of x ¼ 0. The self-rejuvenation is caused by the redistribution of defect dipoles reported in the previous work [36]. The redistribution weakens the pinning effect of defect dipoles. The response of domains under the external field is more simultaneous and instantaneous at stage B than stage A, leading to a drastic increase of polarization (see MHL in the inset of Fig. 5a and b). With defects distributing more uniformly in the bulk, chances for DWs move irreversibly across defects increase a lot. As a result, the maximum r FORC of the irreversible component for x ¼ 0.1 increased drastically from 0.116 mC=kV2 to 0.924 mC=kV2 with a sharpened and centered distribution. We'll verify this via the reversible component later in this section. The increasing of Preisach density of the irreversible component at stage B was not found for x ¼ 0 even if Pr rejuvenates a little at stage B (Fig. 4). Instead the maximum r FORC of the irreversible component dropped from 0.225 mC=kV2 to 0.143 mC=kV2 (stage A to B) and the peak broadened, indicating more dispersive interaction between spontaneous polarizations and defect dipoles at stage B than stage A. The selfrejuvenations of polarization for x ¼ 0 and x ¼ 0.1 exhibit completely different mechanism as shown in Figs. 4 and 5. The slight rejuvenation of Pr for x ¼ 0 stems from the broad distribution of hysterons that respond at wider range of electric field at stage B than stage A, which was caused by fatigue effect. Note that in Fig. 5b, there's a mutual permeation of the negative sub-regions and the minor ridge of irreversible component, showing change of position of the negative sub-regions. It results from the change of system geometry due to the reorientation of defect dipoles verified by the enhanced internal bias during the fatigue process [36]. Next, we will discuss the evolution of the reversible component during the self-rejuvenation process from stage A to B. For x ¼ 0, the evolution of the reversible component mainly results from two aspects: one is the phase evolution that weakens the response as
we will verify in section 4.3; the other is the screening effect of degraded layer induced by fatigue effect (local degraded layer/ phase decomposition, see Supplementary Information), which weakens the sensitivity of reversible process and shifts the peak to high field [22]. For x ¼ 0.1, the reversible peak shifted to high field and broadened slightly from stage A to B. The slight increase of the peak value of the reversible component prev (11.3%) is negligible compared with the increase of the irreversible component (697%), indicating that the reorientation of defect dipoles enhances mainly the irreversible component. The slight increase of the reversible component stems from the enhanced reversible bending of DWs due to the random distribution of defect dipoles. The shift and broadening of the peak stem also from the redistribution of defect dipoles, which enhances the interaction between defect dipoles and spontaneous polarization in the domains. To conclude, the selfrejuvenation was induced mainly by the irreversible component instead of the reversible component. The FORC distribution of x ¼ 0 at stage C (Fig. 4c) is very similar to PZT film [2,23]. For x ¼ 0.1 (Fig. 5c), the irreversible component was stronger and sharper than that of x ¼ 0 (Fig. 4c), indicating weak fatigue effect. The irreversible component for both x ¼ 0 and x ¼ 0.1 shifted to high values along E’C axis, indicating that the switching of polarization becomes hard as the cycles increases. It's hard to further analyze the fatigue mechanism without looking into the reversible component. The reversible component for x ¼ 0 decreased drastically at stage C and no obvious peak was detected due to complex phase evolution induced by fatigue (Fig. 6a), while for x ¼ 0.1, the reversible component deteriorate slightly with a large shift toward high field (Fig. 6b). The change of FORC distribution for fully fatigued sample might be a result of DWs pinning, which weakens the DWs mobility. As a result, a subsequent increase of reversible component should have been detected. But prev for both compositions decreased during fatigue treatment despite a shift to high electric field (see Fig. 6). Hence DWs pinning is not mainly responsible for the fatigue behavior for both doped and undoped PLN-PT ceramics. Most of fully fatigued ferroelectric materials recover the ferroelectric properties after re-annealing [10,13,48,49]. Lou et al. proposed a universal model [50], in which fatigue is induced by local phase decomposition. Perovskite phase decomposes into pyrochlore-like phase to form “dead layers”, which would be restored to perovskite phase after re-annealing. The obvious degraded layer was not detected by SEM (see Fig. S1). But the local phase decomposition has been detected by Raman spectra (see Fig. S2) for PLN-PT-based ceramics without composition change (see Tables S1eS4). The discussion of phase decomposition was presented in Supplementary Information. The FORC distribution got sharper along E'i axis after re-annealing, indicating weaker interactions between spontaneous polarization and defect dipoles, but the distribution of the irreversible component stretched to low values and became dispersive along E’C axis in spite of the partial recovery of the ferroelectric properties as shown in Figs. 4d and 5d. It indicates that some switching units that were hard to be switched somehow recover from the fatigue effect, but some other don't recover and they are still hard to be switched. The interactions between defect dipoles and spontaneous polarization also weaken after the re-annealing. Such weakened dispersion along E'i axis and strengthened dispersion along E’C axis have not been reported to our knowledge. In the regime of local phase decomposition mechanism, the stretching of the irreversible component toward low E’C is essentially the increasing number of available nucleation sites due to the restoration of perovskite phase (supported by Raman spectra as shown in Fig. S2). The reversible component for both x ¼ 0 and x ¼ 0.1 also recovered a bit after re-annealing because of the increasing number of available nucleation sites
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(Fig. 6). It is noted that re-annealing helps re-active the nucleation sites that is frozen by the local phase decomposition induced by fatigue. But the restoration is not complete, which is verified by the FORC distribution at stage D (Fig. 5d) with the dispersed irreversible component. 4.3. Structure evolution The structure evolution of the sample surface was examined as shown in Fig. 7. Complex multi-phase was found for x ¼ 0, while no obvious extra phase was detected for x ¼ 0.1. The undoped sample (x ¼ 0) showed different symmetry at different stages (Fig. 7a and b). The profiles of (200) peaks were decomposed with three Gaussian peaks (Fig. 7b). At stage A, the XRD pattern shows symmetry near the MPB (morphotropic phase boundary) comprised of rhombohedral (R) and tetragonal (T) phases. The two peaks corresponding to (200)T and (002)T merged into one single peak at stage B, showing the main phase of R phase. It is known that the ratio of the intensities of (200)T and (002)T is about 1: 2 for T phase, but the ratio of intensities of (200)T and (002)T at stage B was about 1: 1. It can be speculated that another phase exists except for R and T phase. The third phase is the orthorhombic (O) phase elucidated hereafter. At stage C, the T phase disappeared confirmed by the change of the intensities of (200). Different from T phase, the
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intensities of (200) at low angle was larger than that of (200) peak at high angle (Fig. 7b), indicating O phase. Therefore, a mixture of O phase and R phase coexist at stage C. It can be deduced that the structure change from T phase to O phase results from the fatigue effect. The structure bears much resemblance at stage D and stage B dominated by R phase, except the difference of intensity. For x ¼ 0.1, the R phase kept unchanged and no obvious phase transition was detected all the time, showing excellent phase stabilization under AC-field cycling process. The structure evolution inside the bulk was similar to the surface (see Fig. S3 and analysis in Supplementary Information). Therefore the phase analysis is reliable and persuasive. The ferroelectric ceramics with perovskite structure have excellent piezoelectric properties near the MPB region. But the ferroelectric ceramics with single phase have excellent fatigue endurance. Therefore, the effective way of improvement of the fatigue endurance is to avoid the MPB, sacrificing the piezoelectric properties [10]. It is now very clear that the enhancement of fatigue endurance comes from the phase stability of rhombohedral throughout the whole fatigue process. The reason for the phase stabilization by acceptor doping is the domain stabilization effect of defect dipoles [9], while the complex phase evolution results in the drastic decreasing of reversible component as can be seen in Fig. 6a. 5. Conclusions In conclusion, the polarization switching properties of acceptordoped ferroelectric ceramics (Sn-doped Pb(Lu1/2Nb1/2)O3ePbTiO3) were studied using the first-order reversal curve (FORC) approach. For acceptor doping, the response of polarization switching weakened. The reversible and irreversible switching components both decreased. The abnormal self-redistribution effect for undoped ceramics stemmed from the dispersed responses of switching units to electric field, while that for Sn-doped ceramics resulted from the redistribution of defect dipoles. The abnormal self-rejuvenation was induced mainly by the irreversible contribution. Domain wall pinning was not responsible for the fatigue effect. The structure decomposition was the main reason for fatigue endurance. The enhancement of fatigue endurance comes from the phase stability of structure in acceptor-doped ceramics, while complex phase evolution exists in undoped ceramics with weak fatigue endurance. The present work provided a new aspect to investigate the interaction between spontaneous polarization and defect dipoles influenced by repetitive AC electric field by using the FORC approach, which allow scientists to better understand the behavior of polarization switching. Acknowledgements This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000); the National Natural Science Foundation of China (41874214); the Science and Technology Project of Fujian Province (2018H0044); the Youth Innovation Promotion Association CAS, the “Chunmiao” Talents Program for Young Scientists of Haixi Institute of the Chinese Academy of Sciences (CMZX-2016-006). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.actamat.2019.03.033.
Fig. 7. XRD patterns of (a) x ¼ 0 and (c) x ¼ 0.1 at different fatigue stages (the index (hkl) refers to cubic phase); (b, d) show the enlarged XRD patterns of (200) peak, hollow circles represent the experiment data, green lines and red lines are the fitted results. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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