The influence of A/B-sites doping on antiferroelectricity of PZO energy storage films

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INTRODUCTION
Dielectric materials, an essential part of capacitors, would generate polarization under an electric field, enabling them to be widely used in electrocaloric, actuator, and energy storage devices.According to different polarization behaviors, dielectrics can be divided into linear dielectrics (LDs), ferroelectrics (FEs), and antiferroelectrics (AFEs) [1][2][3][4] .The energy storage performances of dielectric materials could be determined by the polarization-electric field (P-E) curves as follow: where W rec , η, W loss , P max, and P r are the recoverable energy density, the energy efficiency, the dissipated energy, the maximum polarization, and the remnant polarization under an applied electric field E, respectively.Therefore, FE and AFE materials are suitable for energy storage applications due to a large P max , low P r, and moderate E. Meanwhile, dielectric films with much larger breakdown strength E b could attain higher energy density than their bulk counterparts [3][4][5][6] .
AFE materials possess a characteristic known as a double hysteresis loop, which corresponds to four current peaks under an applied electric field.The current peaks represent the AFE-to-FE phase transition at forward switching field E F and FE-to-AFE phase transition at backward switching field E A , respectively [7][8][9][10] .PbZrO 3 (PZO), as a prototype AFE material, exhibits an apparent double hysteresis loop characteristic, while the antiferroelectricity's origin is still controversial [11,12] .Hao et al. used the tolerance factor (t) to evaluate the antiferroelectricity of PZO films, and later an increasing researches focus on chemical doping to adjust antiferroelectricity of Pb-based and Pb-free AFE materials using t value [13] .The equation of tolerance factor (t) of perovskite structure can be expressed as follow: where r A , r B and r O denote the ion radius of A-site, B-site, and oxygen, respectively.It is accepted that the AFE phase is stabilized at t < 1, and the FE phase is stabilized at t > 1.For example, a reduced t value can be found in La-doped PZO and Ca-doped AgNbO 3 materials corresponding to an enhanced E F and E A to stabilize the AFE phase [14,15] .In 2017, Zhao et al. prepared Ag(Nb 1-x Ta x )O 3 ceramics in a similar t value and proposed that enhanced antiferroelectricity should be attributed to reduced polarizability of the B-site [10] .In addition, (Ca, Zr), (Sr, Zr) and (Ca, Hf) modified NaNbO 3 AFE ceramics both possess a double hysteresis loop by decreasing the value of t while keeping the value of electronegativity fixed [16][17][18] .It can be seen that the electric field-induced AFE phase could be affected by a tolerance factor, polarizability, and electronegativity in A/B-sites for Pb-based and Pb-free materials.In the case of only considering the tolerance factor t, whether the role of A/B-sites on influencing antiferroelectricity of PZO films exists difference.

MATERIALS AND METHODS
(Pb 0.95 Bi 0.05 )ZrO 3 (PBZ), Pb(Zr 0.95 Bi 0.05 )O 3 (PZB), and pure PbZrO 3 (PZO) films were prepared on 150-nm Pt/20-nm Ti/100-nm SiO 2 /Si substrate via sol-gel method.Pb (CH 3 COO) 2 •3H 2 O (AR, 99.5 %), Bi (NO 3 ) 3 •3H 2 O (AR, 99 %), and Zr(OCH 2 CH 2 CH 3 ) 4 solution (70 wt%) were used as starting raw materials to prepare a stable 0.2 M precursor solution.Simultaneously, 2-methoxyethanol, acetic acid, and acetylacetone were used as solvents and stabilizers.A 10 % excess lead was added to the solvent to compensate for lead loss during the annealing process.The completed sol was spin-coated on the substrate at 4500 rpm for 30 s, pyrolyzed at 450 °C, and annealed at 650 °C to attain the desired thickness.The annealing process was achieved using an RTP-500 furnace in an air atmosphere.Finally, a P t electrode with a diameter of ~200 μm was deposited through a magnetron sputtering system.
The crystal phase of PZO-based films was examined by grazing incident X-ray diffraction (GIXRD, Empyrean, PANalytical, Netherlands) with Cu Kα1 radiation.The cross-section morphology of the PZObased thin films was measured using a field emission scanning electron microscope (SEM, Zeiss Ultra Plus, Germany).The surface information of thin films was collected by atomic force microscopy (AFM, Nanoscope IV, Veeco, USA).The thicknesses of PZO-based films are about 180 nm.The dielectric and ferroelectric properties of the PZO-based thin films were measured using an impendence analyzer (Agilent 4294) and a ferroelectric workstation (Precision Premier II, Radiant Technologies Inc., USA).

RESULTS AND DISCUSSION
Figure 2A shows the GIXRD patterns of PZO-based films at 2θ = 20°-60°.In the case of a PZO-based film with good crystallinity, the perovskite crystalline structure is orthorhombic phase, and no secondary phase is detectable in the range of accuracy of GIXRD technology.In order to investigate the effect of Bi doping in PZO, the enlarged patterns of PZO-based films at 2θ = 30°-31° are shown in Figure 2B.The diffraction peak (122) around 30.5° of PBZ and PZB compared to pure PZO has a slight shift towards the low angle and high angle, respectively, which demonstrates Bi successfully replaces Pb and Zr at A/B-sites.In terms of the change in diffraction peak, similar phenomena have been observed in La-doped PZO films at low content [15] .
Figure 3A shows the SEM images of PZO-based films.It can be seen that all films possess good density and no pores and crack in the surface morphology.The grain size of PZB films exceeds PBZ and pure PZO films, which should be related to a reduced Pb/Zr ratio compared to PZO and PBZ. Figure 3B displays the AFM images of PZO-based films.The surface roughness R q is 2.91 nm, 3.32 nm, and 4.04 nm for pure PZO, PBZ, and PZB films.PZO-based thin films are characterized by a smooth and flattened surface, indicating high-quality materials.
Due to the valence difference of Bi 3+ , Pb 2+ and Zr 4+ , the chemical defect would be generated.The defect equation of Bi replaces Pb and Zr is given as follows: It can be seen that the point defect of and could be generated when Bi acts as a donor and acceptor dopant, respectively.Figure 4 shows the room temperature frequency dependency of dielectric properties for PZO-based films.As frequency increases, the dielectric properties of PZO-based films maintain stability over the frequency range of 1 kHz-1 MHz. Figure 4A shows that the dielectric constant enhances from ~250 for PZO films to ~325 for PBZ and PZB films, which may be related to an increased point defect contribution.In addition, Figure 4B illustrates that the dielectric losses of PZO-based films are around 0.1.
Figure 5A shows the room temperature P-E loops of PZO-based films at an applied electric field of 800 kV/cm.PZO films exhibit an apparent double hysteresis loop.Bi-doped PZO films at A/B sites still show double hysteresis loops, but the polarization and switching fields have been changed.The polarization differences Δ P between P max and P r of PZO-based films are exhibited in Figure 5C.The P r of all PZO-based films is unchanged, but the P max of Bi-doped PZO films is lower than that of pure PZO films.Therefore, Δ P of Bi-doped PZO films cannot be enhanced effectively at the same electric field.Figure 5B shows the I-E loops of PZO-based films at an applied electric field of 800 kV/cm.Four current peaks correspond to the AFE-to-FE and FE-to-AFE phase transition, respectively [8,10] .Compared to pure PZO films, PBZ and PZB films both possess a lower current density and higher forward switching field E F of 562.2 kV/cm for PBZ and 537.3 kV/cm for PZB than 413.5 kV/cm for PZO films, which illustrates Bi-doping enhances antiferroelectricity of PZO materials in some degree.As discussed above, this result may be related to a reduction in t value to stabilize the AFE phase.Unfortunately, the increased Δ E value sacrifices the E F of PBZ and PZB, as shown in Figure 5D.It can be attributed to an enlarged grain size [Figure 3], which differs from the previous results.As grain size decreases, hysteresis loss in relaxor ferroelectric materials and antiferroelectric materials is reduced due to increased dipole mobility, and therefore PBZ and PZB with large grain sizes would possess a high Δ E. It can be seen that the antiferroelectricity of PZO is no positive  correlation with the tolerance factor in t in A/B-sites doping.In addition, grain size should be taken into account when enhancing antiferroelectricity.
Figure 6A shows the P-E loops of pure PZO films at different electric fields.As the electric field enhances, linear hysteresis loops gradually transform into double hysteresis loops causing P max to increase dramatically.With further improving the electric field, the P-E loops are unchanged and stay in a polarization saturation state.A detailed description of the division of regions into different states can be found in the following content.For PBZ and PZB films, the polarization saturation has a slight delay.Meanwhile, breakdown  strength enhances compared to pure PZO films, as exhibited in Figure 6B and C. The leakage current is a crucial parameter for evaluating dielectric film's electric properties and conduction mechanisms [20][21][22][23] .Figure 6D illustrates the leakage current for PZO-based films as a function of the electric field.It can be seen that the curve of leakage current of pure PZO films can be divided into two parts: For low electric field, the leakage conductivity belongs to the bulk-limited Ohmic mechanism.The Fowler-Nordheim tunneling (FN) mechanism dominates at high electric field.A similar phenomenon also exists in PBZ and PZB films.Note that the transition field from bulk-limited to FN mechanism gradually reduces for pure PZO, PBZ, and PZB films, which may be related to different defect types in aliovalent doping PZO at A/B sites.Compared to the lead vacancy, the oxygen vacancy may easily contribute more leakage currier; hence, the transformation field of PZB from Ohmic into FN mechanism reduces compared to PBZ. Figure 6E shows the recoverable energy density W rec as a function of the electric field for PZO-based films.Similar to other literature [24] , a corresponding curve can be divided into three regions as the electric field increases.For region I, PZO-based films possess a low W rec value, which should belong to the AFE phase stage with a linear polarization curve.For region II, W rec of PZO-based films both sharply enhance, which should correspond to AFE-FE co-existed phase stage.Note that the dashed and dot lines represent different terminal transition fields into region III, which means Bi dope PZO would delay the polarization enhancement.The W rec only slightly enhances into region III, which should be attributed to the polarization saturation phenomenon at the high electric field [25,26] .The energy efficiency η as functions of electric field for PZO-based films is displayed in Figure 6F.Similarly, the curves of η also could be divided into three regions.
As the electric field enhances, the η value gradually decreases and attains a relatively 50%-60% range.PBZ films achieve a maximum W rec of 26.4 J/cm 3 with a η of 56.2 %, which exceeds other reported pure AFE materials [27][28][29] .
It is known that energy storage stability, including temperature and frequency [5,14,30] , is an important parameter for evaluating the material applications, as shown in Figure 7.As temperature enhances, double hysteresis loop characteristics of PBZ films gradually transform into relaxor AFE, as shown in Figure 7A.Meanwhile, the W rec gradually decreases and η value remains essentially unchanged (see Figure 7C), which should be related to the Curie temperature of PZO at about 230 °C, corresponding to the AFE-to-PE phase transition [8,31] .Figure 7B displayed frequency-dependent P-E loops of PBZ films at room temperature.As frequency enhances, polarization decreases and hysteresis loss also reduces.Therefore, the W rec and η of PBZ films display good frequency stability, as shown in Figure 7D.

Figure 5 .
Figure 5. (A) The P-E loops of PZO-based films at 800 kV/cm, and corresponding (C) the polarization difference value of Δ P (P max -P r ).(B) The I-E loops of PZO-based films at 800 kV/cm, and corresponding (D) the switching field value of Δ E (E F -E A ). PZO: PbZrO 3 .