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

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)^[1-4]. 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^[3-6].

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^[7-10]. PbZrO₃ (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 AgNbO₃ 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-xTa_x)O₃ 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 NaNbO₃ AFE ceramics both possess a double hysteresis loop by decreasing the value of t while keeping the value of electronegativity fixed^[16-18]. 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.

Following the above discussion, we choose Bi³⁺ (~1.38 Å for CN = 12 and 1.03 Å for CN = 6) to replace Pb²⁺(1.49 Å for CN = 12) and Zr⁴⁺ (0.72 Å for CN = 6) at A/B-sites respectively^[19], compare the difference of A/B-sites on influencing antiferroelectricity of PZO, and hence fabricate (Pb_0.95Bi_0.05)ZrO₃ (PBZ), Pb(Zr_0.95Bi_0.05)O₃ (PZB) and pure PbZrO₃ (PZO) films. A schematic representation of the crystal structure of the Bi-doping PZO material can be seen in Figure 1. Based on Equation (3), calculated t values are 0.9639, 0.9621, and 0.9569 for PZO, PBZ, and PZB compositions, respectively. Meanwhile, a correlation between the tolerance factor t and the stabilized antiferroelectricity of PZO is discussed. Our work provides a new perspective in improving energy storage properties of AFE materials and prompts the development of AFE materials.

Figure 1. The schematic of 5 mol% Bi³⁺ replaces Pb²⁺ at A-site and Zr⁴⁺ at the B-site of PZO. PZO: PbZrO₃.

MATERIALS AND METHODS

(Pb_0.95Bi_0.05)ZrO₃ (PBZ), Pb(Zr_0.95Bi_0.05)O₃ (PZB), and pure PbZrO₃ (PZO) films were prepared on 150-nm Pt/20-nm Ti/100-nm SiO₂/Si substrate via sol-gel method. Pb (CH₃COO)₂·3H₂O (AR, 99.5 %), Bi (NO₃)₃·3H₂O (AR, 99 %), and Zr(OCH₂CH₂CH₃)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 PZO-based 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 2. (A) The GIXRD patterns of PZO-based films at 2θ range from 20° to 60°. (B) The enlarged patterns at 2θ =30°-31°. GIXRD: Grazing incident X-ray diffraction; PZO: PbZrO₃.

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.

Figure 3. The (A) SEM images and corresponding (B) AFM images (5*5 μm) for PZO, PBZ, and PZB films. SEM: Scanning electron microscope; AFM: atomic force microscopy; PZO: PbZrO₃; PBZ: (Pb_0.95Bi_0.05)ZrO₃; PZB: Pb(Zr_0.95Bi_0.05)O₃.

Due to the valence difference of Bi³⁺, Pb²⁺ and Zr⁴⁺, 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 4. Frequency-dependent (A) dielectric constant and (B) dielectric loss for PZO-based films at room temperature. PZO: PbZrO₃.

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 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: PbZrO₃.

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^[20-23]. 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 6. (A-C) The P-E loops of PBZ, PZB, and pure PZO films at different electric fields, respectively. (D) The leakage current functions of electric field for PZO-based films. (E) The recoverable energy density W_rec and (F) the energy efficiency η of PZO-based films at an applied electric field. PBZ: (Pb_0.95Bi_0.05)ZrO₃; PZB: Pb(Zr_0.95Bi_0.05)O₃; PZO: PbZrO₃.

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/cm³ with a η of 56.2 %, which exceeds other reported pure AFE materials^[27-29].

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 7. (A) Temperature-dependent P-E loops and corresponding (C) W_rec, η of PBZ films at 1 kHz. (B) Frequency-dependent P-E loops and corresponding (D) W_rec, η of PBZ films at room temperature. PBZ: (Pb_0.95Bi_0.05)ZrO₃.

CONCLUSIONS

PZO, PBZ, and PZB films are prepared on Pt/Ti/SiO₂/Si substrate via the sol-gel method. Crystallized PZO-based films with orthorhombic perovskite phases exhibit low roughness and good density. The dielectric constants of the Bi-doped PZO and pure PZO films are approximately 300, and these films possess good frequency stability at room temperature. Compared to pure PZO films, Δ E value increases and Δ P decrease for PBZ and PZB films hindering the effective energy storage. A/B-site doping in influencing the antiferroelectricity of PZO has a similar effect in only considering t value, and A-site doping would be better than B-site one in energy storage properties. PBZ films achieve a high W_rec of 26.4 J/cm³ with a η of 56.2 % under an applied electric field of 1278 kV/cm, accompanying a suitable temperature and frequency energy storage stabilities.