Rapid stability of ferroelectric polarization in the Ca, Ce hybrid doped BaTiO₃ ceramics

Abstract

In this work, we report a rapid stability phenomenon of ferroelectric polarization in the Ca, Ce hybrid doped BaTiO₃ ceramics (BCaxT+BTCe8) (x = 10, 20, 24, 30 mol%) prepared by separate doping Ca²⁺ and Ce⁴⁺ ions. Double hysteresis loops are identified in the aged BCaxT+BTCe8 samples; meanwhile, the polarization of these loops present a rapid decrease within very short aging time (about 1 h), and then the polarization remains almost unchanged over the followed ~1000 h. This phenomenon is not reported in previous researches. Raman scattering spectrum indicates that oxygen vacancies are generated because of Ca²⁺ ions entering into Ti sites partly in the BCaxT+BTCe8 samples, and then the oxygen vacancies are quantitatively characterized by half of the Ce³⁺ content through the XPS test. The emergence of the aging phenomenon is explained through the defect dipole reorientation mechanism. The larger radius of Ca²⁺ ions is further discussed as a possible reason for the rapid stability phenomenon of ferroelectric polarization. It may provide an effective design method from the viewpoint of the ionic radius to accelerate polarization stability, and thus to facilitate the possible practical applications of the aging effect.

Introduction

Ferroelectric materials are widely used as capacitors, transducers, sensors, actuators and refrigerant because of their extensive function effects such as dielectric response, piezoelectric, electrostrictive and electrocaloric effects^1,2,3,4. However, for the application of ferroelectric materials, most of these physical properties are subjected to changes with time (namely aging effect)⁵. On one hand, the aging effect is one of the most ineluctable obstacles to the reliability and stability of ferroelectric materials, because the properties like polarization reduces over time^5,6,7. On the other hand, some studies on the aging behaviour have also found that the utilization of the aging effect can lead to a very large recoverable electro-strain^8,9,10,11. Usually, such kind of aging-associated property requires an aging treatment. In either case, a long aging time (from hundreds of to thousands of hours) is necessary to ensure a stable performance^9,12,13,14. Thus there need much work to predict the lifespan of the physical properties.

Many researchers dedicate to study the lead zirconate titanate (PZT) family^12,15, the barium titanate (BT) family^{6,8,9,13,14,16,17,18,19,20,21,22} and some other lead-free compositions²³ to develop reasonable theories to explain the aging phenomenon. There are three theories that have been proposed: grain boundary effect, domain wall effect and volume effect^5,8. From these aging theories, the “strength” of aging effect can be controlled by choosing type of the doping ions and the base system^13,21. However, the effective ways or compositions to reduce aging time, and mostly importantly, to fast reach the stability time of the properties remain awaiting.

Ca and Ce are those chemical dopants which can be incorporated at the Ba-sites or at the Ti-sites in BaTiO₃ when sintered in air. Some researchers have investigated the aging behavior of the Ca doped, Ca, Mn hybrid doped or Ca, Zr hybrid doped BaTiO₃ ceramics^21,22,24,25. Yun et al. use the Raman spectrum at a high frequency peak about ~827 cm⁻¹ to show that, when the Ca²⁺ ions enter into Ti sites in BaTiO₃ and form defect dipoles, the aging phenomenon with double hysteresis loops will appear²⁴. Similar Raman results are evidenced by Puli et al. when Ca²⁺ ions enter into Ti sites in Bi-BCT system²⁵. While, for Ce doped BaTiO₃ ceramics, the researchers usually focus on their piezoelectric properties²⁶. Lu et al. show Raman evidence at the ~840 cm⁻¹ peak for the Ba-Site Ce³⁺ in BaTiO₃²⁷. Few study are about the Ca, Ce hybrid doped BaTiO₃ ceramics and their aging behavior.

In this paper, we report a rapid ferroelectric polarization stability phenomenon (only aged about 1 h) in the Ca, Ce hybrid doped BaTiO₃ ceramics (BCaxT+BTCe8). The rapid stability about hysteresis loops is discussed from a defect dipole reorientation mechanism and the ionic radius of the dopants. Our results may provide a possibility for accelerating aging treatment, and thus make the utilization of the properties associated with the aging effect more efficient, stable and securer.

Results and Discussion

XRD patterns results

Figure 1(a) showed the XRD patterns of the BCaxT+BTCe8 samples. Splitting peaks of the (002) and (200) appeared in all these samples. Such splitting illustrates the samples are illustrate the ferroelectric perovskite phases with the tetragonal structure at the room temperature. So all the aging-associated studies were performed at the room temperature where the samples are in tetragonal phases. The c/a ratio of the BCa20T+BTCe8 samples is 1.00575, showing the maximal c/a ratio value among all the BCaxT+BTCe8 samples.

Figure 1

The XRD patterns (a), the temperature dependence of relative permittivity at a frequency of 1 kHz (b) of the BCaxT+BTCe8 samples.

Phase transition point

Figure 1(b) showed the temperature dependence of the relative permittivity at a frequency of 1 kHz for the BCaxT+BTCe8 samples. As shown in Fig. 1(b), during cooling process, the first permittivity peak corresponds to the cubic–tetragonal phase transition, and the corresponding temperature is called Curie temperature (T_c); the temperature of the second permittivity peak represents the tetragonal–orthorhombic phase transition temperature (T_o). The T_c value is respectively 52.9, 61.4, 60.0 and 47.8 °C for BCa10T+BTCe8, BCa24T+BTCe8, BCa24T+BTCe8 and BCa30T+BTCe8 samples, all higher than room temperature. It is further found that the T_c increases firstly and then decreases with the increase of the Ca content. The maximum T_c value 61.4 °C appears at BCa20T+BTCe8 sample. The tetragonal–orthorhombic phase transition temperature (T_o) strongly decreases with the Ca concentration increasing. According to the previous reports about the effect of Ca on the transition temperature^28,29, most of Ca in our samples is successfully incorporated as Ca²⁺ into Ba sites in our samples, following the separate doping BCaxT samples.

Rapid ferroelectric polarization stability phenomenon

Ferroelectric property was shown in the Fig. 2. Figure 2(a–d) showed the measured P-E hysteresis loops of the BCaxT+BTCe8 samples before and after their aging treatment. Without aging, all the samples show the squared hysteresis loops; while obvious double loops are identified after the aging treatment. It is worth noting that all these samples with the similar P-E hysteresis loops characteristics show a rapid loop-shrinking along the P-axis within very short aging time 1 h (under E = 30 kV/cm). However, when continuously increasing aging time, the double loops remain almost unchanged. The polarizations present a same trend which have a sharp decrease and then become smooth. Thus an obvious rapid polarization stability phenomenon arises in our samples. Interestingly, Unlike the BCaxT+BTCe8 samples, the P–E loops of those separate doping samples including BCa20T and BTCe8 remain almost unchanged before and after aging treatment (as shown in inset of Fig. 2(b)). They show the squared hysteresis loops after aging 1 h, without the obvious aging phenomenon as the hybrid doped samples. This indicates that the hybrid doped BCaxT+BTCe8 samples may have some differences in the substitution sites from the separate doping samples BCaxT and BTCe8.

Figure 2

The measured hysteresis loops of the BCaxT+BTCe8 samples after aging 1 min, 10 min, 1 h, 12 h, 48 h and 1080 h (a–d), the value of P_max/P_max(1min) (e), and the value of P_r/P_r(1min) (f) of the BCaxT+BTCe8 samples changing with the logarithmic aging time. Inset of the figure (b) shows the measured hysteresis loops of the separate doping samples (represented by BCa20T and BTCe8) before and after aging 1 h at room temperature.

P_max/P_max(1min) and P_r/P_r(1min) were used to characterize the rapid ferroelectric polarization aging stability phenomenon more clearly, as shown in Fig. 2(e,f). In the Fig. 2(e), the degree of the switched domains when applied electric field at different logarithmic aging time was represented by P_max/P_max(1min). It can be seen that, a large and rapid decrease of the P_max/P_max(1min) in a short aging time (about 1 h, shown by the fourth point in the figure) appears at a rate of 15.8~26.0% per hour. And then, with the increase of the aging time, the P_max/P_max(1min) remains almost unchanged at a decreasing rate of 0.002~0.005% per hour for all the BCaxT+BTCe8 samples. This means the domain stabilization time is reduced to 1 hour in our study when applied electric field. Besides, with the increase of the Ca concentration, the decrement of P_max/P_max(1min) becomes smaller, which shows that the switched domain reduces when doping more Ca. In the Fig. 2(f), the degree of the switched domains when removed electric field at the different logarithmic aging time was represented by P_r/P_r(1min). One can see the same laws of P_r/P_r(1min) as the P_max/P_max(1min). That is to say, the samples present a rapid domain stabilization phenomenon (also about 1 h) when removed electric field.

For comparison, some previous works about the stability time of the dielectric properties (under small field signal) and the ferroelectric polarization (under large field signal) are summarized in Table 1^{9,12,13,14,23,30,31,32,33,34,35,36}. As shown in Table 1, for the dielectric properties, the aging stability time of the Fe, La doped BT, and the Fe doped PZT is respectively above 100 min (~1.7 h) and 1000 min (~16.7 h). Compared with this, for the ferroelectric polarization, the aging stability usually goes through a long time in previous reports. For example, for the Mn doped BST, Mn doped BT, Al doped BT, Cu doped KNN, and Fe doped PZT ceramics, the aging stability time is all above hundreds to thousands of hours. Also, there are some researchers have reported that in the Cu, Fe doped PT ceramics, the aging stability time is about hours to days according to the computational simulation results. All in all, the stability time 1 h of the ferroelectric polarization in our work has not been reported before.

Table 1 The stability time of the dielectric properties (under small signal) and the ferroelectric polarization (under large signal).

Raman spectra and XPS results

The BCaxT+BTCe8 samples were investigated by Raman spectroscopy to show the relationship between the lattice dynamics and the doping positions in this study. As shown in Fig. 3(a), there are four Raman bands in all the samples being similar to the reported single-crystal and ceramic BaTiO₃³⁷: A1(TO₂), B1+E(TO+LO), A1(TO₃), and A1(LO₃)+E(LO₃) with peaking at ~249, ~300, ~516, and ~722 cm⁻¹, respectively. Obviously, the higher frequency Raman band at ~840 cm⁻¹ appears in the spectra of all the BCaxT+BTCe8 samples, but does not appear in the separate doping samples (represented by BCa20T and BTCe8). This band at 830~840 cm⁻¹ was found in many inequitable doping BaTiO₃ systems such as La³⁺ ions, Nd³⁺ ions, Eu³⁺ ions, Ce³⁺ ions at Ba sites, and Ca²⁺ ions at Ti sites in the previous reports^{24,25,27,38,39,40}. According to previous works and the ion size factor in BaTiO₃ system, this mode at about ~840 cm⁻¹ is attributed to an internal deformation of the BO₆ octahedron caused by the charge difference of different types of ions at equivalent sites in BaTiO₃^{17,24,25,27,38,39,40,41}. Correspondingly, the weak peak at ~840 cm⁻¹ in BaTiO₃ doping with Ce may be attributed to Ce entering Ba sites as Ce³⁺, while it has no connection with Ce⁴⁺ at Ti sites²⁷. The weak peak in BaTiO₃ doping with Ca may be attributed to partial migration of Ca²⁺ from Ba to Ti sites^24,25. So in our study, the Raman test results indicate that Ca is successfully incorporated as Ca²⁺ into Ba sites, and that Ce is successfully incorporated as Ce⁴⁺ into Ti sites in the separate doping samples, while the Ca, Ce hybrid doped samples have different substitution sites. That is to say, Ce enters into Ba sites partly as Ce³⁺ or Ca enters into Ti sites partly as Ca²⁺ in the BCaxT+BTCe8 samples. Because of the multivalence of Ce³⁺/Ce⁴⁺ at Ba/Ti sites showing different binding energy in the XPS spectra^42,43, we can distinguish the Ce³⁺ peak and the Ce⁴⁺ peak from the Ce3d XPS spectra. Meanwhile, the area ratio of these two peaks can also be obtained. Then from the XPS quantification report, we can know the total molar fraction of the Ce3d. Thus the exact Ce³⁺ content in the BCaxT+BTCe8 samples can be got through the area ratio of the Ce³⁺ and Ce⁴⁺ peak and the total molar fraction of the Ce3d. As show in Fig. 3(b), with the Ca content increasing, the Ce³⁺(3d) XPS content is 0.17, 0.37, 1.05, and 0.70 mol% respectively for the BCaxT+BTCe8 samples.

Figure 3

Raman spectra of the BCaxT+BTCe8 samples and the separate doping samples (represented by BCa20T and BTCe8) after aging 1 h (a), the Ce³⁺ (3d) XPS content with the Ca content increasing (b). The inset in (b) shows the Ce3d XPS spectra and its Lorentzian dividing results for the BCa10T+BTCe8 samples.

Explanation of the emergence of the aging phenomenon based on the charge balance and chemical composition balance

Considering the above research results (obvious double hysteresis loops after aging and the weak 840 cm⁻¹ Raman band), we will propose here four possible situations in the BCaxT+BTCe8 samples as shown in Fig. 4(a), which can explain the emergence of the aging phenomenon in our samples. Firstly, one Ca²⁺ ion is substituted for A-site Ba²⁺ ions, meanwhile one Ce⁴⁺ ion is substituted for B-site Ti⁴⁺ ions. According to the previous reports about Ba(Ti,Ce)O₃²⁶, the Ce⁴⁺ at Ti sites can help to stabilize the Curie temperature (T_c) (as evidenced in Fig. 1(b)). If all the Ce is incorporated at the Ba sites as Ce³⁺, the T_c would drop. And when the T_c reduces to below the room temperature, the ferroelectric aging effect will disappear due to the sample presenting a paraelectric phase rather than a ferroelectric phase at room temperature. Secondly, one Ce³⁺ ion is substituted for A-site Ba²⁺ ions, meanwhile one Ca²⁺ ion is substituted for B-site Ti⁴⁺ ions. Thus a positive charge is lack to meet the charge balance. Thirdly, one Ce³⁺ ion is substituted for A-site Ba²⁺ ions, so that a positive charge is unnecessary for the charge imbalance. The second and the third situation should be neighbors for charge compensation. Fourthly, from the chemical composition balance, there remains a Ca²⁺ ion. The remaining Ca²⁺ ion is substitute for B-site Ti⁴⁺, causing the formation of one oxygen vacancy to balance the charge misfit. Thus the fourth situation about the formation of oxygen vacancy is considered to induce the obvious aging effect in our study.

Figure 4

The four possible situations (a) (Situation1: one Ca²⁺ ion is substituted for A-site Ba²⁺ ions, meanwhile one Ce⁴⁺ ion is substituted for B-site Ti⁴⁺ ions; Situation 2: one Ce³⁺ ion is substituted for A-site Ba²⁺ ions, meanwhile one Ca²⁺ ion is substituted for B-site Ti⁴⁺ ions; Situation 3: one Ce³⁺ ion is substituted for A-site Ba²⁺ ions; Situation 4: one Ca²⁺ ion is substituted for B-site Ti⁴⁺ ions causing the formation of one oxygen vacancy) and the internal electrical field E_i (b) of the BCaxT+BTCe8 samples after aging 1 min, 10 min, 30 min, 1 h, 2 h, 6 h, 12 h, 48 h and 1080 h.

Besides, the quantitative information of oxygen vacancies can be obtained from above proposed situations. Considering the second, the third and the forth situation, if there are two Ce³⁺ ions are substituted for A-site Ba²⁺ ions, there must be two Ca²⁺ ions are substituted for B-site Ti⁴⁺ ions. One of the Ca²⁺ ions is for charge balance; another remaining Ca²⁺ ion which causes the formation of one oxygen vacancy is for the chemical composition balance. Therefore, the oxygen vacancy generated by the Ca²⁺ ions substitution at the B-site can be quantitatively characterized by half of the Ce³⁺ content. So the oxygen vacancy content generated by the Ca²⁺ ions substitution at the B-site is 0.085, 0.185, 0.525, 0.35 mol% respectively with the Ca content increasing from the Ce³⁺ (3d) XPS content in the Fig. 3(b). Thus we can quantitatively characterize the content of oxygen vacancies through charge balance, chemical composition balance, and half of the Ce³⁺ content by XPS test.

The double hysteresis loops related to the aging phenomenon could be well explained by the defect dipole reorientation mechanism, followed by the recent symmetry-conforming short-range ordering (SC-SRO) principle^8,44,45. As the fourth situation mentioned, defect dipoles form in the unit cell. After aging, the defect dipoles align along the spontaneous polarization (P_S) direction because of the oxygen vacancy migration; and the aligned defect dipoles form defect dipole polarization (P_D)^8,16,17. According to the SC-SRO principle, the defect dipoles and the associated P_D cause reversible domain switching, and the double hysteresis loops associated with aging are observed under the periodic electric-field drive. Correspondingly, during the P−E loop measurement, the un-switched defect symmetry and associated defect dipoles generate an internal electrical field E_i for stabilizing the domain pattern. The E_i is calculated by the equation of E_i = (E₁ + E₂)/2, where E₁ and E₂ are the peak field of forward and backward domain switching processes⁴⁶. In our study, E_i was calculated respectively by the loops. As shown in Fig. 4(b), the defect dipoles field E_i build up in all BCaxT+BTCe8 samples. All the E_i present a large and rapid increase over the logarithmic function of aging time in the initial period (about 1 h, shown by the fourth point in the figure), and then saturates to a stable value. This result about E_i and P_D is consistent with the laws of P_max and P_r in sense of the rapid aging stability.

Origin of the rapid ferroelectric polarization stability phenomenon

Then the causes of the rapid ferroelectric polarization stability phenomenon should be discussed. Due to the size of the oxygen octahedral gap having great influence on the movement of oxygen vacancy, the ionic radius is considered to be an important factor on aging stability phenomenon in this study. Here we compare the aging effect of our samples with other acceptor-doped ions in BaTiO₃ systems. We easily find that the aging stability time of polarization (about 1 h) of the BCaxT+BTCe8 samples in our study is much shorter than those reported value in doping with Mn samples (hundreds of hours)^13,14. Here we propose a possible reason for this phenomenon. Although the similar c/a ratios between our samples and the Mn doped samples indicate the similar thermodynamic driving force for the migration of oxygen vacancy¹³, the kinetic migration condition of oxygen vacancy in the oxygen octahedral of these systems is different due to the ionic size difference between Ca and Mn. The similar thermodynamic driving force leads to the initial aging effect in these two systems, and then the driving force reduces due to the redistribution of oxygen vacancy after the initial aging. With the aging time further increases, the reduced force is not large enough for more oxygen migration as to overcoming the barrier from the Ca²⁺ ions, as the radius of Ca²⁺ ions (R_Ca²⁺=1 Å) is larger than the Mn³⁺ ions (R_Mn³⁺=0.645 Å). Therefore, the migration is stopped soon and the aging reaches stabilization soon in our hybrid doped samples. Thus the larger ionic radius is a possible reason for the rapid ferroelectric polarization stability phenomenon. More experimental data and computational simulation can be used to verify these possible causes.

Conclusion

In summary, the ferroelectric polarization aging behavior of the Ca, Ce hybrid doped samples (BCaxT+BTCe8) were investigated in this study by the measurement of the hysteresis loops (under E = 30 kV/cm) and corresponding characterization parameters P_max/P_max(1min), P_r/P_r(1min) and the internal electrical field E_i changing over the logarithmic function of aging time. Obvious double loops were identified in the aged BCaxT+BTCe8 samples. Meanwhile, these hybrid doped samples show a rapid ferroelectric polarization stability phenomenon only after aging about 1 h, which is not reported in previous researches. Raman scattering spectrum indicates that the Ce enters into Ba sites partly as Ce³⁺ or Ca enters into Ti sites partly as Ca²⁺ in the BCaxT+BTCe8 samples. And through the XPS test, the exact Ce³⁺ content in the BCaxT+BTCe8 samples is obtained. Based on the charge balance and chemical composition balance, four possible situations in the BCaxT+BTCe8 samples are proposed: (1) one Ca²⁺ at Ba sites, meanwhile one Ce⁴⁺ at Ti sites; (2) one Ce³⁺ at Ba sites, meanwhile one Ca²⁺ at Ti sites; (3) one Ce³⁺ ion at Ba sites; (4) one Ca²⁺ ion at Ti sites. The fourth situation about the formation of oxygen vacancy because of the Ca²⁺ ions partly entering into Ti⁴⁺ sites induces obvious aging effect. The oxygen vacancies are quantitatively characterized by half of the Ce³⁺ content through XPS test. The content is 0.085, 0.185, 0.525, 0.35 mol% respectively with the Ca content increasing. The emergence of aging phenomenon is explained through a defect dipole reorientation mechanism. Finally, the larger radius of Ca²⁺ ions is considered as a possible reason for the rapid polarization aging stability phenomenon. Our results may provide a possibility for accelerating aging treatment and thus make the utilization of the properties associated with the aging effect more efficient, stable and securer.

Methods

Preparation

The Ca, Ce hybrid doped BaTiO₃ samples were prepared by separate doping Ca and Ce using a conventional solid state reaction method in this study. The separate doping samples Ba(Ti_0.92Ce_0.08)O₃ (denoted as BTCe8) and (Ba_1-xCa_x)TiO₃ (x = 10, 20, 24, 30 mol%) (denoted as BCaxT) used the highly pure BaCO₃ (99.9%), TiO₂ (99.9%), as well as the analytically pure CeO₂ (99.9%) and CaCO₃ (99.9%) as starting materials. These highly pure and analytically pure chemicals are all from Alfa Aesar company. The hybrid doped BTCax + BTCe8 samples used the pre-sintered separate doping BTCe8 and BCaxT powders as starting materials.

Preparation method was as follows: (1) BTCe8 and BCaxT: the BaCO₃, TiO₂ and CeO₂ (BTCe8) or the BaCO₃, TiO₂ and CaCO₃ (BCaxT) were mixed and planet ball-milled in ethanol with agate ball mill media for 6 h and then dried. Then the mixture was calcined at 1300 °C for 4 h and then ball-milled for another 6 h and dried again; (2) BCaxT+BTCe8: through the step of (1), the calcined separate doping powders were gained, then these powder (for example BTCe8 and B20CaT) were used as the starting materials, then were planet ball-milled and mixed in proportion, and dried. The dried powder was added PVA aqueous solution (10 weight%) and then pressed into pellets with diameter of 12 mm. Sintering was done at 1400 °C with the holding time of 4 h in air.

Characterization

The X-ray diffraction (XRD, X’Pert diffractometer (D8 Advance, Germany) with Cu Kα λ-1.5406 Å) was used to determine the phase structure of the ceramics at room temperature. Temperature dependence of the dielectric constant was measured from −150 to 200 °C at a frequency of 1k Hz. The time-dependent P-E hysteresis loops were characterized by Precision Premier II from Radiant Company, together with a high voltage amplifier. For the aging study, the samples were de-aged by heating to 200 °C for 2 h and then cooling to the room temperature to do the aging treatment. The aging time here was from several minutes to 1080 h. Hereafter the aging time of 1 min is denoted as the fresh sample state. Raman spectra were measured at room temperature using a HR800 Raman spectrometer (Horiba Jobin Yvon) with the 633 nm excitation. The X-ray photoelectron spectroscopy (XPS) spectra were obtained by Axis Ultra (UK) using monochromatic Al K_α (150W, 15 kV, 1486 eV).