3.1 Crystal structure
Figure 1(a) shows the room temperature XRD patterns of (1- x) Bi0.85Nd0.15Fe0.98Zr0.02O3- xBaTiO3 (BNFZ-xBT, x = 0, 0.275, 0.3, 0.325, 0.35, 0.375, 0.4) ceramics in the 2θ range of 20° − 80°. The XRD pattern of Bi0.85Nd0.15Fe0.98Zr0.02O3 ceramic agrees very well with those earlier reports, pointing out that the rhombohedral R3c symmetry.[31] Due to the volatility of the Bi element, an impurity peak is detected in the undoped sample indicating the formation of the secondary phase of Bi2Fe4O9 (asterisk in Fig. 1 (a)). Fortunately, the introduction of BaTiO3 effectively stabilizes the BFO structure and inhibits the generation of the second phase.[30] Fig. 1(b) shows the (104) and (110) peaks shift toward low angles slightly with the content of BaTiO3 increasing and then superpose together forming one diffraction peak (110) for the doped samples in the 2θ range of 31° − 33°, which suggests that the ceramics undergo a phase transition.[32] The same situation is found in the 2θ range of 44° − 48° in Fig. 1(c). [33]
Detailed analysis of the crystal structure has been performed using the Rietveld method. Figure 2(a-b) shows the results of the Rietveld refinement for x = 0 and 0.275 ceramics. The refinements of the other samples are found to have the similar results with x = 0.275 sample. According to the results, the crystal structure of the sample changed from rhombohedral phase R3c to a pseudo-cubic structure.[34] The difference curves at the bottom are nearly flat and the reliable factors are reasonable indicating the results of the refinement are credible. The structure parameters of all the ceramics are shown in Table 1. It is obvious to see the lattice parameters increase with the increase of BaTiO3 content (0.275 ≤ x ≤ 0.4). This is because the radius of Bi3+ (1.03 Ǻ) is much smaller than that of Ba2+ (1.35 Å) while the ionic radius of Fe3+ (0.64 Å) is close to that of Ti4+ (0.608 Å). The expansion of the lattice also indicates that the BaTiO3 successfully entered into the crystal lattice.[34]
Table 1
The lattice parameters obtained by Rietveld refinement for (1- x) Bi0.85Nd0.15Fe0.98Zr0.02O3- xBaTiO3 (x = 0, 0.275, 0.3, 0.325, 0.35, 0.375 and 0.4) ceramics.
x
|
a/Å
|
b/Å
|
c/Å
|
V/Å3
|
ρ/g·cm− 3
|
Rwp
|
χ2
|
0
|
5.5876
|
5.5876
|
13.8372
|
374.141
|
8.12
|
17.4
|
1.94
|
0.275
|
3.9729
|
3.9729
|
3.9791
|
62.809
|
7.55
|
11.7
|
1.54
|
0.3
|
3.9759
|
3.9759
|
3.9826
|
62.957
|
7.48
|
13.3
|
1.53
|
0.325
|
3.9781
|
3.9781
|
3.9839
|
63.017
|
7.43
|
11.2
|
1.63
|
0.35
|
3.9825
|
3.9825
|
3.9845
|
63.192
|
7.36
|
11.4
|
1.47
|
0.375
|
3.9859
|
3.9859
|
3.9845
|
63.303
|
7.30
|
13.9
|
1.42
|
0.4
|
3.9881
|
3.9881
|
3.9848
|
63.376
|
7.25
|
14.9
|
1.62
|
3.2 Microstructure
The microstructural morphologies of (1- x)Bi0.85Nd0.15Fe0.98Zr0.02O3- xBaTiO3 (x = 0, 0.275, 0.3, 0.325, 0.350, 0.375 and 0.4) ceramics performed by SEM are shown in Fig. 3. The average grain size shown in Fig. 3(h) was measured using the IMAGE-J by the linear intercept method. The relative density of BNFZ - xBT ceramics are 96.25%, 94.08%, 96.44%, 96.64%, 99.80%, 99.04% and 97.99% for x = 0, 0.275, 0.300, 0.325, 0.35, 0.375 and 0.4 specimens, respectively. As the doping ratio increasing, the relative density of the ceramics is improved while excessive BaTiO3 content deteriorates the density. It can be seen that some obvious pores appear in Fig. 4(a) and 4(b), but not found in the other ceramics. The SEM results indicate that introduction of BaTiO3 into BNFZ has an effect on promoting grain growth and densification. One can observe that the average grain size increases as the BaTiO3 content increasing. With further increasing content of BaTiO3, the average grain size of samples seems not to determine a further increase.
3.3 Ferroelectric properties
Figure 4(a) shows the P-E loops of (1- x)Bi0.85Nd0.15Fe0.98Zr0.02O3- xBaTiO3 (x = 0, 0.275, 0.3, 0.325, 0.35, 0.375 and 0.4) ceramics measured under an electric field of 50 kV/cm. Figure 4(b) shows the BaTiO3 content dependence of the remnant polarization 2Pr of all the ceramics. Compared with the pure BiFeO3, the ferroelectric properties are improved by the Nd3+ and Zr4+ co-doping in Bi0.85Nd0.15Fe0.98Zr0.02O3. As shown in Fig. 4(a) and 4(b), with an further increase in the content of BaTiO3, the values of 2Pr are 0.496 µC/cm2, 0.782 µC/cm2, 0.922 µC/cm2, 0.966 µC/cm2, 1.108 µC/cm2, 1.146µC/cm2 and 1.524 µC/cm2, respectively. It can be seen that the Pr and average grain size have the same change trend which is consistent with the studies before.[35] The ceramics with large grains always exhibit better ferroelectric characteristics. With the grain size increasing, the grain boundary of the sample decreases, which makes the ferroelectric domain more easily flipped under the external electric field.[36] In addition, when BaTiO3 is doped, the Ti-O bonds are established and the Fe-O bonds are disrupted. Owing to the long-range Ti displacement, the samples with more content of BaTiO3 show better ferroelectric properties.
Figure 5 shows the leakage current density (J) versus the applied electric field of all the ceramics. Compared with the previous studies, all the samples have a low leakage current density which is less than 5.9×10− 5 A/cm2 for pure BFO ceramics at an applied electric field of 6 kV/cm.[37] It is evident that the leakage current density decreases with BaTiO3 introduced. A significant decrease for approximately two orders of magnitude is observed for x = 0.375 sample. The improved J value might be associated with the decrease of oxygen vacancies caused by the addition of BaTiO3. The electrons trapped in the oxygen vacancies can be easily activated for conduction by an applied electric field, thus increasing the J value.[8] With the BaTiO3 substitution, the Ba-O bond in the ceramics is hard to destroy compared to the Bi-O bond which greatly inhibits the volatility of Bi element. Besides, the improved morphology and structure transition are also beneficial to suppress the leakage current density. With a further increase of x, the J value increases a little. This is because that the extra electrons introduced by the excess BaTiO3 join in the electronic conduction.
To further understand the roles of Bi and Fe valences on the ferroelectric properties, a detailed X-ray photoelectron spectra analysis for Bi 4f and Fe 2p lines of all the ceramics is shown in Fig. 6. As shown in Fig. 6(a), the peaks around 158.3 and 163.7 eV correspond to the Bi 4f7/2 and Bi 4f5/2 respectively for all compositions. This indicates that the Bi3+ stably exists in the ceramics. The core spectra of the Fe 2p spectrum lines are shown in Fig. 6(b). The peak around 709.5 eV corresponds to the state of Fe2+ and peaks around 723.5 eV and 711.0 eV correspond to the state of Fe3+. As the BaTiO3 content increases, the percentages of Fe3+ ions are 52.3%, 56.8%, 57.2%, 57.5%, 58.2%, 59.0% and 55%, respectively. It is obvious to see that the addition of BaTiO3 increases the Fe3+ concentration which means fewer oxygen vacancies exist in the ceramics. Namely, the Fe2+ formation is suppressed, thus facilitating the improvement of resistivity. This result is consistent with those of the ferroelectric property and the leakage current density. The fewer oxygen vacancies can increase the domain-wall mobility which is beneficial to improve the ferroelectric properties.[38]
3.4 Dielectric properties
The temperature-dependence of dielectric constant (εr) and dielectric loss (tanδ) were measured at 1 kHz and 100 kHz of all the samples from room temperature to 400°C shown in Fig. 7. Note that the dielectric constants and the loss tanδ at 1 kHz are larger than that measured at 100 kHz. This could be attributed to the different polarization mechanisms which include electronic displacement polarization, space charge polarization and orientation polarization. At higher frequencies, only the electronic displacement polarization is effective to the dielectric constant while all the mechanisms are effective at lower frequencies. Another reason for the decrement in dielectric constant is that the high frequency provides sufficient energy for the electron hopping between Fe2+ and Fe3+ ions.[39] In Fig. 7(a), the maximum dielectric constant is obtained for x = 0.4 ceramic at the temperature 329°C (Tm) which corresponds to the Néel temperature (TN) caused by the magnetic ordering transition from antiferromagnetism to paramagnetism. This phenomenon usually indicates the coupling relationship between the magnetic and the electric order parameters. The anomaly near TN is always ascribed to the Landau-Devonshire theory of phase transition.[40, 41] With the increase of BaTiO3 concentration, the dielectric maximum peak shifts gradually toward higher temperature. Further increasing the frequencies in Fig. 7(b), the TN is out of the observation range.
As shown in Fig. 7(a) and 7(b), all the dielectric constants present a similar response trend with respect to the temperature and approach a saturated platform except the x = 0 sample. Compared with the undoped sample, all the other ceramics exhibit a dielectric relaxation character.[39, 42] In addition to the broaden peak of the εr-T peak, the dielectric constant values are slightly decreased and the peak position of εr also changed toward the high temperature with the frequency increasing.[43] It is evident from Fig. 7(a) - (d) that the dielectric constant increases and the dielectric loss decreases dramatically as the BaTiO3 introduced. The improvement can be attributed to the excellent dielectric properties of BaTiO3 which is caused by the large electronegativity difference and tolerance factor between Ba2+ and Ti4+ ions.[44] Another possible cause of the improvement in dielectric property is the oxygen vacancies are suppressed by the addition of BaTiO3. The oxygen vacancies produced by Bi volatilization always result in high conductivity and poor dielectric properties. Doping of Ba2+ and Ti4+ can stabilize the perovskite structure and suppress the defects thus optimize the εr and tanδ value. Simultaneously, the electrons hopping between Fe3+ and Fe2+ ions are weakened because of the suppression for the valence fluctuation from Fe3+ to Fe2+ by BaTiO3 doping, which is in accord with the XPS results shown in Fig. 6.
3.5 Magnetic properties
The magnetization hysteresis loops of (1- x)Bi0.85Nd0.15Fe0.98Zr0.02O3- xBaTiO3 (x = 0, 0.275, 0.3, 0.325, 0.35, 0.375 and 0.4) ceramics for the maximum magnetic field of 30 kOe are shown in Fig. 8. In comparison with the pure BiFeO3 ceramic,[45] the Bi0.85Nd0.15Fe0.98Zr0.02O3 doped with Nd3+ and Zr4+ already exhibits weak ferromagnetism with a lower coercivity. Pure BeFeO3 always presents a G-type anti-ferromagnetic structure due to the space modulated spin ordering of Fe3+ ions[34]. The substitution of Fe3+ ions by the nonmagnetic ion Zr4+ generates the unbalance antiferromagnetic structure thus the spiral arrangement of Fe3+ is disrupted resulting in net macroscopic magnetization. With further doping of BaTiO3, a decrease in magnetization has occurred. The decreased magnetization in Ba2+ and Ti4+ substituted Bi0.85Nd0.15Fe0.98Zr0.02O3 might be caused by the size effect. As shown in Fig. 3(h), the grain size increases as x increasing. The increase in size would decrease the spin canting of magnetic moments at the particle surface, which decreases the Dzyaloshinskii-Moriya (DM) interaction resulting in a lower magnetization.[46] In addition, with BaTiO3 doping, the crystal structure transformed from rhombohedral structure to pseudo-cubic structure which has an untilted FeO6 octahedra. The Fe-O-Fe bond angle is nearly 180° and the strong antiferromagnetic interaction is induced between the neighboring Fe spins, therefore the magnetization is reduced. After the incorporation of BaTiO3, the remnant magnetization and coercivity values first increase and then decrease as the BaTiO3 concentration increases. For x = 0.325, the sample obtains the maximum remnant magnetization (0.055 emu/g) and the coercive field (Hc = 1785.8 Oe).