Pure KNN and 0.98KNN-0.02LF ceramics were prepared using the conventional solid-state sintering method. Reagent-grade oxide and carbonate powders of K₂CO₃, Na₂CO₃, Nb₂O₅, La₂O₃and Fe₂O₃were used as the starting materials. Before being weighed, these powders were first separately dried in an oven at 110℃ for 5 h. They were milled for 24 h using planetary milling with zirconia ball media and alcohol. After drying, the mixed powder was calcined in an alumina crucible at 950℃ for 5 h. These calcined powders were ball-milled again for 12 h, then dried and pressed into disks of 12 mm in diameter and 1mm in thickness under 300 MPa using polyvinyl alcohol (PVA) as a binder. After burning off PVA, the pellets were finally sintered at 1120-1150℃ for 2 h in the sealed Al₂O₃ crucibles. The obtained samples were polished. Silver paste was fired on both sides of the samples at 810℃ for 20 min as the electrodes for the sake of measurements.

The phase structures of the sintered ceramics were examined using X-ray powder diffraction analysis with a Cu Kα radiation (Philips χ’Pert ProDiffractometer, Almelo, and The Netherlands) at room temperatures. The dielectric spectrum measurements were performed using the LCR meter (Agilent E4980, USA) at a heat rate of 3 ℃/min in a temperature range of 0-520℃ and a frequency range of 1-1000 kHz. The polarization-electric field ( P- E) hysteresis loops were evaluated by a Sawyer-Tower circuit at room temperature.

Figure 1 shows the X-ray diffraction (XRD) patterns of pure KNN and 0.98KNN-0.02LF ceramics at room temperature (27℃). As can be seen from these patterns, all samples show a pure perovskite phase and no secondary phase could be certified. This indicates that LaFeO₃ (LF) has completely diffused into the KNN lattice to form new solid solutions. The insert of Fig. 1 shows that the diffraction peaks are split at about 45° for pure KNN, which is the orthorhombic structure^{[ 23]}. However, the structure is a pseudo-cubic perovskite structure for 0.98KNN-0.02LF ceramics. The split peaks around 45º for orthorhombic symmetry were gradually combined into a single peak. The calculated lattice parameters for pure KNN are a=0.564 nm, b=0.394 nm, and c=0.567 nm, respectively, and those for and 0.98KNN-0.02LF ceramics are a= b=0.398 nm, and c=0.399 nm, respectively. This can be explained as follows: the Fe³⁺ ions enter into the six fold coordinated B-site owing to the ion radius (0.065 nm) which is very close to that of Nb⁵⁺ ionic (0.064 nm), and the La³⁺ionic (0.118 nm) enter A-site of the perovskite structure to substitute for (Na_0.5K_0.5)⁺ because of radius matching, and which gave rise to a small shrinkage of the cell volume.

Fig. 1

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	Fig. 1 XRD patterns of pure KNN and 0.98KNN- 0.02LF ceramics sintered at 1120℃ and 1150℃ for 2 h

Figure 2 provides the temperature dependent dielectric permittivity (a) and dielectric loss (b) at 10 kHz from room temperature to 500℃ for pure KNN and 0.98KNN-0.02LF ceramics. For pure KNN ceramics, two sharp dielectric peaks can be observed at 200℃ and 410℃, which correspond to the ferroelectric orthorhombic- tetragonal polymorphic phase transition ( T_O- T) and the tetragonal-cubic transition ( T_c) temperatures. The above analysis is in agreement with the previous reports^{[ 24, 25, 26]}. The dielectric permittivity curves of 0.98KNN-0.02LF ceramics present two anomalies compared with pure KNN: (i) the orthorhombic-tetragonal transition ( T_o-T) and the tetragonal-cubic transition ( T_{T -c}) shifts down to lower temperatures; (ii) The phase transition temperature range around T_m becomes more and more broad, which can be called the broad dispersive dielectric maximum. In addition, it can be observed that 0.98KNN-0.02LF ceramics exhibit a very stable temperature dependence of dielectric permittivity with permittivity maximum near 2000 in the temperature range of 0-400℃. While the tan δ of 0.98KNN-0.02LF ceramics is slightly less than the pure KNN in the range of temperature investigated. The dielectric loss of 0.98KNN-0.02LF ceramics is less than 4% in the temperature range of 0-400℃. The results indicates that 0.98KNN-0.02LF ceramics may have great potential for high temperature capacitors applications.

Fig. 2

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	Fig. 2 Temperature dependence of dielectric permittivity (a) and dielectric loss (b) for pure KNN and 0.98KNN-0.02LF ceramics sintered at 1120 and 1150℃ for 2 h (measured at 10 kHz)

Figure 3 shows the temperature dependence of the dielectric permittivity of pure KNN and 0.98KNN-0.02LF ceramics under various measuring frequencies. For samples of pure KNN ceramics, no obvious frequency dispersion is observed. These results indicate that pure KNN is a normal ferroelectric. However, for samples of 0.98KNN-0.02LF ceramics, a strong frequency dispersion of the dielectric permittivity is clearly found through all the measured temperature range. The temperature T_m corresponding to the maximum value of the dielectric constant shifted to higher temperature and the maximum value of the dielectric constant decreased with increasing frequencies. Therefore, it can be concluded that 0.98KNN-0.02LF ceramics are lead-free relaxor ferroelectrics. The relaxor behavior in 0.98KNN-0.02LF ceramics should be attributed to a cationic disorder induced by both A-site and B-site substitutions, such as Fe³⁺ ionic enter into the six fold coordinated B-site, and the La³⁺ionic enter A-site of the perovskite structure to substitute for (Na_0.5K_0.5)⁺. Heterovalent substitution induces quenched random electric fields (RFs) owing to the local charge imbalance and the local elastic fields in KNN-derived systems. The fields hinder long-range ordering and give rise to polar nano-regions (PNRs). The dielectric properties of relaxors are believed to result from the complex response of all the PNRs and matrix^{[ 27]}.

Fig. 3

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	Fig. 3 Temperature dependence of dielectric permittivity for pure KNN and 0.98KNN-0.02LF ceramics under various measuring frequencies

It is well known that the dielectric constant of a normal ferroelectric should follow the Curie-Weiss law when the temperature exceeds the Curie temperature.

where Cis the Curie-Weiss constant and T_CW is the Curie-Weiss temperature. The plot of the inverse dielectric constant vs temperature at 10 kHz is shown in Fig. 4. It is found that the dielectric permittivity of pure KNN ceramics well obeys the Curie-Weiss law above the Curie temperature. However, the dielectric permittivity of 0.98KNN-0.02LF ceramics obviously deviates from the Curie-Weiss law above the Curie temperature. The deviation from the Curie-Weiss law can be defined by Δ T_m as follows:

Where T_B denotes the temperature from which the dielectric permittivity starts to follow the Curie-Weiss law. This temperature is referred as the Burns’ temperature. Generally, T_B is determined from the inverse dielectric permittivity. It can be observed in Fig. 4 that the T_B is 392℃ for the 0.98KNN-0.02LF ceramic. The T_m represents the temperature at which the dielectric permittivity reaches the maximum. It is found that Δ T_m increases from 0℃ for the pure KNN ceramics to 192℃ for the 0.98KNN-0.02LF ceramic. This indicates that the relaxor behavior have been enhanced in 0.98KNN-0.02LF ceramic.

Fig. 4

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	Fig. 4 Inverse dielectric permittivity at 10 kHz as a function of temperature for pure KNN and 0.98KNN-0.02LF ceramics (the symbols: experimental data; solid line: fitting to the Curie-Weiss law)

Figure 5 shows the typical P- Ehysteresis loops (at 1 Hz) of pure KNN and 0.98KNN-0.02LF ceramics measured at room temperature. One can see that the pure KNN ceramics has well saturated loop, whereas that 0.98KNN-0.02LF ceramics shows relatively slim one and the remnant polarization decreased. It is well known that another typical characteristic of relaxor ferroelectrics is a slim P- Eloop. A slimmer hysteresis loop observed for 0.98KNN-0.02LF ceramics further confirms the relaxor behavior in 0.98KNN-0.02LF ceramics.

Fig. 5

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	Fig. 5 P- Ehysteresis loops (at 1 Hz) of pure KNN and 0.98KNN-0.02LF ceramics measured at room temperature

In conclusion, this work has demonstrated that LF-doping can improve the broadness of dielectric permittivity peak for KNN ceramics. 0.98KNN-0.02LF ceramics exhibited a very stable temperature dependence of dielectric permittivity with permittivity maximum near 2000 and dielectric loss less than 4% in the temperature range of 0-400℃. The result indicates that 0.98KNN-0.02LF ceramics may have great potential for high temperature capacitors applications. The relaxor nature of the 0.98KNN-0.02LF ceramics is attributed to the appearance of polar nanoregions. These results also confirm that the KNN-based relaxor ferroelectrics can be regarded as a new direction for high temperature lead-free relaxor ferroelectrics.