Structure evolution, dielectric, and conductivity behavior of (K0.5Na0.5)NbO3-Bi(Zn2/3Nb1/3)O3 ceramics

(1−x)K0.5Na0.5NbO3−xBi(Zn2/3Nb1/3)O3 ((1−x)KNN−xBZN, x = 0.010, 0.015, 0.020, 0.025, and 0.030) lead-free ceramics were fabricated via a traditional solid-state method. The crystal structure, microstructure, dielectric, and conductivity behavior of this system were studied. Combined with X-ray diffraction (XRD) patterns, Rietveld refinement, and dielectric spectroscopy, an orthorhombic phase was determined for x = 0.010, an orthorhombic-tetragonal mixed phase was identified for x = 0.015, and a rhombohedral symmetry appears in 0.020 ⩽ x ⩽ 0.030. Both 0.98KNN−0.02BZN and 0.975KNN−0.025BZN ceramics exhibit stable permittivity and low dielectric loss tangent (tanδ) in wide temperature ranges owing to the combination of rhombohedral-tetragonal step-like feature and the diffuse phase transition from tetragonal to cubic. The activation energies of dielectric relaxation and conductivity behavior at high temperatures initially decrease slightly, then drop sharply, and finally decline slowly, which could be attributed to microstructure morphologies and the concentration of oxygen vacancies.


Introduction 
Ceramic capacitors as essential electronic components have been largely applied in a variety of fields such as mobile communication, automotive electronics, and military devices [1,2]. Currently, some extreme working conditions including engine control and oil logging

Experimental
(1−x)K 0.5 Na 0.5 NbO 3 -xBi(Zn 2/3 Nb 1/3 )O 3 ((1−x)KNN-xBZN, x = 0.010, 0.015, 0.020, 0.025, and 0.030) lead-free ceramics were fabricated via a traditional sintering method. The raw oxides and carbonates include Na 2 CO 3 (99.99%), K 2 CO 3 (99.99%), Nb 2 O 5 (99.99%), Bi 2 O 3 (99.5%), and ZnO (99.0%) (Shanghai Aladdin Bio-Chem Technology Co., Ltd., China). These powders were first fired to remove moisture at 120 ℃ for 24 h and weighted based on the (1−x)KNN-xBZN ratios, then mixed by a ball mill for 6 h and calcined at 900 ℃ for 4 h at a heating rate of 5 ℃/min. Next, the calcined powders were ball re-milled for 6 h, and dried and ground by adding 5 wt% polyvinyl alcohol (PVA) binder. Finally, the obtained powders were pressed to form discs of 10 mm in diameter under about 350 MPa and the discs surrounded with the powders of the same composition were sintered at 1100-1120 ℃ for 4 h at a heating rate of 5 ℃ /min in alumina crucible, followed by being cooled down with the furnace.
The diameter of sintered ceramics measured by the vernier caliper is 9.04, 9.05, 9.03, 9.12, and 9.13 mm for x = 0.010, 0.015, 0.020, 0.025, and 0.030, respectively. The density was recorded by the Archimedes method. The room-temperature crystal structure was identified using an automatic diffractometer (X'Pert PRO, PANalytical, the Netherlands) with Cu Kα radiation. The crystalline parameters were calculated by the GSAS software. To observe the surface microstructure, the sintered discs were thermally etched at 1000 ℃ for 30 min and then coated with gold to enhance their conductivity. The microstructure morphologies were gained by a scanning electron microscope (SEM, JSM 6380, Japan). Before dielectric measurement, the discs with x = 0.010, 0.015, 0.020, 0.025, and 0.030 were polished to 0.70, 0.75, 0.73, 0.72, and 0.70 mm in different thicknesses, respectively, and painted with silver paste on both surfaces, and then warmed up to 650 ℃ for 30 min to generate electrodes. The ε', tan, and impedance data were collected by the means of an impedance meter (Agilent 4294A, USA) in a frequency range of 40 Hz−1 MHz from room temperature (RT) to 550 ℃. ceramics at RT from 20° to 70°. All samples present a single perovskite structure except that the samples with x = 0.010 and 0.015 include an impurity phase K 6 Nb 10.8 O 30 (ICDD PDF Card No. 01-070-5051), suggesting that BZN has entered into the lattice to produce a stable solid solution. Moreover, the enlarged XRD patterns from 44° to 47° in Fig. 1(b) show that a trace amount of BZN greatly changes the symmetry of KNN. The peak splitting and higher intensity of (022) peak than that of (200) peak around 45° indicates that the sample with x = 0.010 is an orthorhombic phase (Amm2) similar to pure KNN [24]. With increasing BZN, the symmetry for the sample with x = 0.015 changes into an orthorhombic-tetragonal (O-T) mixed phase determined by the intensity of (022)/(002) peak comparable to (200) peak, which is in consistent with the Li-substituted KNN-based solid solution [25]. For the samples with 0.020 ≤ x ≤ 0.030, a single (200) peak in the vicinity of 45° suggests the formation of a rhombohedral symmetry (R3m) since the doping concentration is not very high, and a similar feature can also be observed in the KNN-(Bi 0.5 Na 0.5 )ZrO 3 solid solution [26].

Results and discussion
To verify the symmetry and understand the crystal structure evolution of (1-x)KNN-xBZN ceramics, the Rietveld refinement of XRD data was conducted by the GSAS software [25]. The space groups of all the samples were selected based on the XRD results. The initial crystalline parameters were given according to Ref. [27]. In the process of refinement, the scale factor, background, and cell parameters were revised to minimize the difference between the experimental data and the fitted profile. The refinement results in Fig. 2 show that the experimental data is well fitted. The detailed crystalline parameters, together with R wp (weighted profile residual factor) and R p (profile residual factor), are listed in Table 1. The reasonable R wp and R p factors confirm the validation of crystal symmetry. Figure 2 shows that the unit cell of Amm2 includes two octahedral units, whereas one octahedral unit presents in that of P4mm and R3m. According to the cell volumes in Table 2, the average volume of     [20,28]. As a result, the addition of BZN slightly reduces the volume of octahedral unit. Besides, with increasing BZN content to 0.020 ≤ x ≤ 0.030, the larger size Zn 2+ replacing Nb 5+ at the B site produces a great lattice distortion to make the rhombohedral symmetry stable above RT, which agrees with the XRD results.   [29,30], and thus the decrease of average grain size in this study should be attributed to the role of Bi 3+ . However, with further increasing BZN content up to x = 0.025 and 0.030, the average grain size slightly increases, which could reach a saturation value. In addition, the bulk densities of the samples with x = 0.010, 0.015, 0.020, 0.025, and 0.030 are 4.34, 4.32, 4.30, 4.21, and 4.23 g/cm 3 , respectively. It can be found that the density of 0.010 ≤ x ≤ 0.020 is much higher than that of 0.025 ≤ x ≤ 0.030 in consistent with the result of microstructure morphologies.
The temperature dependence of ε′ of (1−x)KNN-xBZN ceramics at 1 MHz is plotted in Fig. 4. The sample with x = 0.010 exhibits an orthorhombic phase due to the T m and T O-T permittivity peaks above RT. The inset of Fig. 4 shows that T m declines rapidly with the addition of BZN. The T O-T around 125 ℃ in the sample with x = 0.015 suggests the O-T mixed phase occurring at RT, and a similar phenomenon was also found in the 0.94 [31]. It is noted that the samples with x = 0.020 and 0.025 ≤ x ≤ 0.03 exhibit a step-like feature around 171 and 206 ℃, respectively, which is much higher than the T O-T for the sample with x = 0.015. Such a step-like feature is related to the rhombohedral-tetragonal (R-T) phase transition in similarity to the reported KNN-based solid solutions [7,31]. It suggests that a small amount of BZN stabilizes the rhombohedral phase whereas suppresses the orthorhombic phase. The aforementioned dielectric behavior related to the phase transition further verifies the crystal structure, which is in agreement with the XRD analysis and Rietveld refinement. Moreover, the samples with 0.020 ≤ x ≤ 0.030 exhibit a tetragonal-cubic (T-C) diffuse phase transition, which has been frequently reported in the ferroelectrics with relaxor characteristics [16][17][18][19][20]. Consequently, the high-temperature stability of permittivity is greatly enhanced by the combination of R-T step-like feature and the diffuse phase transition from tetragonal to cubic.
The permittivity above Curie temperature (T C ) for typical ferroelectrics generally follows the Curie-Weiss law 1/ε′ = (T -T 0 )/C (T > T C ). Figure 5 displays the temperature dependence of reverse permittivity for (1−x)KNN-xBZN ceramics at 1 MHz. It is obvious that the permittivity for the samples with 0.020 ≤ x ≤ 0.030 deviates from the Curie-Weiss law. The deviation degree can be expressed as ΔT m = T CW -T m [32] (Fig. 5). The linear fitting between reverse permittivity and temperature above T CW is carried out and the fitted parameters are listed in Table 2. The inset of Fig. 5 shows that ΔT m continuously increases with the addition of BZN, which indicates that introducing BZN gradually improves the diffuse phase transition behavior of the system.
To further quantitatively characterize the diffuse phase transition behavior of permittivity above T m for (1−x)KNN-xBZN system, the modified Curie-Weiss law proposed by Uchino and Nomura [33] is adopted by the following equation: where ε′ m is the maximal permittivity when T = T m , C 1 is a constant on Curie-Weiss behavior, and γ = 1 and 2 are the classical ferroelectric state and relaxor ferroelectric state, respectively, 1 ≤ γ ≤ 2. Figure 6 displays the log(1/ε′ -1/ε′ m ) as a function of log(T -T m )  With increasing BZN, the γ monotonously increases from 1.260 to 1.893. Especially, the γ values are close to 2 for the samples with 0.020 ≤ x ≤ 0.030, implying a diffuse phase transition behavior associated with the thermal evolution of polar nano-regions (PNRs) size due to ion substitution [34][35][36]. In this work, since the valence and radius of Bi 3+ at the A site of ABO 3 -type perovskite structure are different from those of K + , Na + , and similar cases also appear at the B site. In comparison with the larger size Zn 2+ , a more pronounced off-center shift for the smaller size Nb 5+ exists in oxygen octahedra, similar to that has been reported in the (Bi 0.5 Li 0.5 )ZrO 3 and (Bi 0.5 Na 0.5 )TiO 3 modified KNN solid solutions [7,19,34]. Then, the introduction of Bi 3+ (including a chemically active lone electron pair) into K + /Na + and Zn 2+ into Nb 5+ not only interrupts the long-range Nb 5+ displacement, but also leads to substantial local lattice strain. The mismatches of valence and size produce local static electric and elastic fields, and thereby the ferroelectric long-range order is prevented which results in the formation of PNRs. When the doping content of BZN is not very high, the PNRs are embedded in the lattice matrix of (1−x)KNN-xBZN ceramics, which gives rise to the diffuse phase transition behavior. The temperature variation of permittivity plays a very important role in practical application of HTCC materials. Figure 7(a) shows the temperature variation of permittivity (Δε′/ε′ 300 ℃, Δε′ = ε′ − ε′ 300 ℃) for (1−x)KNN-xBZN ceramics at 1 MHz. The addition of www.springer.com/journal/40145 BZN greatly enhances the high-temperature stability of permittivity. According to the ±15% tolerance of Electronic Industries Alliance X8R capacitor standards as shown by the gray flame in Fig. 7(a) [7,8], the samples with x = 0.020, 0.025, and 0.030 show broad hightemperature ranges of 75-342, 103-459, and 96-457 ℃, respectively. In Fig. 7(b), the tanδ of (1−x)KNN-xBZN ceramics as a function of temperature at 1 MHz shows that the tanδ for the samples with x = 0.020 and 0.025 remains low (≤ 0.05) in broad temperature ranges of 79-433 and 119-364 ℃, respectively, but it only exists a narrow temperature range of 137-326 ℃ for the sample with x = 0.030. Therefore, 0.98KNN-0.02BZN ceramic with a stable ε′ (≈ 1310, ±15% tolerance) and a low tan (≤ 5%) from 79 to 342 ℃, and 0.975KNN-0.025BZN ceramic with a stable ε′ (≈ 1219, ±15% tolerance) and a low tan (≤ 5%) from 119 to 364 ℃, are the potential materials for HTCC application. It is noted that the introduction of BZN greatly enhances the high-temperature tanδ due to the thermally activated conductivity behavior [37][38][39].
To better understand the influence of BZN content on the high-temperature conductivity behavior of the system, the normalized imaginary parts of impedance (Z″/Z″ max ) for (1−x)KNN-xBZN ceramics as a function of frequency at selected temperatures are shown in Figs. 8(a)−8(e). A systematic loss peak exhibits at the measuring frequency region. With increasing temperature, the peak frequency (relaxation frequency, f max ) moves toward high frequency, which indicates a thermally activated relaxation behavior [39][40][41]. This relaxation behavior indicates that elevating temperature accelerates  the hopping or migration of charge carriers (electrons, holes, and ions). The relationship between f max and temperature can be established by the following equation (Arrhenius law): where f 0 is a constant on relaxation behavior, E rel represents the activation energy on relaxation behavior, k B and T are the Boltzmann constant (eV/K) and absolute temperature (K), respectively. By fitting the Arrhenius plots of relaxation frequency as shown by the red lines in Fig. 9, the E rel can be determined according to the fitted slope: E rel = 1.240 eV for x = 0.010, E rel = 1.154 eV for x = 0.015, E rel = 1.095 eV for x = 0.020, E rel = 0.596 eV for x = 0.025, and E rel = 0.555 eV for x = 0.030. The E rel monotonously declines with increasing BZN. The frequency dependence of AC conductivity (σ AC ) for (1−x)KNN-xBZN ceramics at selected temperatures is displayed in Figs. 10(a)−10(e). With the increase of frequency, the σ AC first exhibits a low-frequency platform that is close to DC conductivity (σ DC ) and then dramatically increases due to the contribution of localized charge carriers by hopping or migration to conductivity. The conductivity behavior describes the many-body mutual actions between dipoles and charge carriers. It can be established by the following equation ("universal dielectric response" (UDR) law [40,41]): where σ 0 , f, and s are a constant on σ AC , the measuring frequency, and a frequency exponent (0 < s ≤ 1), respectively. The σ DC can be determined by the nonlinear fitting between σ AC and frequency based on Eq. (3), as shown by the red lines in Fig. 10.
The relationship between σ DC and temperature can be described based on the the following equation (Arrhenius law): where σ 1 is a constant on σ DC and E con represents the www.springer.com/journal/40145 activation energy on σ DC . By fitting the Arrhenius plots of σ DC as shown by the red lines in Fig. 11, the E con can be determined according to the fitted slope: E con = 1.064 eV for x = 0.010, E con = 0.983 eV for x = 0.015, E con = 0.873 eV for x = 0.020, E con = 0.582 eV for x = 0.025, and E con = 0.543 eV for x = 0.030. The variation of E con is in agreement with that of E rel . Moreover, the E rel values of all the samples are greater than those of E con . This is due to that E rel equals the free energy of charge carriers by hopping or migration between adjacent lattice sites whereas the E con involves both the long-distance hopping/migration free energy of charge carriers and the creation of charge carriers [42,43]. Therefore, the difference between them is associated with the creation of free energy, which indicates that charge carriers (electrons, holes, and ions) are dissociated from the traps and defects exist in the samples.
In perovskite oxides, oxygen vacancies are one of the most common charged carriers and have a crucial effect on the high-temperature conductivity behavior. It was reported that the activation energy of single-ionized oxygen vacancies is in the range of 0.3-0.5 eV and it is 0.6-1.2 eV for that of doubly-ionized oxygen vacancies [44]. According to the magnitude of E con and E rel , the charged carriers in (1−x)KNN-xBZN ceramics are considered to be doubly-ionized oxygen vacancies. Generally, the volatilization of K/Na/Bi from the lattice during sintering could be compensated by oxygen vacancies. In addition, the possible Nb 5+ substitution by Zn 4+ generates the defect dipoles with regard to the acceptor ions and the charge compensation of oxygen vacancies. The corresponding Kroger-Vink notations can be expressed as the following equations:  (7) show that both the formation of Bi vacancy and creation of negatively charged defect Zn' Nb produce oxygen vacancies. Thus, the addition of BZN enhances the concentration of oxygen vacancies, allowing activation energy to be reduced, which agrees with the variation of E con and E rel . It is noted that with increasing BZN, both E rel and E con initially decrease slightly, then drop sharply at x = 0.025, and finally decline slowly. The change of activation energy generally depends on the concentration of oxygen vacancies, lattice distortion, and microstructure morphologies [20][21][22]. Since the volume change of octahedral unit for (1−x)KNN-xBZN system is almost independent of BZN content, it is assumed that such a decrease of activation energy could be related to microstructure morphologies and the concentration of oxygen vacancies. With the increase of BZN, all the samples with 0.010 ≤ x ≤ 0.020 are dense and the higher concentration of oxygen vacancies plays a major role in the slight decrease of activation energy. For the sample with x = 0.025, the formation of a large number of pores provides the large surface on the grains to make activation energy of oxygen vacancies decreased greatly [42]. Further increasing BZN content up to x = 0.030, the microstructure morphology is almost unchanged and the slight decrease of activation energy is associated with the improvement of oxygen vacancy concentration.

Conclusions
(1−x)KNN-xBZN ceramics were fabricated via the traditional solid-state method. The crystal structure, microstructure, dielectric, and conductivity behavior of the system were investigated systematically. The crystal structure dependence of BZN was determined by XRD patterns, Rietveld refinement, and dielectric spectroscopy. The decrease of octahedral unit is associated with the ion substitution. Due to the combination of R-T step-like feature and T-C diffuse phase transition, 0.98KNN-0.02BZN ceramic exhibits a stable ε′ (≈ 1310, ±15% tolerance) and a low tan (≤ 5%) in a broad temperature range of 79-342 ℃. In addition, a stable ε′ (≈ 1219, ±15% tolerance) and a low tan (≤ 5%) from 119 to 364 ℃ are obtained for 0.975KNN-0.025BZN ceramic. The activation energies of dielectric relaxation and conductivity behavior at high temperatures initially decrease slightly, then drop sharply, and finally decline slowly, which could be attributed to microstructure morphologies and the concentration of oxygen vacancies.
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