Complex-Ions on Structure and Electrical Properties of Bi0.5Na0.5TiO3-BaTiO3 Ceramics

The (Bi0.5Na0.5)0.94Ba0.06Ti1-x (Nd0.5Ta0.5)xO3 (0.01≤x≤0.05) lead-free ceramics (BNBT-xNT) used (Nd0.5Ta0.5) 4+ complex-ions to modify its structure and electrical properties. The BNBT-xNT ceramics exhibit the coexistence of tetragonal and rhombohedral phase. The (Nd0.5Ta0.5) 4+ complex-ions prohibit grain growth, and its average grain size decreases from 2.53 μm to 0.84 μm with increasing complexion content. The NT doping not only induces the transformation from ferroelectric phase to relaxor ferroelectric phase but also decreases the coercive field and remnant polarization. The permittivity curves are broadened at heavily doping content. The energy storage and strain properties are improved by complex-ions. The maximum energy storage density of 0.475 J/cm3 is obtained at x = 0.035 and 60 kV/cm, the energy storage efficiency achieves the maximum efficiency of 61.5% at x=0.05. As increasing complex-ion content, the typical butterfly-shaped strain curve develops into a sprout-shaped one, and the maximum strain of 0.199% is obtained at x=0.02.


Introduction
The growing level of environmental awareness makes the researchers try their best to find out lead-free material to take place of the poisonous lead-based ferroelectric ceramics with excellent electrical properties.Bi 0.5 Na 0.5 TiO 3 (BNT) ceramics have ABO 3 perovskite structure which is deemed to be one of the most potential lead-free candidates for lead-based ceramics due to its outstanding ferroelectric and piezoelectric properties at room temperature 1 .
The Ca 0.61 Nd 0.26 Ti 1-x (Cr 0.5 Ta 0.5 ) x O 3 microwave ceramics were modified by (Cr 0.5 Ta 0.5 ) 4+ complex-ions, which show the good and stable comprehensive microwave dielectric properties 15 .The ionic radius of Ta 5+ (0.640 Å) is close to that of Ti 4+ (0.605 Å).However, the Nd 3+ with larger ionic radius (0.983 Å) was used to induce lattice distortion.So the (Nd 0.5 Ta 0.5 ) 4+ complex ions were designed to modify the structure of BNT-based ceramic and improve its electrical properties.

Experiments
The BNBT-xNT (x=0.01,0.02, 0.03, 0.035, 0.04, 0.05) ceramics were prepared by using Bi 2 O 3 (99.95%),Na 2 CO 3 (99.8%),BaCO 3 (99%), TiO 2 (99.9%),Nd 2 O 3 (99.95%)and Ta 2 O 5 (99.99%) dried powders as the starting raw materials.All oxide powders were weighed according to the stoichiometric formula.And according to our previous work 16 , after ball milling in ethanol with ZrO 2 balls for 12 hours, the powders were dried at 90 °C for 24 hours and then calcined at 880 °C for 2 hours.The powders added polyvinyl alcohol aqueous solution (7 wt%) as binder were granulated by 100-mesh sieve.The green bodies with a diameter of 13 mm and a thickness of 1.0 mm were pressed under 40 MPa.The sintered bodies were sintered in air at 1150 °C for 2 hours.The electrical test samples were polished to 0.5mm in thickness and fabricated Ag electrodes on both sides at 580 °C for 30 minutes.
The crystal structure of BNBT-xNT samples was characterized by X-ray diffractometer (XRD, D8-Advance, Bruker).The surface morphology and grain size were analyzed by Filed-Emission Scanning Electron Microscope (FESEM, qunata 450 FEG, FEI).The electric-field-induced polarization (P-E), bipolar strain (S-E) and current-electric field (I-E) were measured by ferroelectric test system (TF Analyzer-2000E, aixACCT).The energy storage properties were calculated by integrating the P-E loop.The dielectric constant and loss were measured by impedance analyzer (4294A, Agilent) with a heating rate of 2 °C/min from room temperature to 400 °C, and the impedance spectrum was also measured by impedance analyzer.

Results and Discussion
Figure 1 is the XRD patterns of BNBT-xNT ceramics with x=0.01-0.05.The XRD patterns reveal all of the diffraction peaks are indexed by (Na 0.5 Bi 0.5 )TiO 3 (JCPDS No.46-0001), which indicates the formation of perovskite single phase structure without other impurity phases.Therefore, the Nd 3+ and Ta 5+ ions diffused into the lattice of matrix and formed solid solution.As previous report 17 , the pure Bi 0.5 Na 0.5 TiO 3 and BaTiO 3 are rhombohedral and tetragonal structure, respectively.As shown in Figure 1 (b), the (111) peak at about 40° splits into the (003) and (021) peaks which proves the existence of rhombohedral phase structure.The (200)/(002) splitting peaks at 46°-47° as shown in Figure 1 (c) indicates tetragonal phase structure 18 .The peaks shift to lower diffraction angles with the increasing amounts of NT complex-ions.The reason of peak shifting is that the ionic radius of Ti 4+ (0.0605 nm) at B-site are smaller than Nd 3+ (0.0983 nm) and Ta 5+ (0.0640 nm) 19,20 .Thus the lattice distortion induces the change of lattice constant, and the diffraction peak shifts to lower angle according to the Bragg law.According to the two splitting peak behaviors, it can be concluded that the rhombohedral and tetragonal phase structure coexist in the BNBT-xNT ceramics 21,22 .
The surface morphologies and grain size distribution of BNBT-xNT ceramics with x=0.01-0.05 is shown in Figure 2. The grain size of BNBT-xNT ceramics was measured by the calculation software "nano measurer".The grains and grain boundaries observed from SEM images are clear.The shape of grains from top view shows circular-like structure.However, the ceramic samples show a small number of pores.These pores can affect breakdown electric field because these defects are preferentially broken down.When x=0.01, the grain size illustrates a wide range from 1.0 µm to 6.0 µm.As the complex-ion content increase, the range of grain size gradually becomes smaller, and the same phenomenon can be observed in Nb/In co-doped Bi 0.49 La 0.01 Na 0.49 Li 0.01 TiO 3-δ ceramics 23 .And the average grain size of BNBT-xNT ceramics decrease slightly from 2.53 µm at x=0.01 to 0.84 µm at x=0.05, which implies the (Nd 0.5 Ta 0.5 ) 4+ complex-ions are grain growth inhibitor for BNBT ceramics.The grain growth of 0.94BNT-0.06BTceramics was not evidently affected by Ta-doping 24 .However, the grain size of Nd doped BNT ceramics shown an obvious decrease 20 .And the grain size of Nd doped 0.82BNT-0.18BKTceramics decreases slightly 9 .The reason is that the segregation of some Nd 3+ ions at grain boundaries, which prevents grain boundary movement during sintering and inhibits grain growth 9 .Therefore, the grain growth behavior of BNBT-xNT ceramics in this work can attribute to the same reasons.
Owning to the breakdown electric field of BNBT-0.01NTceramic is under 70 kV/cm, so the P-E hysteresis loops of the BNBT-xNT ceramics were tested at 60 kV/cm and 1 Hz as shown in Figure 3(a).And the Figure 3(b) is the evolution of the coercive field (E c ), remnant polarization (P r ), maximum polarization (P max ) and ΔP (P max -P r ) for BNBT-xNT ceramics.It can be seen that the P max and P r simultaneously decrease with increasing NT complex-ion content, and the change of the E c is not obvious.When x=0.01, the E c , P r and P max observed from the P-E loop are 29.4 kV/cm, 26.9 µC/cm 2 and 34.4 µC/cm 2 , respectively.And then the P r and E c suddenly decrease to 8.4 µC/cm 2 and 14.7 kV/cm at x=0.02.After that the P r , P max and E c gradually decrease along with the increase of complex-ion content.Lastly, the BNBT-xNT ceramics obtain the minimum P r of 1.97 µC/cm 2 and E c of 10.61 kV/ cm at x=0.05.The ferroelectric behavior transformation lies in that the NT complex-ions disturb the long-range ferroelectric order and the relaxor characteristics increased 11 .The following formula can be used to calculate the energy storage density (W) and energy storage efficiency (η) 25 : (1) Where W 1 is the electrical energy storage density, E refers to the applied external electric filed, P r and P max are the remnant and maximum polarization.W 2 is the energy absorption density and η is energy storage efficiency.
Figure 4 exhibits the energy storage density and efficiency of BNBT-xNT ceramics which are calculated from the P-E loops at 60 kV/cm and 1 Hz.The improvement of energy storage properties is attributed to the transition of ferroelectric behavior.For x=0.01, the energy storage density is 0.126J/cm 3 and efficiency is 8.3% (P max =34.4 µC/cm 2 , P r =26.9 µC/cm 2 , ΔP=7.5 µC/cm 2 , E c =29.4 kV/cm).Then as shown in Figure 3, the P-E loop becomes slim and the ΔP sharply increases when x=0.02, so the ferroelectric properties decrease.Thus, according to the formula (1) and ( 2), the energy storage density and efficiency have an obvious improvement to 0.418J/cm 3 and 35.6%.At x=0.035, the energy storage density goes up to the maximum value of 0.475 J/cm 3 (P max =23.69 µC/cm 2 , P r =4.01 µC/cm 2 , ΔP=19.67 µC/cm 2 , E c =12.78 kV/cm).With the further increase of complex-ion content, the energy storage density gradually decreases.At the same time, the energy storage efficiency is always increasing and achieves the maximum efficiency of 61.5% at x=0.05.
Figure 5 shows the current-polarization-electric field (I-P-E) of BNBT-xNT ceramics tested at 60 kV/cm and 1 Hz.When x=0.01, there are only two current peaks which correspond to +E c and -E c for the P-E curve.The reason for appearance of two current peaks is the domain switching of typical ferroelectric at the E c value of the external electric field 26 .Thus, the current peaks, large P r and E c indicate that the ferroelectric phase is dominant for BNBT-xNT at x=0.01 27 .However, four peaks I 1 and I 2 be appeare for each I-E loops when x>0.01 and indicates the phase transition of BNBT-xNT ceramics.The current peak of I 1 represents the relaxor-ferroelectric transition, while the current peak of I 2 corresponds to the ferroelectric-relaxor transition 28 .Therefore, BNBT-xNT ceramics are dominated by relaxor ferroelectric phase when x>0.01.With the increase of complex-ion content, all the four current peaks become weak, and the P max declines.The reason is that the high concentration complex-ions lead to the rising proportions of relaxor ferroelectric phase than that of the ferroelectric phase 29 .
Figure 6 reveals the bipolar field-induced strains of BNBT-xNT ceramics tested at 60 kV/cm.It can be seen that the positive strain shows a little increase from 0.191% to 0.199% when the NT content increased to 0.02.Further increasing NT content, the positive strain shows an obvious decrease from 0.177% to 0.061%.Meanwhile, the negative strain (S neg =0.186%)only appears at x=0.01 and exhibits butterfly characterization, which is mainly related to the dominant ferroelectric phase for the structure 30 .But the butterfly-shaped curve is apparently asymmetric under the effect of internal bias electric field resulting from the formation of defect dipoles during sintering, while the defect dipoles come from the NT doping 31 .The internal bias field and external electric field interact together during bipolar strain test.The actual electric field in samples is the result of their interaction.The internal bias field plays different role to strengthen or weaken external electric field according to their direction.As the doping content increase, it can be seen that the typical butterfly-shaped strain curve develops into a sprout-shaped one and the S neg disappears, which indicates the BNBT-xNT ceramics with ferroelectric phase transform to the relaxor ferroelectric phase 32 .The temperature dependence of relative permittivity (ε r ) and loss (tanδ) of BNBT-xNT ceramics at frequencies of 1 kHz, 10 kHz and 100 kHz are exhibited in Figure 7.As it is shown, there are two dielectric anomaly peaks (T p and T m ) for each permittivity curve owing to the thermal evolution of the symmetric ferroelectric polar nanoregions (PNRs) of R3c and P4bm structure 33 .The T p dielectric anomaly peak shows a strong frequency dispersion behavior, while the T m dielectric anomaly peak illustrates weak frequency dispersion 22 .And at the same temperature, the dielectric constant decreases as the frequency increasing.On the other hand, with increasing complex-ion content, the frequency dependence of dielectric permittivity is weakened.The dielectric peaks of T m indicate the phase transition temperature of BNBT-xNT ceramics from "anti-ferroelectric-like" to paraelectric phase 34 .For the heavily doped BNBT-xNT ceramics, the dielectric peaks of T m become broader and slightly shift to the lower temperature.These broad dielectric behavior and dependent frequency are characteristic of relaxor ferroelectric 35 .In addition, the value of dielectric loss (tanδ) increases with the rising of frequency at the same temperature.Meanwhile, at the same frequency, the dielectric loss gradually decreases at the temperature range of T p to T m .It is probably related to the tiny distortion in crystalline structure after depolarization 36 .
To describe the dielectric dispersion and diffuseness of phase transition, the following modified Curie-Weiss law was used in many researches 37,38 . (3) The letter C is the Curie constant, ε m and ε are the maximum dielectric constant and the dielectric constant, and γ is the degree of diffuseness.The range of γ value is from 1 to 2, which corresponds to a normal ferroelectric to an ideal relaxor ferroelectric 39 .Figure 8 The Modification of (Nd 0.5 Ta 0.5 ) 4+ Complex-Ions on Structure and Electrical Properties of Bi 0.5 Na 0.5 TiO 3 -BaTiO 3 Ceramics and 2.08, confirming that the phase transition has a diffuse characteristic, consistent with the P-E results shown in Figure 5.The similar relaxor behavior was observed in 0.8(Bi 0.5 Na 0.5 )TiO 3 -0.2(Bi0.5 K 0.5 )(Hf x Ti 1-x )O 3 ceramics with γ value between 1.89 and 2 40 .
Figure 9 shows the Cole-Cole plots of impedance with different temperature from 200 °C to 540 °C for x=0.035.Firstly, the impedance curves are nearly parallel to the ordinate at lower temperature.And then the curves bend towards abscissa and form semicircles as the increasing temperature.The BNBT-0.035NT ceramic presents an excellent insulating capacity below the temperature of 300 °C.Afterwards, the radii of the semicircles become smaller and smaller.The radii of semicircles in Cole-Cole plot represent the resistive behavior of ceramic 16,41 .So it indicates that the conductivity increases and the impedance decreases with heating 11,42 .The increase of conductivity of BNBT-0.035NTceramic is attributed to the thermal activated carriers.

Conclusions
The (Nd 0.5 Ta 0.5 ) 4+ complex-ions were used to modify the structure and electrical properties of the (Bi 0.5 Na 0.5 ) 0.94 Ba 0.06 Ti 1-x (Nd 0.5 Ta 0.5 ) x O 3 lead-free ceramics.The BNBT-xNT ceramics are single-phase perovskite structure without impurity phases, and show the coexistence of tetragonal and rhombohedral phase.The average grain size decreases from 2.53 µm to 0.84 µm with increasing complex-ion content.The NT complex-ions decrease the remnant polarization and coercive field.Meanwhile, the relaxor ferroelectric phase of heavily doped BNT-BT ceramics were verified by the evolution of current peak, the disappearing negative strain and diffuseness coefficient.The doping of complex-ions decreases the dielectric constant and broadens the permittivity curves.The maximum energy storage density of 0.475 J/cm 3 is obtained at x=0.035 and 60 kV/cm, and the optimal value of strain is 0.199% obtained at x=0.02.These results reveal that the structure and electrical properties of BNT-BT ceramics can be modified by complex-ions introducing.

a
School of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China b Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China

Figure 7 .
Figure 7. Dielectric constant and loss of BNBT-xNT ceramics as a function of temperature at 1 kHz, 10 kHz and 100 kHz