Effect of lanthanum substitution on structural, dielectric and piezoelectric properties of (Na0.41K0.09Bi0.5)TiO3: A lead-free piezoelectric material

Solid State Communications 298 (2019) 113637

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Communication

Effect of lanthanum substitution on structural, dielectric and piezoelectric properties of (Na0.41K0.09Bi0.5)TiO3: A lead-free piezoelectric material

T

Gurvinderjit Singha,b,∗, A.K. Singhc, Rachna Selvamania, V.S. Tiwaria,b, A.K. Karnala,b a

Laser and Functional Materials Division, Raja Ramanna Centre for Advanced Technology, Indore, 452 013, India Homi Bhabha National Institute, Anushakti Nagar, Mumbai, 400094, India c Department of Physics, IIT-Banaras Hindu University, Varanasi, 221005, India b

ARTICLE INFO

ABSTRACT

Communicated by T. Kimura

The effect of lanthanum doping (0.5–5 mol%) on structural, dielectric and piezoelectric response of (Na0.41K0.09Bi0.5)TiO3 ceramic has been investigated. The x-ray studies reveal that there is no structural transformation with incorporation of lanthanum. However, a gradual lowering of tetragonal-to-rhombohedral transitions and depolarization temperature was observed in the dielectric response. For lower lanthanum contents (0–2 mol%) the piezoelectric coefficients i.e d33 (150 pC/N) and kp (0.31) are nearly same. A significant decrease of d33 and kp was observed for higher lanthanum doping with emergence of pinched P-E hysteresis loop. These results are explained on the basis of disruption of long-range ferroelectric ordering in (Na0.41K0.09Bi0.5)TiO3 with lanthanum doping as well as with emergence of anti-ferroelectric ordering. A structural phase diagram has also been derived for lanthanum doped (Na0.41K0.09Bi0.5)TiO3 using dielectric measurements.

Keywords: Ferroelectrics NBT X-ray diffraction Dielectric Piezoelectric

1. Introduction Sodium bismuth titanate (Na0.5Bi0.5TiO3), NBT, is one of the promising lead-free piezoelectric materials having perovskites structure with two different ions at A-site [1–3]. It bears reasonably high Curie (Tc ∼ 320 °C) and depolarization (Td ∼180 °C) temperature. The depolarization temperature (Td) is an important parameter in view of its device application, because the piezoelectric signals disappear above this temperature [4]. NBT also shows dielectric anomaly associated with frequency dependent dielectric response near 260 °C and it is attributed to the onset of tetragonal to rhombohedral transition (TR-T) [2,5]. A reasonably high remnant polarization (38 μC/cm2) and piezoelectric constant; d33 = 78 pC/N was detected below this temperature [6]. In spite of this, high coercive field (Ec = 7 kV/mm) and relatively large conductivity of NBT make difficult to pole this material which limits its practical application. In order to reduce the coercive field and improve the piezoelectric properties, investigations were carried out by making solid solutions of NBT with other perovskites like; BaTiO3 (BT) [7], KNbO3 [8], (K0.5Bi0.5)TiO3 [KBT] [9], and Ba(Ti, Zr)O3 [10]. Among these, the solid solutions of NBT with BT and KBT are of interest due to presence of MPB with reasonably high piezoelectric properties. The solid solution (1-x)NBT-xBT was first investigated by Takanaka et al. [7] and it shows MPB near x = 0.06. The MPB composition has relatively lower coercive field (3 kV/mm) and higher d33 value ∗

(125 pC/N) compared to pure NBT. The solid solution of (1-x)NBTxKBT i.e. (Na0.5(1-x)K0.5xBi)TiO3, investigated by Elekechai et al. [9], reports d33 value as 96 pC/N at x = 0.16. Later, Sasaki et al. [11] have observed MPB like nature in the range x = 0.16 to 0.20. The x-ray diffraction studies conjectured the presence of MPB near x = 0.18 i.e. (Na0.41K0.09Bi0.5)TiO3 [12]. The reported Tc, TR→T and Td for MPB composition are 300, 200 and 140 °C, respectively. The d33 and dielectric constant at room temperature for this composition are nearly 148 pC/N and 7000, respectively. Studies have been conducted to further improve the piezoelectric properties of this MPB composition by incorporating different rare earth ions such as neodymium [13], samarium [14], holonium [15] and gadolinium [16] at Bi-site. However, the measured d33 values (130–140 pC/N) are lower than reported for (Na0.41K0.09Bi0.5)TiO3 without any doping [12]. The reason could be the poor ceramic density or change of Na/Bi ratio due to loss of bismuth during sintering. Both these factors are known to influence the piezoelectric as well as electrical properties of the NBT ceramics [17–21]. It is known [22–24] that La3+ improves the densification as well as piezoelectric properties in PLZT ceramics. As per our knowledge, the effect of lanthanum doping on piezoelectric properties of (Na0.41K0.09Bi0.5)TiO3 ceramic is not studied yet. The striking difference between PLZT and the solid solution composition (Na0.41K0.09Bi0.5)TiO3 is that La3+ act as donor in PLZT by occupying Pb site, whereas it could act as a donor if replaces Na+ or K+ ions and isovalent substitution by

Corresponding author. Laser and Functional Materials Division, Raja Ramanna Centre for Advanced Technology, Indore 452013 India. E-mail address: [email protected] (G. Singh).

https://doi.org/10.1016/j.ssc.2019.05.008 Received 6 March 2019; Received in revised form 16 May 2019; Accepted 17 May 2019 Available online 20 May 2019 0038-1098/ © 2019 Elsevier Ltd. All rights reserved.

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replacing Bi3. It is reported [25] that Bi3+ ion at A-site of NBT is mainly responsible for ferroelectric behavoiur in this material. Therefore, lanthanum (La) ion is selected to substituted the ferroelectric active Bi3+ ion in the present work. In the present work the compositions are synthesized using the formula (Na0.41K0.09Bi0.5-xLax)TiO3 where La3+ substituted at Bi3+ site. The purpose of the study is to investigate the effect of isovalent lanthanum (La3+) substitution at Bi-site on, structural, dielectric and piezoelectric properties of (Na0.41K0.09Bi0.5)TiO3.

900 °C (data not shown here). Higher temperature accelerates the interatomic diffusion leads to single phase formation. Fig. 1b shows the powder XRD of sintered samples. It is clear from the figure that all the ceramics exhibit single perovskite phase and no second phase was observed. As per literature (Na0.41K0.09Bi0.5)TiO3 composition is expected to have a co-existence of rhombohedral and tetragonal phases [27]. The structural distortion (tetra-gonality or rhombohedricity) in this locally disordered system is small. As a consequence, no peak splitting related to rhombohedral (R3c) or tetragonal phases (P4bm) phases was observed in the powder x-ray diffraction and it shows average cubic structure. Therefore, the diffraction data was indexed with pseudo-cubic structure (Pm3m3 space group). This is consistent with the structure of unpoled NBT based ceramics [27,28]. The appearance of pseudo-cubic structure is described by considering the presence of polar nano-domains with R3c and P4bm local symmetry near MPB composition in an overall cubic matrix [28]. The XRD measurements indicate that there is no structural transformation with lanthanum doping. However, a closer view of diffraction data shows slight shift in the diffraction peaks towards higher 2θ side with increase in La3+ content. This is clearly observable in the diffraction profile of (110) plane presented in Fig. 1c. The shift of diffraction peak implies contraction of the lattice with lanthanum content. The lattice parameters, calculated using standard software shows a decrease from 3.910(3) Å for x = 0.00 to 3.886(4) Å for x = 0.05. The decrease in the lattice parameter is expected because of substitution of smaller La3+ ion (1.36 Å for co-ordination number 12) in place of larger Bi3+ ion (1.45 Å for co-ordination number 12) [29,30].

2. Experimental Lanthanum (La) doped (Na0.41K0.09Bi0.5-xLax)TiO3 ceramics with x = 0.005, 0.01, 0.02, 0.03 and 0.05 were prepared by conventional solid state reaction technique along with pure (Na0.41K0.09Bi0.5)TiO3 i.e. x = 0.000 was also prepared for the sake of comparison of properties. High purity (>99%) Na2CO3, K2CO3, Bi2O3, La2O3, and TiO2 were taken as the precursors for synthesis of (Na0.41K0.09Bi0.5-xLax)TiO3 powders. These raw materials were wet mixed in stoichiometric composition using ethanol in a 3D-mixer (Turbula, Japan) for 10 h and the slurry were dried at 85 °C for 12 h. These mixed powders were taken in alumina crucible and calcined at 800 °C for 3-h. The x-ray diffraction (Rigaku, CuKα) was performed on calcined powders for verifying the phase formation. The dried powders were mixed with a PVA binder and pellets of diameter 15 mm were prepared at 100 MPa using a semi-automatic hydraulic press. The green pellets were placed over platinum foil and sintered at 1160 °C for 3 h with intermediate dwelling at 550 °C (1hrs) to remove the binder. In all the compositions 2 wt% extra Bi2O3 was added to compensate the Bi loss at high temperature during sintering [19]. The density of the sintered pellets was more than 97% of the theoretical values as measured by liquid displacement method. Dielectric measurements on sintered ceramics were carried out using an automated HP4194A impedance analyzer over a frequency range of 100 Hz-100 kHz at different temperatures. All the data were collected during heating cycle and temperature was measured with an accuracy of ±1 K. Polarization versus electric field (P–E) hysteresis loop were recorded using Radiant Technology's Precision Material Analyzer Workstation based on virtual ground system. For piezoelectric measurements the samples were poled in silicon oil under an electric field of 3 kV/mm for 20 min at 130 °C and cooled to room temperature under the applied field. The planar coupling coefficient (kp) was determined from the resonance-antiresonance method with IRE criteria. The longitudinal charge coefficient (d33) was measured using Piezotest make d33 m (model PM-200).

3.2. Temperature dependent dielectric studies Figure-2 shows the temperature dependent dielectric permittivity for different compositions. Although the measurements were carried out at twenty different frequencies (100 Hz–100 kHz) but data of 1, 10, 50 and 100 kHz are presented for the sake of clarity. For x = 0.00 welldefined shoulder in the dielectric curve followed by a diffuse peak near 300 °C (Tc), where dielectric constant attains a maximum value, was observed. In fact, dielectric constant exhibits frequency dispersion at the shoulder which diminishes near 200 °C. The tanδ curve shows welldefined anomalies with incisive decrease of loss near 140 °C. By taking the analogy of pure NBT [2,3], the anomaly at 300 °C is due to crossover from high temperature paraelectric phase to anti-feroelectric phase. The beginning of dielectric dispersion near 200 °C is attributed to the emergence of ferroelectric phase inside the anti-ferroelectric matrix. On the other hand, the structural evaluation suggests the inception of ferroelectric rhombohedral (R3c) phase inside the tetragonal matrix (P4bm) near onset of dielectric dispersion near 200 °C [5]. Because of nano-meter size of ferroelectric domains a frequency dispersion with relaxor like behavior was observed. The content of ferroelectric phase increases with lowering of temperature. The temperature at which tanδ curve falls rapidly (during heating) is the depolarization temperature (Td ∼140 °C) and the macroscopic ferroelectric nature exists only below this temperature. In addition to these, an increase in the dielectric loss was observed at high temperature (>400 °C) and low frequency region (≤10 kHz), probably due to significant space charge conduction [31–33]. The dielectric response agrees well with the previous results and the temperatures are identical to the phase transition temperatures of pure (Na0.41K0.09Bi0.5)TiO3 [31]. Similar anomalies in the dielectric response were observed for lanthanum doped compositions with variation in transition temperatures. At room temperature, the dielectric constant and tanδ are approximately 1500 and 0.04 (at 1 kHz) for all the compositions. Low value of tanδ indicates highly insulating nature of the ceramic samples. It is also clear from the figure that the maximum value of dielectric constant decreases gradually with increase in the La3+ content. For example, the maximum dielectric constant for x = 0.00, 0.02 and 0.05 are 7300, 5400 and 4400, respectively. The lowering of dielectric constant could be attributed to the

3. RESULTS and DISCUSSION 3.1. Phase formation Fig. 1a shows the x-ray diffraction pattern of powders calcined at 800 °C. It is clear from the figure that x = 0.02, 0.03 and 0.05 contain a secondary phases whose intensity increases with increase in ‘x’. These secondary phases are identified as Bi4Ti3O12 and Bi2O3. The presence of secondary Bi4Ti3O12 phase along with Bi2O3 and NBT was also observed by Aksel et al. [26] in solid state synthesis of NBT. The Bi4Ti3O12 phase is considered as transient phase which appears near 500 °C along with NBT phase and gradually transforms to perovskite NBT through solid state diffusion with remnant sodium and bismuth oxide. The process of transformation from Bi4Ti3O12 to NBT completes near 700 °C [26]. The existence of Bi4Ti3O12 and Bi2O3 (for x ≥ 0.02) phases in calcined powders, implies that lanthanum promotes this secondary phase formation by slowing down the solid state diffusion mechanism. Therefore, the transformation to single phase NBT requires either increase in calcination time or temperature. In present study, the temperature of calcination was increased and it has been observed that secondary phase disappears at higher calcination temperatures. For example, the secondary phase for x = 0.05 disappears when powder is calcined at 2

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lower polarizability of lanthanum as compared to bismuth. The reported polarizability of Bi3+ and La3+ are 6.12 and 4.82 Å3, respectively [20,34]. In addition to this, the reported bond strength of Bi and La with oxygen are 343 and 799 kJ/mol, respectively [20,34]. The higher bond strength with oxygen and lower polarizability of La result in lowering of dielectric constant.

diminishes and antiferroelectric nature develops. It is evident from Fig. 3 that the temperature range of coexistence of the ferroelectric and anti-ferroelectric phases increases monotonically with increase in x. Above this temperature of dielectric maximum (Tm) i.e. region-IV the anti-ferroelectric character diminishes with emergence of ferroelastic tetragonal phase (P4bm) [5]. This ferro-eleastic phase (FL) is para-electric in nature i.e. do not possess any polarization.

3.3. Phase diagram

3.4. Ferroelectric and piezoelectric behavior

The transition temperatures (Td, TR-T and Tc) measured from the dielectric response are employed to draw a partial phase diagram for (Na0.41K0.09Bi0.5-xLax)TiO3 ceramics and shown in figure-3. It reveals that the transition temperatures Td and TR-T decreases gradually with increase in lanthanum content. The temperature of dielectric maximum (Tm) is nearly the same for all the compositions. Here, the tetragonal to rhombohedral phase transition temperature (TR-T) was determined from the onset of dispersion in dielectric constant curve. The decrease of Td and TR-T are, estimated on the basis of a linear fit to the data in Fig. 6 are 9 °C and 7 °C per mol% of lanthanum doping, respectively. The downward shift in the Td and TR-T implies that long-range ferroelectric state is disrupted by lanthanum substitution. The incorporation of lanthanum ion (La3+) ions weaken the dipolar coupling between ferro-electrically active oxygen octahedra (BO6). The suppressing of interactions between oxygen octahedral reduces the depolarization temperature (Td) as well as onset of tetragonal to rhombohedral transition temperature (TR-T). On the other hand, the anti-ferroelectric state results from short-range interactions between dipoles and with addition of lanthanum this short-range interaction is not further suppressed. As a consequence, the temperature of dielectric maximum (Tm) do not show any major change with lanthanum concentration. The regions between phase transition temperatures Td, TR-T and Tc marked as I, II, III and IV for different phases. Based on present work and taking structural analogy with pure NBT [34] it can be concluded that region-I, which lies below the depolarization temperature, is ferroelectric (FE) in nature. In this region macroscopic polarization exists and sample shows piezoelectricity. In region-II, the ferroelectric domains split into nano-domain and exhibit relaxor like behavior. In this region anti-ferroelectric (AFE) ordering also develops with increase in temperature. Therefore, region-II is a mixture of relaxor ferroelectric (RFE) and anti-ferroelectric. In region-III the relaxor behavior

Figure-4 shows the room temperature P-E hysteresis loops measured on unpoled (Na0.41K0.09Bi0.5-xLax)TiO3 ceramics under an applied electric field of 40 kV/cm at 10 Hz. The P-E curve for four different compositions i.e x = 0.00, 0.01, 0.03 and 0.05 are presented in the figure. The samples with composition x = 0.00 to x = 0.03 show a normal hysteresis loop confirming ferroelectric nature of (Na0.41K0.09Bi0.5-xLax)TiO3 ceramics. For x = 0.05, a pinched P-E loop was observed. Generally, the pinched loop is associated to the pinning of domain wall due to structural defect present in the material [35]. In the present study, La3+ is an isovalent substitution and will not lead to defect formation owing to charge imbalance. Moreover, the sintering temperature for all the compositions is the same, therefore, the thermal defects are of the same order for each composition. In view of this, it is believed that presence of pinched hysteresis loop for x = 0.05 is not merely due to pinning of domains with structural defects. Recently, Xu el al [36] using atomistic effective Hamiltonian scheme have demonstrated the existence of pinched hysteresis loop for the systems having ferroelectric-antiferroelectric phase co-existence. It is suggested that AFE ordering exists above depolarization temperature in NBT based materials. But the presence of AFE ordering below the depolarization temperature was observed by Guo et al. [37] and Zhang et al. [38] in NBT-BT and NBT-KNN solid solutions, respectively. The presence of AFE feature was attributed to the broad and frequency dependent phase transition. This, the manifestation of pinched hysteresis curve for x = 0.05 could be related to phase co-existence of EF and AFE. It is observed that depolarization temperature moves toward room temperature with increase in lanthanum content. It is also clear from figure4 that with increasing lanthanum content a slight decrease in the remnant polarization from 27 μC/cm2 for x = 0–22 μC/cm2 for x = 0.03 was observed. For x = 0.05 significantly low remnant 3

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polarization is observed (10 μC/cm2). An abrupt decrease in the remnant polarization for x = 0.05 could be due to emergence of AFE ordering in this composition. The value of Ec also found to decrease gradually with ‘x’ and for x = 0.05, has pinched hysteresis loop, an abrupt decrease in the coercive filed was observed. This behavior suggests that the long-range ferroelectric order is disrupted with lanthanum content and lowering of depolarization temperature with emergence of AFE phase contributing to a decrease of the macroscopic polarization and coercive field. These results are in concurrence with the dielectric measurements. Figure-5 depicts the variation of piezoelectric charge coefficient (d33) and coupling coefficient (kp) as a function of lanthanum content. It is clear from the figure that d33 and kp have insignificant variation till

x = 0.02. The d33 and kp values for these compositions is ∼150 pC/N and 0.31, respectively. For x = 0.03 a valuable decrease in the d33 (120pc/N) and kp (0.23) are observed. The composition x = 0.05 shows significant decrease in d33 (30 pC/N) and kp (0.10). The decrease in the piezoelectric coefficients for x = 0.03 and 0.05 could be associated to the presence of AFE order at room temperature. It is observed that for x = 0.03 a normal ferroelectric like hysteresis loop was observed. However, the lower d33 and kp for this composition suggests existence of AFE phase along with major FE phase. In order to understand the temperature stability of piezoelectricity, the temperature dependence of d33 was measured. The experiment was conducted ex-situ where direct d33 was recorded at room temperature after heating the sample at a particular temperature. The de-poling 4

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Fig. 3. Variation of Td, TR-T and Tc for (Na0.41K0.09Bi0.5-xLax)TiO3 ceramics with lanthanum content.

4. Conclusions

temperature is defined as the point where measured d33 value reduced to half of its room temperature value. Figure-6 shows temperature dependence of normalized d33 for three compositions i.e. x = 0.00, 0.03, and 0.05. Although d33 for all the six compositions was recorded but only three were plotted for the sake of clarity. The temperature range in which depolarization occurs found to reduce with increasing lanthanum content. For example, the depolarization for x = 0.00 occurs near 137 °C and it lowers to 100 °C for x = 0.05. The depolarization temperature measured from temperature dependence d33 lies in close vicinity of ‘Td’, measured from the peak of dielectric loss. However, complete depolarization of the ceramics (d33 = 0) occur about 30 °C higher than depolarization temperature.

The structural, dielectric and piezoelectric investigations were carried on (Na0.41K0.09Bi0.5-xLax)TiO3 ceramics. It has been observed that single phase formation temperature of NKBT ceramic increases with lanthanum content. The powder XRD measurements of sintered ceramic sample suggests that there is no structural transformation with lanthanum doping. However, a lowering of lattice parameter with lanthanum content was observed. The dielectric measurements reveals that dielectric constant decreases with lanthanum content. The depolarization temperature Td and TR-T decreases gradually with increase in La content and is attributed to disruption of long-range ferroelectric

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G. Singh, et al.

d33 (normalized)

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x 0.00 0.02 0.05

1.00 0.75 0.50 0.25 0.00 30

60

90

120

150

180

o

Temperature ( C) Fig. 6. Temperature dependence of normalized d33 of (Na0.41K0.09Bi0.5xLax)TiO3 ceramics for x = 0.00, 0.02 & 0.05.

state by lanthanum substitution at the bismuth sites. The P-E measurement shows the presence of pinched hystersis loop for x = 0.05 with significantly lower remnant polarization and for lower lanthanum contents a normal ferroelectric like hysteresis loop was observed. The piezoelectric constant (d33 and kp) are almost same till x = 0.02, but for x = 0.05 a significantly reduction in their values was observed. The ferroelectric state is found to be more stable for lower lanthanum contents. These measurements indicate that lanthanum doping induces antiferolectric ordering even at room temperature for higher lanthanum contents. References [1] G.A. Smolensky, V.A. Isupov, A.I. Agranovskaya, N.N. Krainik, Sov. Phys. Solid State 11 (1960) 2982. [2] C. Tu, I. Siny, V. Schmidt, Phys. Rev. B 49 (1994) 11550. [3] Y. Himura, H. Nagata and T. Takenaka, 104 (2008) 124106.

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