Effect of stoichiometric addition of La on Bi0.5 Na0.5TiO3 and large strain in thus modified (Bi1-xLax)0.5 Na0.5TiO3 at room temperature and above

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Highlights

  • Synthesis of non-lead (Bi1-xLax)0.5Na0.5TiO3 system and reported their structural and electrical ferroelectric properties.

  • Coexistence of FE - AFE phases at room temperature is due to the decrease in depolarization temperature of the system.

  • Large electric field induced strain (EFIS) ~0.50% is observed for x = 0.05.

  • Strain behavior of x = 0.05 with temperature show large EFIS values even at higher temperature (~0.4% at 125 °C).

  • Considering the toxicity of lead based system, this Non-lead system can be a promising candidate for actuator application.

Abstract

The electromechanical behavior of non-lead La modified BNT ((Bi1-xLax) 0.5Na0.5TiO3 is investigated for 0 ≤ x ≤ 0.10 where La is introduced stoichiometrically in parent material BNT. A large bipolar strain of ~0.50% is achieved for composition x = 0.05 which has both antiferroelectric (AFE) and ferroelectric phases (FE) at room temperature, indicating electric field induced phase transition from AFE to FE as the key reason for such large strain. We suggest that addition of La > 0.03 at Bi-site leads to the reduction in lattice distortion of unit cell and resulting in lowering of depolarization temperatures Td (where FE-AFE phase transition occurs) in these modified BNT samples. Moreover Temperature dependent bipolar strain measurement show that bipolar strain has low temperature dependent tendency for particular composition, suggesting that it is very promising non-lead material for practical actuation applications.

Introduction

Ferroelectric materials which show strain on application of electric field or vice versa are widely used in actuator and sensors applications [1]. Since the Electric field induced strain (EFIS) levels in normal piezoelectric and electrostrictive compositions are rather low ~0.1%–0.2% [2]; usually compositions at “morphotropic phase boundary (MPB)” of AFE and FE phases are explored in literature for this purpose [[4], [5], [6]]. These MPB compositions are known to show significantly large strain due to the difference in AFE and FE phase's lattice volume (An FE form has larger lattice volume in comparison to AFE form for the same composition [3]).

In light of the above, “Bi0.5Na0.5TiO3 (BNT)” is a promising candidate for showing EFIS properties, as in this system at 200 °C (designated as Td) the ferroelectric rhombohedral phase transform to AFE tetragonal that finally change to cubic paraelectric phase at ~ 360 °C. Appropriate modification of the system to lower the Td to ~ room temperature, could result in large EFIS at room temperature. In 2007 Zhang et al. investigated “(0.94−x) Bi0.5Na0.5TiO3–0.06BaTiO3xK0.5Na0.5NbO3 ceramics where they observed a large strain of 0.45% for x = 0.02 due to the electric field induced AFE-FE transition [7]. In 2008 Kounga et al. investigated “(1−x) Bi0.5Na0.5TiO3xK0.5Na0.5NbO3 demonstrated EFIS ~0.25% for x = 0.12 resulting from electric field-induced AFE-FE transition [8]. After that researchers have been studying BNT based systems for large EFIS values [[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]]. Recently we have also demonstrated a large EFIS ~0.8% in “1-x(Bi0.5Na0.5TiO3)- x(Bi0.5K0.5TiO3)-y(K0.5N0.5NbO3)” system for x = 0.20 y = 0.01 [13]. In our study, KNN was added to BNT-BKT system in stoichiometric proportion and depolarization temperature Td values of these series were observed to be significantly lower with respect to the BNT-BKT system and consequently large strain at room temperature for some compositions was also observed. Seifert et al. also investigated “(1–x) (0.8BNT–0.2BKT)–x (0.97KNN–0.03BKT)” and observed EFIS ~0.48% for 1 mol% KNN content [11]. In 2012, we investigated “((1−x) Bi0.5Na0.5TiO3xK0.47Na0.47Li0.06Nb0.74Sb0.06Ta0.2O3)” system and demonstrated large strain ~0.40% for composition x = 0.08 [15]. Interestingly, recently Guo et al. investigated “(Nay,Biz)Ti1−xO3(1−x)-xBaTiO3” ceramics [14]. They observed that an antiferroelectric phase can be achieved by changing Na+ and Bi3+ mole ratio. They also observed a strain of 0.48% due to the electric field facilitated phase transition from AFE to FE. It is clear from all these studies [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]] that large EFIS are observed for those compositions which show pinched ferroelectric loops due to the coexistence of “FE-AFE”.

In recent years La modified at A-site in BNT systems have been studied by different researchers with formula [(Bi0.5Na0.5)1-1.5xLax) TiO3] leading to Bi and Na vacancies (A-site vacancies) and [(Bi0.5Na0.5)1-xLax) Ti1-0.25xO3] creating Ti vacancies (B-site vacancies) [[30], [31], [32], [33], [34], [35]]. Yi et al. investigated phase transition behavior of non-stoichiometric La-modified BNT system with formula [(Bi0.5Na0.5)1−1.5xV0.5xLax]TiO3 (NBLT) [33,34]. They observed modulated antiferroelectric phase originated due to the disordering effects of the La ions and cation vacancies. They suggested that the due to the competition between rhombohedral ferroelectric phase and tetragonal paraelectric phase, a modulated antiferroelectric phase was induced in NBLT ceramics.

These studies suggest that AFE phase at ambient temperature in BNT based system can be achieved in both stoichiometric and non-stoichiometric compositions. Therefore La addition in BNT in stoichiometric way i.e. (Bi1-xLax) 0.5Na0.5TiO3 and EFIS properties in thus modified BNT system cannot be ignored.

In this direction, we have investigated La modified BNT, where La is stoichiometrically replaced at Bi-site with chemical formula (Bi1-xLax)0.5Na0.5TiO3 (x varies as 0 ≤ x ≤ 0.10). Here we discuss the results of La modified BNT using X-ray analysis, ferroelectric, strain and dielectric measurements. Through temperature dependent ferroelectric measurement we clearly demonstrate that compositions (x ≤ 0.03) show pure ferroelectric behavior at room temperature and both AF and FE phases at higher temperatures. Whereas, in compositions 0.05 ≤ x < 0.1 we have been able to achieve the pinched P-E loops at room temperature due the presence of both FE and AFE phases and consequently high strain values. We suggest the presence of both FE - AFE phases at room temperature is due to the reduction in lattice distortion of unit cell by stoichiometric addition of La ≥ 0.05 at Bi-site leading to the decrease in depolarization temperature and related effects. We exhibit large bipolar strain ~0.50% with Ec = 27 kV/cm and Pmax/Pr = 36/18 μC/cm2 for x = 0.05 composition at room temperature.

Section snippets

Experimental details

A series of compositions with formula (1−x)BNT–x (La) (BNLT) where x varies from 0.0 to 0.15 was synthesized via solid state route using Bi2O3, TiO2, Na2CO3, and La2O5 powders (Aldrich with purity > 99.5%). These raw powders were weighed stoichiometrically followed by ball milling in acetone medium for 24 h. After ball mixing, powder was calcined at 900 °C for 3 h. Pellets were formed by pressing the calcined powders uniaxially under 80 MPa and sintered at ~1150 °C for ~2 h. To suppress the

Results and discussions

Fig. 1(a) illustrates the XRD patterns of all compositions in the 2θ ranges 20°–80° at room temperature. The crystal structures of all compositions are observed to be perovskite and no other phase is observed. The magnified views of peaks in the 2θ ranges 36°–50° are also shown here (Fig. 1 (b)). BNT shows rhombohedral structure and match well with standard JCPDS File number 36-0340.

Fig. 1(b) clearly depicted that addition of La, split in (003)/(021) peaks decreases and these two peaks start

Conclusions

Stoichiometric addition of La at Bi-site of BNT reduces the lattice distortion of unit cell, leading to a decrease in Td, which in turn facilitates the coexistence of FE - AFE phases in samples at lower temperature. In fact for compositions 0.05 ≤ x < 0.1 we have been able to achieve the pinched P-E loops as well as large strain values at room temperature. For composition x = 0.05 a large bipolar strain and good thermo stability comparable is also achieved which suggest that it is very

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

One of the Author Amrita Singh would like to acknowledge “Department of Science and Technology (DST)”, India for financial help through WOS-A program (Ref: SR/WOS-A/PM-31/2018(G))

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