DIELECTRIC AND PIEZOELECTRIC PROPERTIES OF 0.8Na_0.5Bi_0.5TiO₃-0.2BaTiO₃ MODIFIED WITH SODIUM NIOBATE

1. Introduction

High-performance lead-free piezoelectrics have been sought after for more than two decades. The  demands for ecological environmentally friendly piezoelectric materials have arisen in the  European Union due to the  RoHS directive. It was expected that the  lead-free alternatives would replace Pb-based piezoceramics by now. As a  result of extensive efforts, some outbreak was reached [1–3]. However, the performance of the lead-free perovskite oxides is still not sufficient for commercial applications. The high demand for new research in the area is still necessary to obtain excellent materials.

The search for lead-free piezoelectrics was concentrated on the  morphotropic phase boundary (MPB). The  MPB compositions in PbZr_xTi_1-xO₃ (PZT) [4] or (1-x)PbMg_1/3Nb_2/3O₃-xPbTiO₃ (PMN-xPT) [5, 6] piezoelectrics proved to have superior piezoelectric and dielectric constants. The  first case is the mixture of ferroelectric lead-titanate [7, 8] with antiferroelectric lead zirconate  [9]. These materials have two different crystallographic structures. If these two components are mixed, the  symmetry in a  particular composition range (i.e. the MPB) becomes monoclinic, lower than for its counterparts [5, 10, 11]. In the case of PZT, this composition range lies around x = 0.5 [4].

(1-x)PbMg_1/3Nb_2/3O₃-xPbTiO₃ system is a mixture of canonical relaxor PbMg_1/3Nb_2/3O₃ [12] and ferroelectric lead titanate. The  MPB compositions are around x = 0.3 [5]. The excellent properties in the MPB compositions were explained by the flattening of the thermodynamic potential. This means that the polarization direction in the system can easily change on the  particular monoclinic plane  [13–15]. Due to this change, the  system is very sensitive mechanically and exhibits both high piezoelectric coefficient and high electromechanical coupling factor (k > 0.9).

The main research of lead-free materials is focused on sodium bismuth titanate (Na_0.5Bi_0.5TiO₃ – NBT)  [16, 17] and potassium sodium niobate (Ka_xNa_1-xNbO₃  –  KNN)  [18]. NBT is a  unique material which has a  compositional disorder at the  A-site of the  perovskite lattice. However, there are still uncertainties about the  chemical ordering of sodium and bismuth ions  [19–22]. Due to the  compositional disorder, NBT has a  very unique sequence of phase transitions. At room temperature, NBT has a  polar R3c space group [21]. Upon heating, this structure gradually transforms into an orthorhombic phase through a  modulated phase (in 550–600  K temperature interval) [21, 23–25], which could possess some antipolar ordering [26]. This transformation takes place in a quite broad temperature range in which both rhombohedral and orthorhombic phases coexist  [23, 25]. Above 600  K, NBT undergoes the phase transition to a tetragonal phase and finally – to a cubic phase at 800 K [21].

The macroscopic properties of sodium bismuth titanate are also quite complicated. The room temperature phase shows a  ferroelectric behaviour, but, at certain depolarization temperature T_d, it loses ferroelectric features. The reason of the depolarization is quite robust. There are claims in the  literature that it is due to the  occurrence of a modulated phase in the 416–470 K temperature interval [25, 27, 28]. Below T_d, a relaxor-like dispersion is observed in NBT [28]. The material itself is widely called a  relaxor ferroelectric, although its dielectric dispersion lies below 1  MHz frequency [29], while, in a canonical relaxor, the dispersion extends to the THz frequency range [30]. The whole complex dielectric spectrum of sodium bismuth titanate ceramics has been published previously  [31]. The  polarization dynamics in NBT was attributed to the anharmonicity of Bi³⁺ ions.

However, sodium bismuth titanate has some flaws that are not desirable for the  piezoelectric applications. Its piezoelectric coefficients are significantly lower compared with those of PZT [32]. The dielectric properties are highly dependent on the stoichiometry of sodium and bismuth ions at the A-site of perovskite lattice [33]. Even a small deviation from stoichiometry causes a significant change in the  ionic conductivity of NBT  [34]. Thus, it is necessary to look for mixed materials to overcome the flaws of NBT. Luckily, NBT can form many solid solutions with substitutional ions both in A- and B-sites of perovskite lattice. Particularly, extensive studies of some ternary solid solutions have shown promising results [35, 36].

The most popular solid solutions of NBT is a mixture with ferroelectric barium titanate (Ba-TiO₃ – BT). (1-x)NBT-xBT can form solid solutions in the  whole range of concentrations. The  morphotropic phase boundary was found in these solid solutions to be around x = 0.06 [37]. The phase diagram of NBT-xBT was summarized in several review papers  [38, 39]. The  increase of the  concentration of barium titanate towards MPB develops a rather sharp jump in dielectric permittivity above which relaxor features persist. The  high-temperature (around 550  K) dispersionless peak persists in the material, independently of the concentration. Its position gradually shifts to lower temperatures upon increase of x, but, above MPB, it is weakly temperature-dependent. In the compositions above MPB, the tetragonal ferroelectric phase is detected at room temperature  [40] and the phase transition to a phase of cubic symmetry takes place in the range of T_d [41]. The dielectric behaviour transforms to the  more classical ferroelectric one. The only difference from the compositions below MPB is that the relaxor-like dispersion is observed above the  phase transition temperature  [42]. Thus, the  Curie–Weiss law in the cubic phase is violated – the material cannot be considered a classical ferroelectric.

Dielectric and piezoelectric properties of the tetragonal side of the  phase diagram have not been so extensively studied, since it was expected to have excellent piezoelectric properties only in the vicinity of MPB. Compared to the lead-based piezoelectrics, the  MPB compositions of NBT-BT solid solutions do not have such drastic improvement in dielectric and piezoelectric properties. Most of them are summarized in the review paper by Rödel [1, 39].

In this contribution, we have chosen to investigate the  dielectric and piezoelectric properties of ternary solid solutions based on the 0.8NBT-0.2BT composition, which is located in the  tetragonal side of the  phase diagram. This composition has the  phase transition temperature around 470  K. It does not show the  peculiar relaxation that is a  signature feature of NBT  [40]. Sodium niobate (NaNbO₃ – NN) was introduced to the 0.8NBT-0.2BT system as the third component. We expected that, like in (0.94-x)NBT-0.06BT-xK_0.5Na_0.5NbO₃  [43], NN will decrease the phase transition temperature to the  room temperature range, allowing one to increase electric field-induced strain and exploiting the jump of the lattice parameter at the phase transition.

2. Experiment

The (1-x)[0.8Na_0.5Bi_0.5TiO₃-0.2BaTiO₃]-xNaNbO₃ samples were prepared by the  conventional solid state reaction method from chemical-grade oxides and carbonates. Four different compositions with x = 0, 0.04, 0.05 and 0.08 were prepared. They were coated with gold electrodes for the electrical contact.

Broadband dielectric properties of all the compositions were measured in two different experimental setups. The low-frequency experiments were carried out in the first setup by placing a sample in between two metal plates to form the parallel-plate capacitor. Capacitance and loss tangent were measured by an HP4284A LCR meter.

In the  second setup, a  sample was placed at the end of the coaxial line to form a capacitor. A vector network analyzer was used to measure the complex reflection coefficient from such a  structure. The detailed description of the experimental procedure can be found in the literature [44]. All dielectric experiments were performed on cooling cycling at a rate of 1 K/min after heating the sample to 500 K.

Polarization and displacement hysteresis loops were measured by an AixaCCT TF2000 analyzer which is equipped with a 4 kV voltage supply and a single beam interferometer. The triangular-shaped voltage signal of 10 Hz frequency was applied on all the samples under test. The temperature was measured with a 100 Ω platinum resistor. Temperature was stabilized for 10 min before each measurement.

3. Results and discussion

Figure 1 summarizes the  temperature dependences of complex dielectric permittivity for all the  compositions at frequency f  =  100  kHz. The  initial composition (0.8NBT-0.2BT) shows a  typical ferroelectric-like behaviour. The  sharp increase above 450 K indicates the 1st order ferroelectric phase transition. The high-temperature phase is cubic, while below the phase transition it is tetragonal. Even though the dielectric properties resemble the  classical ferroelectric material, we have a deviation from the Curie–Weiss law due to an intrinsic disorder.

The addition of sodium niobate changes the dielectric properties quite drastically. First of all, the sharp increase of permittivity that would resemble a  phase transition is absent. Instead, we have a broad anomaly in the 325–450 K temperature interval. This anomaly resembles the one observed in pure sodium bismuth titanate. It is not related to any phase transition, but it is known that above the anomaly a ferroelectric phase cannot be induced. Further increase of sodium niobate concentration shifts the  anomaly to lower temperatures, but it becomes concentration-independant in the x = 0.05–0.08 composition range. It is important to note that the breadth of the anomaly is barely dependent on the composition as well.

In order to understand the  impact of sodium niobate, we present the temperature dependence of 0.96[0.8NBT-0.2BT]-0.04NN composition at different frequencies in Fig. 2. It is evident that the dielectric anomaly around 350 K is mostly expressed at low frequencies. It is barely observed above 1 MHz. This is nearly the same scenario as in pure sodium bismuth titanate  [29]. The  temperature dependence of the imaginary part of dielectric permittivity shows a quite similar behaviour to the canonical relaxor lead magnesium niobate [45]. It seems that there is another anomaly above 500  K. However, this temperature region is out of our experimental capabilities. So, all the features in the dielectric data show that the NBT-like behaviour have been restored with the addition of sodium niobate.

Figure 3 represents the displacement and polarization hysteresis for all the compositions at room temperature (295 K). Ferroelectric behaviour is observed for the composition below x = 0.05 concentration. Obviously, the  displacement hysteresis is due to the piezoelectric effect. The displacement at larger electric fields exceeds 0.3%, which is a rather high value for the  lead-free piezoceramics. It is comparable to the morphotropic phase boundary compositions of NBT-BT.

The increase of sodium niobate concentration reduces the  coercive field at room temperature. The  ferroelectric behaviour at room temperature is supressed in the  x  =  0.05–0.08 compositional range. The electric field dependence of polarization does not retain the character of typical ferroelectric loop. The  compositions containing 5 and 6% of sodium niobate show inclined double hysteresis loops. The remanent polarization is rather small in these compositions. However, the  maximum polarization is close to the values at lower temperatures, in a stable ferroelectric state. Such behaviour indicates that the  ferroelectric long-range order can be induced by the external electric field. This ferroelectric phase does not sustain after the electric field is switched off. It is often observed at some temperature range in canonical relaxor ferroelectrics and antiferroelectrics. The sample containing 8% of sodium niobate show a much slimmer loop. The double hysteresis is not so evident from Fig. 3. The  displacement hysteresis resembles the  electrostrictive behaviour.

In order to understand the development of ferroelectric order, polarization and displacement hysteresis at different temperatures was studied. The temperature dependences of remnant polarization for all the compositions are depicted in Fig. 4. The ferroelectric phase in the 0.8NBT-0.2BT sample persists at least up to 440 K. Some of the curves exhibit a maximum of remnant polarization. This maximum results from the  fact that the  coercive field increases when the temperature is decreased. In order to get fully saturated hysteresis loops, it is necessary to apply much stronger electric field than the  coercive field E_c (usually 1.5 E_c is sufficient). The decrease of remnant polarization with the decrease of temperature indicates that the field is too weak and the polarization is not fully switched on.

The steepest part of P_rem(T) dependence shifts to lower temperatures if the concentration of NN increases. This means that the range of stability of the  ferroelectric phase in these compositions is shifted to lower temperature. The only composition that has the steepest part of P_rem(T) dependence below room temperature is 0.92[0.8NBT-0.2BT]-0.08NN. The  ferroelectric phase range is shifted out of our experimental temperature range.

The temperature evolution of polarization and strain hysteresis is depicted in Fig.  5 by choosing the  0.94(0.8NBT-0.2BT)-0.06NN composition as an example. Above 400  K temperature, we observe the slim loop-like dependence of polarization vs electric field. The displacement curve shows the  pure electrostrictive behaviour. Upon cooling, double hysteresis occurs. The electromechanical response in this double-hysteresis regime resembles the electrostrictive behaviour at a weak electric field regime. After the phase transition is induced, this character is no longer electrostrictive. Finally, at 200  K, we see ferroelectric-like loops and the  displacement is due to the  piezoelectric effect. However, the remnant polarization is not as large as in the compositions with a lower NN content and a significant polarization relaxation is evident – the hysteresis starts at non-zero polarization because the  sample was pre-poled before the  measurement. The  measurement cycle ends up with a  much larger remnant polarization. After removing electric field, the remnant polarization drops quite drastically. The  drop in 0.8NBT-0.2BT is much smaller (see Fig. 3). This feature can arise due to several reasons. One of them is the  finite conductivity in the  materials, but this can be safely neglected since the dielectric data at low frequencies does not show a typical conductivity behaviour. It seems that, after removing the  external electric field, some of the  polarization persists in the  system. It is unclear how long it stays stable. It might have some longer relaxation time.

From the  displacement hysteresis loops, we have calculated a large-signal piezoelectric coefficient d₃₃ of the ferroelectric samples at room temperature. d₃₃ for 0.8NBT-0.2BT is 200 pm/V, while for 0.96[0.8NBT-0.2BT]-0.04NN it is 280  pm/V. These are much higher coefficients than the ones found for the  MPB compositions of NBT-BT. However, these values are not as superior as for the lead-free materials with additional co-doping.

4. Conclusions

We have revealed that sodium niobate impacts the  ferroelectric phase of 0.8NBT-0.2BT by shifting depolarization temperature to lower temperatures. In the  compositions (1-x)[0.8Na_0.5Bi_0.5TiO_3- 0.2BaTiO₃]-xNaNbO₃ with x  =  0.05 and 0.06, the  reversible electric field-induced phase transition is observed at room temperature. However, the contribution of the jump of lattice parameter at this induced phase transition does not create a significantly higher field-induced strain, compared with the 0.80NBT-0.20BT composition where such strain is apparently created by domain reorientation. Most probably it is related with reducing of polarization upon increase of x. The maximal strain value at room temperature is observed for the composition x = 0.06 and exceeds 0.25% at 80 kV/cm. The dielectric anomalies in the system become more similar to the peculiar ones observed in pure NBT.

Our data has also revealed that the  morphotropic phase boundary of NBT-BT system is not so extraordinary in terms of superior piezoelectric properties. The  tetragonal composition has similar or larger piezo coefficients than the  MPB compositions at room temperature. The  ternary compositions despite their complexity show electromechanical properties quite similar to the neat composition (i.e. 0.8NBT-0.2BT), despite the  fact that the displacement has an electrostrictive character at a weak electric field regime. Nevertheless, large displacements in the mixed compositions are preserved due to the 1st order electric field-induced phase transition.

Acknowledgements

This work was supported by the Research Council of Lithuania (Project S-LLT-20-4). This work has been supported by Mutual Funds Taiwan–Latvia–Lithuania Cooperation Project Application LV-LT-TW/2020/10.

References

0,8Na_0,5Bi_0,5TiO₃-0,2BaTiO₃, MAIŠYTO SU NATRIO NIOBATU, DIELEKTRINIŲ IR PJEZOELEKTRINIŲ SAVYBIŲ TYRIMAS

^a  Vilniaus universiteto Fizikos fakultetas, Vilnius, Lietuva

^b  Latvijos universiteto Kietojo kūno fizikos institutas, Ryga, Latvija

^c  Nacionalinio Cheng Kung universiteto Elektros inžinerijos fakultetas, Tainanas, Taivanas

Santrauka

Šiame darbe buvo tiriamos tetragoninio 0,8Na_0,5Bi_0,5 TiO₃-0,2BaTiO₃, maišyto su NaNbO₃, dielektrinės ir pjezoelektrinės savybės. Tyrimai atskleidė, kad didinant NaNbO₃ koncentraciją x, feroelektrinė fazė yra slopinama kambario temperatūroje. Maišytose kompozicijose išryškėja plati anomalija 325–450 K temperatūrų intervale. Ši anomalija primena feroelektriniams relaksoriams būdingus bruožus. Detaliau ištyrus poliarizacijos ir poslinkio histerezės kilpas buvo nustatyta, kad maišytose kompozicijose formuojasi dvigubos histerezės kilpos, būdingos elektriniu lauku indukuojamiems 1-os rūšies faziniams virsmams. Buvo nustatyta, kad kuo didesnė natrio niobato koncentracija, tuo žemesnėje temperatūroje stebimos feroelektrinės poliarizacijos kilpos. Elektromechaninės sistemos savybės skirtingose kompozicijose yra panašios, pasiekiami gana dideli pjezoelektriniai poslinkiai. Grynoje medžiagoje elektromechaninis atsakas yra susijęs su pjezoefektu, o maišytose kompozicijose – su dideliu gardelės parametrų pokyčiu, indukuojant 1-os rūšies fazinį virsmą elektriniu lauku.