Synthesis, Structural, Electrical, and Thermal Studies of Pb_1−𝑥Ba_𝑥Nb_𝟐O_𝟔 (𝑥=𝟎.𝟎 and 𝟎.𝟒) Ferroelectric Ceramics

Abstract

The polycrystalline ceramic samples of lead barium niobate with general formula Pb_1−𝑥Ba_𝑥Nb₂O₆ (𝑥=0.0 and 0.4) were prepared by conventional solid state reaction method. The room temperature X-ray diffraction patterns reveal that both of the samples have orthorhombic crystal structure with space group Cm2m. The dielectric constant and dissipation factor were measured as a function of frequency (100 Hz-2 MHz) and temperature (RT-660K). The DC electrical conductivity of both the samples was measured from RT to 660 K. The activation energies calculated from log σ versus 1000/T curves in ferroelectric phase of the compounds are 1.09 eV for pure (𝑥=0.0) sample and 1.36 eV for Ba-substituted (𝑥=0.4) sample. The values of activation energies show that the substitution of Ba²⁺ ion on Pb²⁺ ion site increases the resistivity of pure PbNb₂O₆ (𝑥=0.0) ceramic. The modulated differential scanning calorimetry (MDSC) has been used to investigate the phase transition temperature of both the compounds and also to see the effect of Ba²⁺ ion substitution on the phase transition temperature, specific heat, and other thermal parameters of the compound.

1. Introduction

Lead niobate (PbNb₂O₆) is one of the first crystals of the tungsten bronze-type structure which was first reported as ferroelectric [1]. It is well known that the tungsten bronze niobates generally indicate a relaxor ferroelectric nature. These materials crystallize in a variety of structures including tetragonal tungsten bronze (TTB), hexagonal tungsten bronze (HTB), and intergrowth tungsten bronze (ITB) [2].

The synthesis and phase identification of the piezoelectric/ferroelectric phase of lead niobate is are difficult, that a few competing phases and compounds tend to form during the preparation [3–10]. Crystallographic studies revealed that at room temperature material is found in ferroelectric orthorhombic phase and above the Curie temperature (843 K) it shows paraelectric tetragonal phase [11–14]. The activation energies calculated from dc conductivity measurement from different formalism showed the ionic-type conduction in doped niobate Pb_0.74 K_0.13Y_0.13Nb₂O₆ compound [15]. Thermal characterization through DSC technique for pure PbNb₂O₆ showed the Curie temperature at 843 K in heating profile and at 826 K in cooling process [16].

Due to high Curie temperature and low-quality factor, the material is useful for fabrication of ultrasonic transducers for high-temperature applications where the PZT and other piezoelectric materials cannot be used [5, 17]. The present paper report on the synthesis, structural, electrical, and thermal properties of pure and 40% Ba-substituted lead niobate compounds.

2. Experimental

The polycrystalline ceramic samples of Pb_1−𝑥Ba_𝑥Nb₂O₆ (𝑥=0.0 and 0.4) were prepared by high-temperature solid state reaction technique. The constituent oxides, PbO (High Media, 99.9%), BaO (CDH Mumbai, 99.9%) and Nb₂O₅ (High Media, 99.9%) were mixed together according to stoichiometric amounts in a mortar and pestle under liquid medium to make a homogeneous mixture. The mixtures were calcined at 1073–1273 K for 4 hours in a silica crucible in air atmosphere. The process of firing and mixing was repeated for a number of times. The product was then pressed into pellet form by applying pressure of 498 MPa in a hydraulic press. Finally, these pellets were sintered at 1553–1573 K for 1 hour and cooled to RT very fastly. The room temperature (RT) X-ray diffraction patterns of the sintered pellets were collected using a Rigaku X-ray diffractometer with CuKα radiation and nickel filter in a wide range of 2θ from 10° to 60° with a scanning rate of 2° per minute and step size of 0.02°. The instrument was calibrated using the pure silicon sample provided with the instrument. Before measuring the electrical properties both the samples were electroded with silver paint.The dielectric constant and dissipation factor as a function of frequency and temperature were measured in air atmosphere using Hioki LCR 3532-50 Hi Tester. The dc electrical conductivity measurement was carried out on laboratory made set up using two probe method. The modulated DSC measurement was carried out on TA instruments model 2910 MDSC equipment from RT to 853 K in inert (N₂) atmosphere with a heating rate of 5 K/min and ±0.75 K modulation per 60 second.

3. Results and Discussion

Figure 1 shows the X-ray diffraction patterns of Pb_1−𝑥Ba_𝑥Nb₂O₆ (𝑥=0.0 and 0.4) compounds. All the diffraction peaks, indexed with space group Cm2 m in orthorhombic crystal structure using Rietveld refinement program Fullprof, are well matching with JCPDS card no. 11–0122 of the PbNb₂O₆ compound. The pseudo-Voigt function was chosen for the profile shape refinement. The refined cell parameters are found to be 𝑎=17.682Å, 𝑏=17.953Å, and 𝑐=3.841Å for pure compound (𝑥=0.0) and 𝑎=17.254Å, 𝑏=17.505Å, and 𝑐=3.736Å for Ba-substituted compound (𝑥=0.4). From the figure, it is clear that the X-ray diagram of the substituted compound is quite similar to that of the pure compound except few reflection peaks which are not similar in both the cases. It has also been observed that there is little shift in the peak positions of the substituted compound which is responsible for the corresponding minor change in the structural parameters.

Figure 1 

X-ray diffraction pattern of Pb1−𝑥Ba𝑥Nb2O6 (𝑥=0.0 and 0.4).

There is a good agreement between the observed and calculated interplanar spacings (𝑑-values) for both the compounds and are shown in Table 1. From the table, it is found that with the substitution of Ba²⁺ ion the change in 𝑑-values are not much different insignificant.

Table 1 

Comparison between 𝑑-observed and 𝑑-calculated values for Pb_1−𝑥Ba_𝑥Nb₂O₆ (𝑥=0.0 and 0.4).

The average crystallite sizes as determined from the XRD using the Scherrer formula 𝑡=0.89𝜆/𝛽cos𝜃, where 𝛽= full width at half maxima and 𝜆= wavelength of radiation used, are shown in Table 1.

The frequency variation of the dielectric constant (ε′) and dissipation factor (ε′′) of both samples are plotted in Figures 2(a) and 2(b). It was found that with the increase of frequency both the dielectric constant and dissipation factor decrease very fast up to frequency of 10 kHz which is due to the space charge contribution. After 10 kHz, ε′ and ε′′ decrease slowly due to the ionic and electronic charge contribution up to the measured frequency range of 2 MHz. These variations of ε′ and ε′′ with frequency indicate the characteristics of relaxor behavior of the material. Figures 3(a) and 3(b) show the temperature dependence of the ε′ and ε′′ for both the compounds at a frequency of 100 kHz. From the Figure 3(a), it is observed that the values of ε′ of both the compounds are almost temperature independent or constant up to the temperature of 450 K and then increases. The pure compound (𝑥=0.0) does not show any transition/dielectric anomaly up to the measured temperature range. With the substitution of Ba²⁺ ion in place of Pb²⁺ ion (𝑥=0.4), the overall dielectric constant decreases and shows a broad and diffuse peak at 560 K and then decreases again up to the measure temperature range of 660 K. This broad and diffuse dielectric response may be correlated with the fine grain size of the compound consistent with the literature report [18, 19] as well as observed in the case of PZT [20]. The temperature variation of the dissipation factor (ε′′) for both the compounds at a frequency of 100 kHz is shown in Figure 3(b). The dissipation factor shows almost same trend as dielectric constant with no indication of any peak/anomaly up to the measured temperature range.

(a)

(b)

Figure 2 

(a) Dielectric constant versus log f curves of Pb1−𝑥Ba𝑥Nb2O6 (𝑥=0.0 and 0.4). (b) Dissipation factor versus log f curves of Pb1−𝑥Ba𝑥Nb2O6 (𝑥=0.0 and 0.4).

(a)

(b)

Figure 3 

(a) Dielectric constant versus temperature curves of Pb1−𝑥Ba𝑥Nb2O6 (𝑥=0.0 and 0.4). (b) Dissipation factor versus temperature curves of Pb1−𝑥Ba𝑥Nb2O6 (𝑥=0.0 and 0.4).

Figure 4 shows the temperature dependence of dc conductivity (𝜎_dc) for both of the compounds where log of conductivity was plotted against inverse of temperature. Since, the conductivity is too small to be measured below the temperature of 463 K for both the compounds and hence the results are plotted for elevated temperature range from 463 K to 660 K. Above 463 K, the conductivity of both samples increase up to the measured temperature range. By measuring the slope of the curves and using the relation 𝜎=𝜎_𝑜exp[−Ea/kT], (where k is Boltzmann constant, 𝜎_𝑜 is preexponential factor and Ea is the activation energy) the activation energies of both the samples were determined. The average activation energy calculated for pure compound is 1.09 eV while the same for Ba-substituted compound are 1.36 eV in ferroelectric phase (below transition temperature, 560 K) and 1.22eV in paraelectric phase (above transition temperature, 560 K). The low value of activation energy in high temperature region (Ea=1.22eV) may be due to the oxygen ion vacancies which might be the main carrier in the electrical conduction process.

Figure 4 

DC conductivity versus 1000/T curves of Pb1−𝑥Ba𝑥Nb2O6 (𝑥=0.0 and 0.4).

The MDSC curves of the Pb_1−𝑥Ba_𝑥Nb₂O₆ (𝑥=0.0 and 0.4) compounds are shown in Figures 5(a) and 5(b). From the Figure 5(a), it is clear that the pure compound (𝑥=0.0) shows an exothermic peak at 841 K in heat capacity curve and an endothermic peak at 840 K in heat flow curve. These exo- and endothermic peaks around 568°C/567°C (841 K/840 K) are consistent with the reported ferro to paraelectric phase transition temperature (Tc=843K) [2]. When the Ba²⁺ ion (40%) is substituted in the Pb²⁺ ion site, the Tc peak shifted from 568°C (841 K) to 277°C (550 K) Figure 5(b). The shift in Tc value from 841 K to 550 K is consistent with our dielectric result. It is surprising that the heat capacity curve does not show any anomaly/transition around 550 K. This change in Tc for Ba-substituted compound may be due to the difference in ionic radii of Pb²⁺ and Ba²⁺ as discussed in our earlier papers [21–23] as well as their different thermal characteristics. The calculated values of entropy change (Δ𝑆) are 1.05 JK⁻¹  g⁻¹ and 0.88 JK⁻¹ g⁻¹ for 𝑥=0.0 and 𝑥=0.4 compounds, respectively, at phase transition temperature.

(a)

(b)

Figure 5 

(a) Heat capacity and heat flow versus temperature curves of Pb1−𝑥Ba𝑥Nb2O6 (𝑥=0.0). (b) Heat capacity and heat flow versus temperature curves of Pb1−𝑥Ba𝑥Nb2O6 (𝑥=0.4).

4. Conclusion

From the above studies, it is concluded that both samples exhibit single ferroelectric orthorhombic phase with space group Cm2m. Although the Ba-substitution is responsible for shifting in peak positions but both of the compounds shows same crystal structure. The high dielectric constant and high Curie temperature of pure lead niobate is suitable for high temperature piezoelectric transducers applications. With the substitution of isoelectronic cation (Barium) on Pb-ion site, the binding strength and thermal stability increases as well as the lattice parameters, crystallite size, dielectric constant, transition temperature, and dc conductivity decreases. The increase in binding strength and thermal stability as well as the decrease in transition temperature are useful parameters for eventual functionality of the materials for device application.

Acknowledgments

The authors are thankful to Dr. A. M. Awasthi and Suresh Bhardwaj of UGC-DAE-CSR Indore for providing MDSC measurement facility.