Structural, dielectric and AC conductivity studies on 0.8Ba0.2(Bi_0.5K_0.5)Ti_1-xZr_xO₃ lead free ceramic system

Structural characterization of the Zr substituted 0.8Ba0.2(Bi_0.5K_0.5)Ti_{1 - x}Zr_xO₃ lead free ceramics was done by using X-ray diffraction in the 2Θ range of 20° to 70°. Room temperature powder X-ray diffractograms of BT-BKT20 substituted with different amounts of Zr are shown in the Fig. 1. All the samples show the perovskite structure and no secondary phases are observed within the detection limit of X-ray diffraction. It implies that Zr⁴⁺ has entered into the BT-BKT20 lattice to form a solid solution. Tetragonal symmetry of the Zr substituted samples was confirmed by the splitting of the reflections (1 0 1)/(1 1 0), (0 0 2)/(2 0 0) and (11 2)/(2 1 1) observed at 31°, 45° and 56°, respectively, and these were in good agreement with the JCPDS data [13, 14].

Fig. 1

Substitution of Zr causes shifting of the reflection/diffraction peaks towards the lower angles. This is due to the substitution of large size Zr⁴⁺ cation (0.072 nm) in the place of comparatively small size cation Ti⁴⁺ (0.0601 nm). Substituting with the bigger size cation expands the lattice and results in an increase of lattice parameters. The values of lattice parameters (a [Å], c [Å]) are shown in Table 1. In the present composition range, no change in the structure was observed. As the Zr substitution was increasing, the tetragonality (c/a ratio) and density were decreasing. The low values of density (theoretical and experimental) are due to the evaporation of bismuth and potassium during the high temperature sintering process [12]. Microstructure images of Zr substituted samples are shown in Fig. 2.

Measurement results of dielectric properties of Zr substituted 0.8Ba0.2(Bi_0.5K_0.5)Ti_{1 - x}Zr_xO₃ (x = 0.01 to 0.06) lead free ceramics as a function of temperature at different frequencies are shown Fig. 3. From Fig. 3, it is seen that at a particular temperature, dielectric constant decreases as the frequency increases for all the compositions. Also as the Zr content increases, the room temperature dielectric constant and maximum dielectric constant values are also decreasing. It is well known that as the Zr content in BaTiO₃ increases the dielectric constant values decrease [1, 2, 5, 7, 9].

Fig. 3 depicts the dielectric loss measured for all the Zr substituted samples as a function of temperature at different frequencies. Dielectric loss is decreasing with an increase of Zr content. Dielectric loss is higher at lower frequencies and decreases with an increase of frequency. It shows that at higher frequencies dipoles were unable to follow the applied AC field and hence, a decrease in dielectric loss at higher frequencies occurred [68, 11]. Zhi et al. [16] reported that dielectric constant value also depends on the density of the samples. The decrease in density or increase in porosity (Fig. 2) with the increase in Zr substitution is responsible for the decrease of dielectric constant and loss.

Fig. 2

From Fig. 3, it is also seen that ferroelectric to paraelectric phase transition temperature (Curie temperature) is decreasing with the increase in Zr content. This is due to the substitution of large size Zr⁴⁺ cation (0.072 nm) in place of small size Ti⁴⁺ cation (0.0605 nm) as this substitution depresses the oriented displacement of B-site ions in the oxygen octahedra. Therefore, the interaction between B-site ions and oxygen ions in the oxygen octahedra would become weaker, resulting in the decrease of phase transition temperature [1, 2, 8, 9, 11].

All the Zr substituted samples exhibit the diffused and dispersive ferroelectric to paraelec-tric phase transition, indicating relaxor behavior. The relaxor behavior is not just due to the Zr substitution, but even the Zr unsubstituted sample itself shows the relaxor behavior [13]. But the relaxor behavior increases with the increase of Zr substitution. In the case of Zr substituted BaTiO₃, the relaxor behavior is observed only at the higher values of Zr substitution (>0.3 mole) [17]. In the present study on the composition 0.8Ba0.2(Bi_0.5K_0.5)Ti_{1 - x}Zr_xO₃, the relaxor behavior is observed for all Zr doped samples (x = 0.01 to 0.06). This is due to the multiple occupation of different valence cations (Ba²⁺, Bi³⁺, K⁺) at the A-site and different homovalent cations (Ti⁴⁺, Zr⁴⁺) at the B-site [13, 18, 19]. Diffuseness of the phase transition can be calculated from the modified Curie-Weiss law [20]: 1ε−1εmax=(T−TC)Cγ$$\begin{equation} \frac{1}{\varepsilon } - \frac{1}{{{\varepsilon _{\max }}}} = {\frac{{\left( {T - {T_C}} \right)}}{C}^\gamma } \end{equation}$$(1)

where γ is degree of diffuseness/relaxation and the value of γ lies between 1 and 2. When γ = 1, the material is normal ferroelectric and follows an ideal Curie-Weiss law and when γ = 2, the material is diffused ferroelectric material. Degree of diffuseness has been calculated for all the Zr substituted samples and it was found to be increasing. Fig. 4 shows the Curie-Weiss law fit for the x = 0.01 and x = 0.06. The values of degree of diffuseness are shown in the Table 1.

Effect of Zr substitution on conductivity can be understood by studying AC conductivity. So, the frequency dependent electrical conductivity was calculated at different temperatures for all the Zr substituted 0.8Ba0.2(Bi_0.5K_0.5)Ti_{1 - x}Zr_xO₃ lead free ceramics by using the relation:

Fig. 3

where ε’ is the real part of dielectric constant, ε₀ is permittivity of free space, ω is angular frequency and tanδ is dielectric loss tangent [13]. AC conductivity was calculated at temperatures 375 °C, 400 °C, 425 °C and 450 °C. It is observed that at the above mentioned temperatures, frequency dependent conductivity was found to obey universal Jonscher’s power law [21]: σA.C=σ0+Aωs$$\begin{equation}{\sigma _{A.C}} = {\sigma _0} + A{\omega ^s}\end{equation}$$(3)

where σ₀ is the frequency independent conductivity (DC conductivity), A is a constant, ω is the frequency of AC field and s is the frequency dependent factor in the range of 0 < s < 1. Fig. 5 shows the frequency dependent AC conductivity at different temperatures for 0.8Ba0.2(Bi_0.5K_0.5)Ti_{1 - x}Zr_xO₃lead free ceramics with x = 0.01 and x = 0.06.

Fig. 4

Fig. 5

It is clear from Fig. 5 that conductivity increases as the frequency and temperature increase. Dispersion in conductivity was observed at lower frequencies for all the Zr substituted samples and all the curves merged at higher frequency region for all temperatures indicating less defect mobility. The increase of conductivity with temperature at high frequencies indicates possible release of space charges, due to which reduction in the barrier height of the material occurs. Plateau region is observed in the low frequency region (frequency independent conductivity, i.e. DC/bulk conductivity) and this plateau region of frequency is increasing with an increase of temperature. This is observed for all the Zr substituted samples. From Fig. 6, it is clear that as the Zr substitution is increasing AC conductivity is decreasing. This is due to the substitution of chemically more stable Zr which reduces the electronic hopping between Ti⁴⁺ and Ti³⁺, thereby, increasing the resistivity of the material. Activation energy (E_AC) was calculated by using Arrhenius relation and it was found to be decreasing from 0.28 eV for x = 0.01 to 0.14 eV for x = 0.06 at 1 kHz. The values are shown in the Table 1.

Fig. 6