Structural and Dielectric Properties of (1-x) (Al0.2La0.8TiO3) + (x) (BiZnFeO3) (x = 0.2 - 0.8) Nanocomposites

(1-x) (Al 0.2 La 0.8 TiO 3 ) + (x) (BiZnFeO 3 ) (x = 0.2 - 0.8) [ALTBZFO] nanocomposites were synthesized via hydrothermal method. The X-ray diffraction patterns indicated the phase transformation from tetragonal to cubic for x = 0.2 to 0.4 - 0.8 samples, respectively. The surface morphology showed the existence of nanospheres like structures. At 1 MHz frequency also, the dielectric constant was increased from 230 to 710 for x = 0.2 – 0.6 samples, respectively. But, interestingly, x = 0.6 nanocomposite exhibited the negative dielectric behavior having the dielectric constant ( ε' ) ~ -58.5 and dielectric loss ( ε" ) ~ -417 at 8 MHz. Likewise, x = 0.6 sample showed ac-electrical conductivity ( σ ac ) -0.159 S/cm at 6 MHz. Hence, these kinds of materials can provide high charge stored capacitor, and perfect absorber applications. and dielectric properties,


Introduction
It was a familiar fact that the perovskite nanoceramic composites performed advanced structural, and dielectric properties [1]. Even the applications of several perovskite nanocomposites were extended to biomedical, magnetic, electromagnetic, microwave, ferroelectric, charge storage etc., [1]. Few of the above merits were seen in the case of LaTiO3 [2]. In the literature [1 -4], AlLaTiO3 was studied extensively by several researchers for structural, optical, morphological, electrical, and dielectric properties. The results indicated that the huge dielectric response was obtained from the compositions of AlLaTiO3.
Further, the nanocomposites were synthesized including the AlLaTiO3, and studied for the similar properties. These materials were of ALTBT [1], ALTBFO [5,6], LCET [7], and AELTO [2] revealing the nanorod formation, negative dielectric behavior, and defect dipole formation, respectively. This indicated a fact that the more advanced properties were achieved on mixing the barium titanate, bismuth iron oxide, and europium oxide.
Similarly, for ALTBFO nanocomposites, the zinc element was substituted within the BiFeO3 system. That is, the BiZnFeO3 was mixed in the ALTBZFO nanocomposite system.
In general, the ZnO is a compound having variety of applications such as wide band gap semiconductor, high conducting due to high mobility, good luminescence, etc., [8]. Due to these factors, we planned to select this compound as the substituent in the composite system thereby achieving more advanced structural, morphological, dielectric, and impedance spectroscopy properties. In addition, BiFeO3 showed the biosensor [9], magnetoelectric [10], optical [11], magnetic [12], multiferroic [13], enhanced electrical properties [13], and high dielectric constant [14]. Further, it was also noticed in the literature that the BiZnFeO3 provided photovoltaic [15] and progressed magnetic properties [16]. In case of Zn-doped BiFeO3, the leakage, and high polarization behavior was observed predominantly [17].
Similarly, there was a significant influence on optical, electrical, and magnetic properties upon doping the zinc in the bismuth iron oxide perovskite system [18 -20]. Uniyal et al. [21], reported the multiferroic nature of ZnFe2O4-BiFeO3 nanocomposites. Hence, the ALTBZFO nanocomposites were prepared using hydrothermal method owing to the benefits such as low operating temperature, less time & power consuming, well crystallinity, low particle size, etc., [1].
Then, the autoclave was kept in a hot air oven, and further the reaction was taken place at 150 o C/6 hr. After completion of reaction, the autoclave was quenched to room temperature.
Then, the Teflon bowl was removed from the autoclave to extract the nanocomposite solution. This solution was washed for 10 times, and further dried for 2 hr., at 60 o C. Thus, the final nanocomposite product was obtained for x = 0.2 -0.8 samples (Fig.1). In the next step, the nanocomposites were subjected to various analysis works like X-ray diffractometer  whereas the x = 0.4 -0.8 samples showed the cubic phases. In addition, the secondary phases related to the BiFeO3 (indicated by *) and La2TiO5 (indicated by +) were noticed. Usually, this kind of secondary phase formation was attributed to the vast difference of ionic radii of cations present in the composite system. That is, La +3 : 0.136 nm, Bi +3 : 0.096 nm, Al +3 : 0.039 nm, Ti +4 : 0.068 nm, Fe +3 : 0.0645 nm, and Zn +2 : 0.060 nm [22]. Herein, the bismuth, and lanthanum cations have larger ionic radii as compared with the rest of the cations. Therefore, bismuth, and lanthanum cations can form their associated compounds (BiFeO3 and La2TiO5).
The high crystallinity was also identified for all diffraction planes. Further, the average crystallite diameter was calculated using the Scherrer relation [23]: Da: 0.9λ/βCosθ, where λ indicates the wavelength of X-rays used (0.15418 nm), β is full width half maxima (FWHM) and θ is the angle of diffraction. The results indicated that the Da was found to be increasing   Fig.3 at 100 nm scale. In Fig.3, it was observed that the larger numbers of cotton spheres like grains were formed for x = 0.2.
Moreover, few nanofibers like grains were also deposited in the microstructure. It was also noticed that the distribution of cotton spheres was almost homogeneous. The dielectric behavior of ALTBZFO nanocomposites was explained using the dielectric constant (ε'), and dielectric loss (ε") versus frequency (log ω) plots. In Fig.6 (ε' -log ω), it was seen that the ε' was very high for x = 0.2 -0.8 samples. Usually, this kind of behavior can be attributed to the space charge species. That is, the charges will be piled up at the grain boundary interface at low log ω. Thus, the polarization can be developed progressively, and it can be treated as Maxwell-Wagner's polarization [26]. It leads achieving the high dielectric constant. It was seen from the ε' -log ω plots that the ε' was found to be increasing from 3727 to 14166 for x = 0.2 -0.6 samples (Table.2), respectively at 1 kHz. But for x = 0.8 sample, it was decreased to 5081. Similarly, the ε" (in Fig.6 (ε" -log ω)) also  (Table.2). This indicated a fact that for x = 0.6 sample, the transition was happened from positive to negative dielectric behavior at 6 MHz frequency. In general, it was known that the positive dielectric behavior is a usual property while the negative dielectric behavior is an unusual property. This kind of property was earlier observed in the case of perovskite materials [27,28] as well as the metamaterials [2, 7, 29 -33]. However, the present nanocomposites are not the metamaterials. But nevertheless, the negative dielectric behavior can be obtained due to the geometrical effect thereby providing the negative polarization.
That is, for the provided electric field frequency, the charge carriers will be aligned in the opposite direction of the applied electric field. Therefore, the polarization will be developed having negative sign. This can lead to the formation of negative polarization. Consequently, the negative dielectric constant, and dielectric loss will be achieved. This behavior can be considered as diaelectricity nature which is like the diamagnetism [2]. That means, in case of diamagnetic nature of a magnetic specimen, the magnetization will be developed opposite to the direction of applied magnetic field. Likewise, the in case of diaelectricity, the polarization will be developed opposite to the direction of applied electric field frequency. The similar observations were evidenced in the case of x = 0.6 nanocomposite during 6 -8 MHz frequency. From these results, it was understood that the nanocomposites (x = 0.2, 0.4, and 0.8) exhibiting the high dielectric constant can be suited for high charge storage capacitor applications [2]. Likewise, the x = 0.6 sample offering negative dielectric behavior can provide applications in waveguides, filters, antenna, electromagnetic cloaking devices, and perfect microwave absorbers [2]. Further, the ac-electrical conductivity was calculated using a relation σac: εoε"ω, where εo is the permittivity of free space, and the other symbols have their usual meaning [34]. To explain the electrical conductivity as a function of composite and frequency, the log σac versus log ω plots (Fig.7) were drawn. It was clear that at low log ω value, the σac was very small while going to the high log ω values, the conductivity was increased progressively. This was happened for y = 0.2, 0.4, and 0.8 samples. But for x = 0.6 sample, the ac-electrical conductivity was noticed to be negative from 6 to 8 MHz frequency.
This behavior was obtained owing to the space charge effect. That is, the charge carriers will be accumulated at the interface, and therefore, this trend can lead to the high magnitude of Z', and Z" values. Moreover, at high log ω values, the Z', and Z" parameters were decreased to small values. This can indicate a fact that the effect of charge species was decreased at high frequencies. The Cole-Cole plots (Z' versus Z") were shown in Fig.9. For all the samples, the Cole-Cole plots showed the formation of single, and partial semicircular arcs. In general, the partial arcs will be formed due to the long-range motion of the charge carriers [34]. But for x = 0.6, the reverse nature of arc was observed at high frequencies. This manner was occurred due to the diaelectricity behavior at frequencies. The dielectric modulus spectrum was were noticed in the literature [2,7,27,28]. The partial arcs were formed due to the partial relaxation strength.

Conflicts of Interest:
The authors declare that we have no conflicts of interest.
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