Investigation of microstructure and dielectric behavior of Bi2/3Cu3−xMgxTi4O12 (x = 0, 0.05, 0.1 and 0.2) ceramics synthesized by semi-wet route

In this manuscript, we have reported the synthesis and characterization of Mg-doped and undoped BCTO ceramic (Bi2/3Cu3−xMgxTi4O12, x = 0, 0.05, 0.1 and 0.2) sintered at 1173 K for 8 h, which have been prepared by the semi-wet route. The single-phase formation of ceramic was approved by the XRD pattern. The microstructural properties were studied by TEM and AFM. The samples were characterized by dielectric and impedance spectroscopic properties. The dielectric constant (εr) was calculated to be 3024 for BCTO ceramics at 423 K and 100 Hz. The tangent loss (tan δ) value was calculated to be 0.45 for BCTO ceramic at 423 K and 10 kHz. The internal Barrier Layer Capacitance (IBLC) mechanism was responsible for the high value of the dielectric constant. XPS spectroscopy confirmed the oxidation state of the elements present in the ceramic. It was observed from Impedance and modulus studies that there was the existence of the Maxwell–Wagner form of relaxation in the ceramics. In the temperature range 300–500 K, the Bi2/3Cu3−xMgxTi4O12 (where x = 0, 0.05, 0.1, 0.2) ceramic follows Arrhenius behavior with an almost single slope.


Introduction
ACu 3 Ti 4 O 12 (where A = Ca, Y 2/3 , Bi 2/3 , Gd 2/3 ) type of perovskite oxide was the first time revealed in 1967 [1], which was able to produce high dielectric constant. The detection of ultrahigh dielectric response in body-centered CaCu 3 Ti 4 O 12 (CCTO) has induced vast research to understand the underlying physics behind the unusual phenomenon [2][3][4]. For many technical uses such as condensers, resonators, filters and memory chips, high dielectric materials have been fascinated very much [5][6][7][8][9][10][11]. Regularly, the size and execution of such electronic gadgets can be managed by their material properties. Further, the dielectric constant of the material should be independent of temperature and frequency [12]. Recently, high dielectric permittivity (e * 10 4 ) with weak temperature and frequency-dependent permittivity have been noticed in TiO 2 -based ceramics and CCTO (CaCu 3 Ti 4 O 12 )-like compounds [13,14]. The search for ultrahigh permittivity (e r [ 10 3 ) materials is one of the most popular research topics in the fields of microelectronics and energy storage devices. CaCu 3-Ti 4 O 12 (CCTO), a typical ACu 3 Ti 4 O 12 (ACTO, where A = Ca, Cd, La 2/3 , Y 2/3 , Bi 2/3 , Na 1/2 La 1/2 , Na 1/2 Y 1/2 , or Na 1/2 Bi 1/2 , Sr) ceramic, have been widely studied due to its ultrahigh dielectric permittivity and good frequency/thermal stability. However, other members of the ACTO family with isostructures, have also been widely studied. Impedance spectroscopy estimation shows that the high dielectric study is a direct result of the Internal Barrier Layer Capacitance (IBLC) impact [15][16][17][18][19][20][21][22][23]. Bi 2/3 Cu 3 Ti 4 O 12 (BCTO) is isostructural with CCTO, already been reported in the literature. It indicated the temperature and frequency dependence of dielectric as CCTO [21][22][23][24]. Particles covered with a thin coating of other materials have improved properties compared to nonfunctional uncoated particles, such as the Lu 3 Al 5-O 12 ÁCe@SiO 2 core-shell composite, which reduced the thermal/chemical degradation of LEDs [25]. Because of their possible applications in various fields such as engineered multiferroics, catalysts, and biomedical applications, core-shell structured materials have been mostly studied for a few years [26]. The sources of high permittivity in ACTO materials would include extrinsic processes. Some mechanisms have been proposed so far to explain the origin of high dielectric constant and low dielectric loss (tan d), which eventually help to build up CCTO perovskite material applications. These mechanisms are internal domain, electrode polarization effect, bimodal grain size model, internal barrier layer capacitance (IBLC), and Nanoscale barrier layer capacitance (NBLC) model. An IBLC model, i.e., semiconducting grains separated by isolating grain boundaries, is currently the most accepted way to explain this phenomenon [27][28][29][30][31][32]. Polarization at the interfaces of the substance electrodes may also lead to this extrinsic effect [33]. The reasons behind the presence of semiconductivity in the bulk, however, are not yet clear. Cation nonstoichiometry mechanism relating to a Cu deficiency in the interior grain area was alleged to responsible for this effect [33][34][35].
In this present study, we have reported the effects of Mg doping on Cu sites on dielectric properties and Impedance studies of BCTO Ceramic, which was synthesized through the semi-wet method. Its microstructure, dielectric and electric properties were described. O was prepared in distilled water. Solid TiO 2 was added to this solution. The amount of citric acid (99.5% Merck, India) equivalent to metal ion as a chelating agent, was dissolved in distilled water and added to the mixture. The resultant solution was heated on a hotplate magnetic stirrer at 343-353 K to vaporize water permitted for self-ignition. A fluffy mass of BCTO and BCMgTO powders were obtained after the removal of a lot of gasses. Citric acid was also applied as fuel in the ignition step. The resulting BCTO and BCMgTO powders were ground with the help of a pestle and mortar to make a fine powder. The powders were calcined at 1073 K for 6 h. the cylindrical pellets were made with calcined powders using 2% PVA as a binder on applying 5 tons of pressure using hydraulic pressure for 90 s. This binder was burnt out at 773 K for 3 h. Finally, the BCTO and BCMgTO pellets were sintered at 1173 K for 8 h.

Material characterization
The crystalline phase of the sintered samples was identified using X-ray Diffractometer (MiniFlex2 goniometer, Rigaku, Tokyo, Japan under 30Kv/ 15mAX-Ray, 2h/h-continuous scanning mode within the scanning range 20°-90°) technique employing CuK a radiation (k = 1.54059 Å ). Bright-Field TEM images were obtained by transmission electron microscopy (TEM, FEI Tecnai-20G) with an accelerating voltage of 200 kV. Samples were characterized also by XPS (X-ray Photoelectron Spectroscopy) and AFM (Atomic Force Microscopy). For dielectric measurements, sintered pellets were polished by silver paste on both sides and dielectric data were taken by LCR meter (PSM 1735-NumetriQ, Newton 4th Ltd.UK) with a variation of temperature (300-500 K) and frequency (10 2 -10 7 Hz).

Transmission electron microscopic (TEM) studies
The bright-field TEM image of BCMgTO-0.2 ceramic sintered at 1173 K for 8 h is shown in Fig. 2a. The TEM image confirms that the BCTO ceramic possesses crystalline particles of cubical shape. The particle size was observed by TEM analysis with the help of Image-J software and was found to be 102.6 nm for BCMgTO-0.2 ceramic. Figure Fig. 3c. The average roughness (R a ) and Root mean square roughness (R q ) was calculated using NOVA software for a three-dimensional image and found to be 86.46 nm and 109.91 nm for BCMgTO-0.2 ceramic sintered at 1173 K for 8 h, respectively. Figure 3d shows the histogram plots for particle size which shows most of the roughness of particles was calculated in the range between 0.4 and 0.7 lm for BCMgTO-0.2 ceramic.   Fig. 3 a Three-dimensional image for surface roughness; b 2D image for grain; c 2D image for grain boundaries, and d histogram of three-dimensional particle roughness of BCMgTO-0.2 ceramic sintered at 1173 K for 8 h the peak of 933.9 eV and 953.6 eV that corresponds to Cu 2 p 3/2 and Cu 2 p 1/2 , respectively [36]. Figure 4d depicts the XPS spectrum of BCMgTO-0.2 ceramic at 457.9 eV, 463.7 eV which corresponding to the Ti 2 p doublet namely Ti 2 p 3/2 and Ti 2 P 1/2 confirmed the presence of ? 4 oxidation state of Ti ion [37]. The binding energy peak of Oxygen of BCMgTO-0.2 ceramic was found to be 529.6 eV and 530.9 eV corresponding to lower and higher binding energy is shown in Fig. 4e. The above result confirmed the presence of a ? 2 oxidation state of Oxygen ion in BCMgTO-0.2 ceramic. Thus, all ions present in the ceramic were found in the required oxidation states, which was confirmed by XPS studies.  [38]. The dielectric constant (e r ) decreases rapidly in the lower frequency range, whereas in the higher frequency range, it decreases slowly as shown in Fig. 5a. Figure 5b shows the variation of tangent loss (tand) with frequency at 303 K. It is obvious from the figure that the value of tand decreases with an increase in frequency in lower frequency regions, while it decreases smoothly in the higher frequency region. The dielectric loss of BCTO, BCMgTO-0.05, BCMgTO-0.1, and BCMgTO-0.2 ceramics was found to be 0.45, 0.27, 0.26, and 0.28, respectively, at 303 K and 10 kHz. Figure 6a and b displays the variation of e r and tangent loss with the temperature at 10 kHz frequency. It is noted from the figure that the value of e r and tand first increases, attains a maximum, and then decreases slowly to a lower value. The variations of e r and tand are qualitatively analogous showing dielectric distribution at lower frequencies. Both show a decrease and then become nearly independent of frequency. This type of frequency behavior can be described based on the space charge model of Maxwell-Wagner [39].

Impedance spectroscopic studies
The complex impedance plot (Z 00 vs Z 0 ) of Mg-doped and undoped BCTO ceramics sintered at 1173 K for 8 h at 303 K is shown in Fig. 7a. The figure reveals the presence of semicircular arcs with different intercepts, which may be due to the grain and grain boundaries contribution in higher frequency, and no contribution due to electrode specimens is observed in this frequency range. The arcs for grains at high frequency get suppressed due to the high value of grain boundary resistance, which is usually observed for IBLC, a characteristic of semiconducting grain with insulating grain boundaries. Thus, the dielectric property of BCTO ceramics (Mg-doped and nondoped) is due to the combined effect of grain and grain boundary. The variation of the imaginary part of impedance (Z 00 ) and the real part of impedance (Z 0 ) with frequency at 303 K is shown in Fig. 7b and c, respectively. It shows the appearance of relaxation peaks. The peak relaxation was observed at a lower frequency and peak suppression occurred at a high frequency, which confirms the presence of the relaxation phenomenon of Bi 2/3 Cu 3 Ti 4 O 12 ceramics.

Electric modulus studies
The cole-cole plot (complex impedance plot) between M 00 and M 0 for Mg-doped and undoped BCTO ceramic is shown in Fig. 8a. Semicircular arcs present in M 00 vs M 0 plot, show grain boundaries separated by grains. This type of behavior supports the long-range reduction phenomenon. To explain the modulus spectra of BCTO ceramics (Mg-doped and undoped), the variation of imaginary and real part of electric modulus M 00 and M 0 is plotted against frequency at 303 K and is shown in Fig. 8b and c, respectively. In the low-frequency region, the magnitude of M 0 tends to zero, which confirms a negligibly small contribution of the electrode effect [40,41]. The modulus plot displays non-Debye behavior and is asymmetric concerning the peak maxima, and that the peaks are considerably broader on both sides of the maxima and suggest dielectric relaxation in the material [42]. M 00 peaks also show dielectric relaxation. The results obtained in Modulus coincide with impedance results.

Conductivity measurements
Conductivity is calculated by the following formula: where r is conductivity, R is the grain boundary resistance (ohm), l is the thickness of the ceramic and A is the area of the electrode. The variation of ln of conductivity (r) as a function of 1000/T (K -1 ) at 10 kHz frequency for Mg-doped and undoped BCTO ceramics sintered at 1173 K for 8 h, is shown in Fig. 9a. It is observed from the figure that the value of lnr decreases with a decrease in temperature. Meanwhile, the conductivity (r) obeys the Arrhenius law where r is the conductivity at the given temperature T, r 0 is the pre-exponent factor, E a is the activation energy, k B is Boltzmann constant and T is the absolute temperature in K. The following relation with ln r and 1/T can be obtained by Eq. (3).
whereas conduction activation energy. Conduction activation energy is calculated by the slope of the plot of ln r vs 1000/T. The calculated conduction activation energy values for BCTO, BCMgTO-0.05, BCMgTO-0.1 and BCMgTO-0.2 ceramics are 0.27 eV, 0.16 eV, 0.14 eV and 0.13 eV, respectively. It should be noted that, by Mg doping in BCTO ceramic, its conduction activation energy decreases. The frequency-dependent conductivity can be described by Jonscher's power law, which is given by [43] rðxÞ where A is a constant and s is the power law exponent, 0 \ s \ 1.
The transport mechanism (conductivity phenomenon) is explained by the thermally activated hopping process between two sites separated by an energy barrier. AC conductivity [43,44] mainly depends upon frequency. The variation of AC conductivity with frequency is shown in Fig. 9b. The value of Power law exponent(s) was calculated by the slope of ln r ac vs ln x. It was calculated to be 0.94, 0.89, 0.64 and 0.65 for BCTO, BCMgTO-0.05, BCMgTO-0.1 and BCMgTO-0.2 ceramics, respectively.

Conclusion
Mg-doped and undoped BCTO ceramics were successfully synthesized via a semi-wet method using metal acetate, metal nitrate solutions, and solid TiO 2 powder. BCTO phase formation was confirmed by XRD. The crystalline size of Mg-doped and nondoped BCTO ceramics were found to be in the range of 35-47 nm by XRD. The particle size of BCMgTO-0.2 ceramic (higher Mg-doped sample) was found to be 102.6 nm by TEM studies. The average Roughness (R a ) value and root mean square (R q ) value was found to be 86.46 nm and 109.91 nm, respectively, for BCMgTO-0.2 ceramic by AFM studies. The oxidation state was confirmed by XPS. At low frequency, a high value of dielectric constant (e r ) and at high frequency, low tangent losses were recorded. The impedance and Modulus analysis of BCTO ceramics confirmed the presence of the phenomenon of grain and grain boundaries effects. The conductivity of BCTO ceramics increases with an increase in temperature obeying the Arrhenius law. AC conductivity of BCTO ceramics increases with increasing frequency satisfying Jonscher's power law.