Transport and Dielectric Properties of Mechanosynthesized La2/3Cu3Ti4O12 Ceramics

La2/3Cu3Ti4O12 (LCTO) powder has been synthesized by the mechanochemical milling technique. The pelletized powder was conventionally sintered for 10 h at a temperature range of 975–1025 °C, which is a lower temperature process compared to the standard solid-state reaction. X-ray diffraction analysis revealed a cubic phase for the current LCTO ceramics. The grain size of the sintered ceramics was found to increase from 1.5 ± 0.5 to 2.3 ± 0.5 μm with an increase in sintering temperature from 975 to 1025 °C. The impedance results show that the grain conductivity is more than three orders of magnitude larger than the grain boundary conductivity for LCTO ceramics. All the samples showed a giant dielectric constant (1.7 × 103–3.4 × 103) and dielectric loss (0.09–0.17) at 300 K and 10 kHz. The giant dielectric constant of the current samples was attributed to the effect of internal barrier layer capacitances due to their electrically inhomogeneous structure.

Liu et al. reported on the preparation of LCTO powder by sol-gel technique [25]. A room temperature GDC value of 0.9-1.6 × 10 4 at 10 2 -10 5 Hz could be obtained. The fabrication process included calcination of the powder at 750-950 • C for 10 h, followed by conventional sintering at 1100-1110 • C for [10][11][12][13][14][15][16][17][18][19][20] h. In the current study, we investigate the possibility of reducing the preparation process of LCTO ceramics by using a comparatively shorter time and lower temperatures in the process. The process is based on the mechanochemical synthesis of the LCTO powder followed by conventional sintering. Mechanochemical milling is considered to be a versatile method of producing almost all forms of materials in the nanosize scale [23][24][25][26][27][28]. The advantages of mechanochemical milling are that almost every material is accessible and materials can be produced in large amounts. In the current study, we prepared LCTO powder by mechanochemical milling and the calcination step that is usually done in solid state reaction technique was skipped, leading to a simpler process. Moreover, the sintering temperature in the current work was limited to 1025 • C, which is lower than the sintering temperatures of 1100 • C that is usually reported in the literature for LLCT ceramics [23,25,27]. The prepared ceramics were then studied for the microstructure, dielectric, and relaxation behavior using field-emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), and impedance spectroscopy (IS) measurements in a wide range of frequencies and temperatures.

Materials and Methods
High-purity CuO (99.9%), TiO 2 (99.95%), and La 2 O 3 (99.95%) raw powders were weighed according to the chemical formula La 2/3 Cu 3 Ti 4 O 12 and mixed with isopropyl alcohol for 30 h at room temperature with a rotation speed of 500 rpm in Fritsch P-7 premium line machine. Tungsten carbide pot and tungsten carbide balls were used, with a powder to balls mass ratio of 1:8. The synthesized powder was then dried for 1 hour at 300 • C. Afterwards, a suitable amount of the powder was pressed into pellet disks of~1.5 mm thickness and 10 mm diameter under static pressure of 32 MPa. Pellets were conventionally sintered at 975, 1000, and 1025 • C in air for 10 h using a heating rate of 5 • C/min. After the sintering process, the ceramics were allowed to naturally cool down in the furnace. For simplicity, these ceramics are denoted as CS-975, CS-1000, and CS-1025, respectively. Materials characterization was performed by XRD and FE-SEM. XRD measurements were performed by using a Bruker D8 Advance X-ray powder diffractometer (CuKα radiation, Karlsruhe, Germany) in the 10 • ≤ 2θ ≤ 90 • range with a step of 0.02 • . The graphs of the microstructure were obtained by FE-SEM (Joel SM7600F, Tokyo, Japan) without prior sputtering of the samples. For impedance measurements, the studied ceramics were gold sputtered. Impedance spectroscopy measurements were conducted for the prepared ceramics in the 120-460 K temperature range over the 1 Hz to 10 MHz frequency range using Novocontrol concept 50 system with an applied ac voltage of 0.1 V. The IS measurements were performed in dry nitrogen atmosphere where the temperature was controlled by the Quatro Cryosystem (Novocontrol Technologies, Montabaur, Germany). Figure 1 shows the XRD pattern of mechanosynthesized LCTO powder. This figure suggests the formation of CCTO-like structure, with the major peaks indexed in the figure, which confirms the synthesis of LCTO material by the mechanochemical synthesis technique. Figure 2 represents the X-ray diffraction profiles of the CS-975, CS-1000, and CS-1025 LCTO samples analyzed using the Rietveld refinement process with the Im3 space group by FullProf software [29]. The experimental data in Figure 2 are represented with black points, whereas the calculated spectra are shown by the red line. The difference curves between the experimental and fitted spectrum are highlighted by the blue line. The pink vertical lines represent the exact location of Bragg's position.    Table 1. It is found that lattice parameters and unit cell volume increase slightly with the increase in the sintering temperature. The goodness factor (χ 2 ) values of fitting were obtained in between 1.17 to 1.22, which clearly convince a very good agreement between calculated and experimental spectra as shown in Table 1. Besides the main CCTO-like phase, the minor reflections marked by # at 27.78 • ,~36.2 • , and 54.62 • correspond to the impurity phase of rutile TiO 2 [JCPDS card No. 89-4920 with tetragonal structure, space group P42/mnm]. Figure 3 shows FE-SEM images of LCTO powder and the fractured surfaces of the corresponding ceramics. Mechanosynthesized LCTO powder exhibits particle size in the 50-150 nm range (Figure 3a). After conventional sintering, all the studied ceramics showed uniformly distributed microstructure with the average grain size of 1.5 ± 0.5, 1.8 ± 0.4, and 2.3 ± 0.5 µm for CS-975, CS-1000, and CS-1025 ceramics, respectively, as can be seen from Figure 3b-d.  Figure 3 shows FE-SEM images of LCTO powder and the fractured surfaces of the corresponding ceramics. Mechanosynthesized LCTO powder exhibits particle size in the 50-150 nm range (Figure 3a). After conventional sintering, all the studied ceramics showed uniformly distributed microstructure with the average grain size of 1.5 ± 0.5, 1.8 ± 0.4, and 2.3 ± 0.5 µm for CS-975, CS-1000, and CS-1025 ceramics, respectively, as can be seen from Figure 3b-d.   The complex impedance plots for CS-975, CS-1000, and CS-1025 LCTO ceramics are given in Figure 4. The complex impedance plane plot at a given temperature is composed of two semicircular arcs. The semicircle at high frequency is related to the grain response while the grain boundary response is correlated with the low frequency semicircle. This is supported by the capacitance values associated with these semicircles. The capacitance could be estimated from the impedance data by the relation; RC = 1/2πf max , where f max is the frequency at the top of the semicircles. The estimated values of the capacitance at different temperatures are in the range of 1.4-4 nF for the low frequency semicircles, and 55-85 pF for the high frequency semicircles. These values of the capacitance confirm that the high-and low-frequency semicircles are related to grain and grain boundary contributions, respectively. The complex impedance plots for CS-975, CS-1000, and CS-1025 LCTO ceramics are given in Figure 4. The complex impedance plane plot at a given temperature is composed of two semicircular arcs. The semicircle at high frequency is related to the grain response while the grain boundary response is correlated with the low frequency semicircle. This is supported by the capacitance values associated with these semicircles. The capacitance could be estimated from the impedance data by the relation; RC = 1/2πf , where fmax is the frequency at the top of the semicircles. The estimated values of the capacitance at different temperatures are in the range of 1.4-4 nF for the low frequency semicircles, and 55-85 pF for the high frequency semicircles. These values of the capacitance confirm that the high-and low-frequency semicircles are related to grain and grain boundary contributions, respectively.  It can be seen from Figures 4 and 5 that the overall resistivity of LCTO ceramic decreases with an increase in the measuring temperature, which is typical semiconductor behavior. Figure 5 shows the complex impedance diagram at 300 K for all three samples. As the sintering temperature increases, the overall resistivity of LCTO ceramics decreases. Moreover, the ratio of resistivity of grain-boundary to the grain in LCTO ceramics is found to be four (CS-975) to three (CS-1000 and CS-1025) orders of magnitude, which indicates the electrical heterogeneity of the ceramics. The temperature dependency of the grain and grain boundary conductivity extracted from the complex impedance plots of the current LCTO ceramics is shown in Figure 6. It can be seen from Figures 4 and 5 that the overall resistivity of LCTO ceramic decreases with an increase in the measuring temperature, which is typical semiconductor behavior. Figure 5 shows the complex impedance diagram at 300 K for all three samples. As the sintering temperature increases, the overall resistivity of LCTO ceramics decreases. Moreover, the ratio of resistivity of grain-boundary to the grain in LCTO ceramics is found to be four (CS-975) to three (CS-1000 and CS-1025) orders of magnitude, which indicates the electrical heterogeneity of the ceramics. The temperature dependency of the grain and grain boundary conductivity extracted from the complex impedance plots of the current LCTO ceramics is shown in Figure 6. The solid lines in this figure represent the linear fit according to the Arrhenius relationship:

Results
where σ 0 is the pre-exponential factor, k B is Boltzmann constant, and E a is the activation energy for conduction. Table 2  where σ0 is the pre-exponential factor, kB is Boltzmann constant, and Ea is the activation energy for conduction. Table 2    The frequency dependence, at selected temperatures, of the dielectric constant ε' of each LCTO sample is shown in Figure 7, whereas Figure 8 represents the frequency dependence of ε' and tanδ at 300 K for all LCTO ceramics. As seen in these figures, all the ceramics showed the same behavior where at low temperature a first dielectric plateau exists followed by a large drop in the ε' value to the bulk dielectric constant value of about 100 at high frequency. This behavior is commonly reported in CCTO-like materials and attributed to the Debye-like relaxation in the grain [27]. With increasing temperature, this plateau shifts towards higher frequency and a second plateau with higher dielectric value appears at low frequency. This second plateau is thought to be due to the Maxwell- where σ0 is the pre-exponential factor, kB is Boltzmann constant, and Ea is the activation energy for conduction. Table 2    The frequency dependence, at selected temperatures, of the dielectric constant ε' of each LCTO sample is shown in Figure 7, whereas Figure 8 represents the frequency dependence of ε' and tanδ at 300 K for all LCTO ceramics. As seen in these figures, all the ceramics showed the same behavior where at low temperature a first dielectric plateau exists followed by a large drop in the ε' value to the bulk dielectric constant value of about 100 at high frequency. This behavior is commonly reported in CCTO-like materials and attributed to the Debye-like relaxation in the grain [27]. With increasing temperature, this plateau shifts towards higher frequency and a second plateau with higher dielectric value appears at low frequency. This second plateau is thought to be due to the Maxwell- The frequency dependence, at selected temperatures, of the dielectric constant ε of each LCTO sample is shown in Figure 7, whereas Figure 8 represents the frequency dependence of ε and tanδ at 300 K for all LCTO ceramics. As seen in these figures, all the ceramics showed the same behavior where at low temperature a first dielectric plateau exists followed by a large drop in the ε value to the bulk dielectric constant value of about 100 at high frequency. This behavior is commonly reported in CCTO-like materials and attributed to the Debye-like relaxation in the grain [27]. With increasing temperature, this plateau shifts towards higher frequency and a second plateau with higher dielectric value appears at low frequency. This second plateau is thought to be due to the Maxwell-Wagner polarization (M-W) effect which manifests in electrically inhomogeneous materials [23,28,30].
As approved by impedance measurements, the current LCTO ceramics have the structure of semi-conductive grains surrounded by resistive grain-boundary. Therefore, at high temperatures and suitable frequencies, the moving charge carriers are piled up at the resistive grain-boundary, which creates internal barrier layer capacitance (IBLC).
Wagner polarization (M-W) effect which manifests in electrically inhomogeneous materials [23,28,30]. As approved by impedance measurements, the current LCTO ceramics have the structure of semi-conductive grains surrounded by resistive grain-boundary. Therefore, at high temperatures and suitable frequencies, the moving charge carriers are piled up at the resistive grain-boundary, which creates internal barrier layer capacitance (IBLC).  The frequency dependence of ε and tanδ for LCTO samples is shown in Figure 8. One relaxation peak is observed in the spectra of tanδ at high frequency, which is related to the grain relaxation process. The peak related to the grain-boundary relaxation is not well resolved due to the high conductivity of the sample at room temperature. The dielectric loss for the samples at room temperature and 10 4 Hz is summarized in Table 2. The sample CS-975 showed the lowest dielectric loss of~0.09, which increases to 0.17 for CS-1025 sample. These values of tanδ are higher than the values reported previously for LCTO ceramics as in Ref. [25].
The frequency dependence of the electric modulus (M * = M + jM" = 1/ε * where ε * is the complex permittivity) is commonly used to study the relaxation properties of the materials. The spectra of the imaginary part of the electric modulus M" at selected temperatures for CS-975, CS-1000, and CS-1025 ceramics are shown in Figure 9. For all the samples, two relaxation peaks could be detected in the studied temperature and frequency range. A first peak is observed in the temperature range (120-300 K) and high frequency (>10 5 Hz). A second peak is seen at a higher temperature range (~240-450 K) and lower frequency. The peak maximum of M" is inversely proportional to the capacitance [31,32]. Thus, considering the higher resistivity and capacitance of the grain-boundary compared to the grain, the low and high frequency relaxation peaks are attributed to the response of grain-boundary and grain contributions, respectively.
(c)   The relaxation time τ is determined from the peak frequency f max as τ = 1/(2πf max ). Figure 10 shows the inverse temperature dependence of the relaxation time determined from the modulus spectra for the grains and grain boundaries. The activation energy of the relaxation processes was therefore calculated according to the Arrhenius relationship: where τ 0 is the pre-exponential factor and E R is the activation energy for the relaxation process. The values of the relaxation energy in the grain and grain-boundary for the LCTO ceramics are given in Table 2. These values of the activation energies are similar to the reported values for the LCTO ceramics prepared by other techniques [24,25].

Conclusions
La2/3Cu3Ti4O12 (LCTO) ceramics were synthesized by mechanosynthesis and conventional sintering (CS) at a comparatively low temperature range (975-1025 °C) without calcination step. All the sintered LCTO ceramics showed CCTO-like body-centered cubic structure with space group Im3. The grain size of the LCTO ceramics is found to increase

Conclusions
La 2/3 Cu 3 Ti 4 O 12 (LCTO) ceramics were synthesized by mechanosynthesis and conventional sintering (CS) at a comparatively low temperature range (975-1025 • C) without calcination step. All the sintered LCTO ceramics showed CCTO-like body-centered cubic structure with space group Im3. The grain size of the LCTO ceramics is found to increase from 1.5 ± 0.5 to 2.3 ± 0.5 µm with increasing CS temperature from 975 to 1025 • C. The giant dielectric constant was obtained for the LCTO ceramics with the dielectric constant (1.7 × 10 3 -3.4 × 10 3 ) and dielectric loss (0.09-0.17) at 300 K and 10 kHz. The sintered sample at 1025 • C showed the highest dielectric constant, while the sintered sample at 975 • C exhibited the lowest dielectric loss. The giant dielectric response of the current samples is believed to be related to the Maxwell-Wagner polarization effect of the ceramic samples.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.