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Study of LaFe1−xMnxO3 (x = 0.05, 0.1, 0.15, 0.2) perovskite materials by impedance spectroscopy

We use room-temperature impedance spectroscopy to investigate LaFe1−xMnxO3 (x = 0.05, 0.1, 0.15 and 0.2) perovskite-type oxides. The samples were synthesized using the sol-gel method followed by a sintering process to form bulk samples. The microstructural and chemical composition were analyzed using scanning electron microscopy and x-ray fluorescence, with the results indicating a grain size of 373-305 nm and an atomic composition compatible with the chemical formula LaFe1−xMnxO3. Applying impedance spectroscopy to LaFe1−xMnxO3 at room temperature reveals a decreasing impedance and relaxation time with increasing Mn content. The dielectric spectra show a increasing dielectric constant with increasing of Mn content.


Introduction
Perovskites have an ABX3 structure where A and B are cations and X is usually an anion such as LaFeO3, LaMnO3, etc. Lanthanum orthoferrite (LaFeO3), an orthoferrite perovskite compound, is the focus of the present work because of its structural stability and suitability in gas-sensor applications [1]. Previous research has reported substitution into the La and/or Fe sites of LaFeO3, which enhances its electronic properties by producing an increased dielectric constant, decreased dielectric loss, and high thermal stability [2,3]. Cao et al. [2] reported that Na substitution into the La site of La1−xNaxFeO3 (x = 0, 0.1, 0.2) results in a higher dielectric constant, with the maximum being 10 5 for x = 0.2. Phokha et al. [3] reported that LaFe1−xTixO3 (x = 0, 0.1, 0.2) has a higher dielectric constant, with the maximum at x = 0.2 being to the order of 10 4 . In addition, Fe substitution into the Mn site of LaMn1−xFexO3 (x = 0.15, 0.3, 0.5, 0.7, 1) was investigated by Karmakar et al. [4], who reported an increasing dielectric constant with increasing Fe content. Zeng et al. [5] reported that LaMn0. 5Fe0.5O3 nanocrystalline materials have crystallite sizes and a dielectric constant of ~25 nm and ~4300 at room temperature, respectively.
Building on previous research, we investigate how Mn doping into the Fe site of LaFe1−xMnxO3 with x = 0.05, 0.1, 0.15, 0.2 affects the microstructural and dielectric properties at room temperature. The samples were synthesized by using the sol-gel method followed by sintering to form bulk samples. The chemical composition and microstructural properties were examined by using x-ray fluorescence and scanning electron microscopy (SEM), respectively. RLC-Meter FLUKE-PM 6303 with frequency range of 100 Hz to 1 MHz is used to characterized the electrical properties of the samples. The data were analyzed by using impedance spectroscopy and the parallel-plate method.

Experimental
By using the sol-gel method, LaFe1−xMnxO3 perovskite compounds were prepared with x = 0.05, 0.10, 0.15, and 0.20 with lanthanum (III) oxide La2O3, manganese (II) chloride tetrahydrate MnCl2.4H2O, and  iron (III) nitrate nanohydrate Fe(NO3)9H2O (analytical grade) as raw materials. The materials were mixed stoichiometrically with aquades and citric acid monohydrate as a solvent and stirred at 393 K until a gel formed. The gel was dried at 473 K for 5 h then calcined at 1173 K to obtain the nanoparticle form. To obtain nanocrystalline form, the material was pressed into bulk samples by applying 3 kN/m 2 and sintered at 1273 K for 12 h. The chemical composition was checked by x-ray fluorescence. The morphology was analyzed by using SEM (FEI QUANTA 650). To apply the parallel-plate capacitor model, the materials were coated with carbon by using a Quorum K975x Turbo Evaporator. RLC-Meter FLUKE-PM 6303 with frequency of 100 Hz to 1 MHz is used to measured the electrical properties of the samples at room temperature.
3. Results and discussion X-ray fluorescence analysis confirms an atomic composition consistent with the chemical formula LaFe1−xMnxO3. Table 1 shows the composition of LaFe1−xMnxO3 with x = 0.05, 0.10, 0.15, 0.20 after calcination at 1173 K. The results of these measurements differ slightly from the calculation, which may be attributed to preparation conditions and insufficient measurement accuracy. Figure 1 shows typical SEM images of Mn-doped LaFeO3 sintered at 1273 K for 12 h. The average grain sizes are in the range of 373-305 nm. The SEM images show a homogenous surface morphology with spherical grains [6]. Zeng et al. [5] reported a similar morphology for LaFe0. 5Mn0.5O3.   Figure 2b shows a Bode plot of the impedance Z(f) for LaFe1−xMnxO3. The impedance magnitude ! decreases with increasing Mn content at low frequency, and decreases at high frequency for all materials.
Impedances of LaFe1−xMnxO3 at room temperature are shown in figure 3. The real part ! ! (!) of impedance decreases with increasing frequency for all materials (figure 3a). Figure 3b shows the imaginary part ! !! (f) of LaFe1−xMnxO3, which increases to a maximum value and then starts to decrease at high frequencies, indicating that a relaxation phenomenon is active in the system. The peak of ! !! gives the relaxation time for the dielectric-relaxation process. Figure 4a shows the dielectric constant ! ! (f) for all materials, which increases with increasing frequency and Mn content. This phenomenon is similar to that in BaTi1−xMnxO3 (with x = 0.00, 0.01, 0.02, 0,03, 0.04) [7], where the dielectric constant also increases with increasing Mn content. Figure  4b shows the dielectric loss tan !(f) of LaFe1−xMnxO3 at room temperature, which shows that the dielectric loss depends on Mn content. In addition, all materials exhibit the dielectric-relaxation phenomenon. The varying dielectric constant and dielectric loss may be due to the number of grain boundaries and to the Mn doping. LaFe0. 89Mn0.20O3 has a higher dielectric constant than the others, which indicates that it has a smaller grain size and fewer grain boundaries. Riaz et al. [8] reported variations in dielectric constant and dielectric loss due to several factors, including grain size and grain boundaries. The decrease in grain size reportedly changes the dielectric constant in BiFe1−xMnxO3 [8]. In addition, Islam et al. [7] reported a decrease in the dielectric constant of BaMnxTi1−xO3 with increasing Mn content and increasing grain size, and Verma et al. [9] reported a small dielectric constant resulting from a large number of grain boundaries. Riaz et al. [8] also reported increased dielectric parameter values due to a large number of grain boundaries and Mn doping.