During the last decade there have been carried extensive searches and investigations of various oxide systems, which can be used as multifunctional materials, possessing combination of electric, dielectric, ferroelectric, ferroelastic, magnetic and other properties. Such materials can be used in different kinds of micro- and nanostructured materials of the new generation such as thin films, super lattices, nanofibers, nanotubes and nanowires [1-5]. The number of catalysts used in modern chemical industries are based on mixed oxide including perovskite oxide ABO3 where A is a rare earth element, B is 3d transition metals remain predominant [6] perovskite oxides crystals can have broad applications in advanced technologies such as catalysts, oxide fuel cell, chemical sensors, magnetic materials, electrode materials [7-9]. Perovskites, especially LaMnO₃, have also been used in environmental applications, e.g., the oxidation of hydrocarbons [10-12], chlorinated organics [13] and H₂O₂ reactions [14]. LaMnO₃ shows good stability, flexible oxygen stoichiometry (δ) and the different Mn oxidation states i.e., Mn²⁺, Mn³⁺, Mn⁴⁺, which strongly affects the catalytic behavior. Also, isomorphic substitution of metals in the perovskite structure allows some control the catalytic properties of the material and its several derivatives have been previously investigated [15,16]. Lanthanum-based perovskites containing transition metal in B-site, (LaBO₃, B=Co, Fe, Ni or Mn), show catalytic activity close to the noble metal, presenting low cost and high thermal stability. The efficiency of these materials depends on the synthesis method [17]. Many methods are available for the synthesis of perovskite oxide materials such as solid state reaction [18], sol gel [19], solution combustion synthesis, electrospinning, hydrothermal synthesis, EDTA glycine process, and reverse micro emulsion process, etc. In this work, we studied the isomorphic substitution of Mn in the LaMnO₃ structure by different amounts of Fe to produce LaMn_1-xFexO₃. The Fe also vary the oxidation states and this property can improve the catalytic activity in oxidation processes. The sample has been prepared by coprecipitation method. The average crystallite size and morphology of the sample obtained have been investigated.

LaMn_1-xFexO₃ (x=0, 0.1, 0.2) nanoparticle were synthesized by co-precipitation method. In this method, stoichiometric amount of Lanthanum nitrate [La(NO₃)₃.6H₂O], Manganese chloride [MnCl₂.4H₂O], Ferric nitrate [Fe(NO₃)₃.9H₂O] were dissolved in distilled water. After complete dissolution, the solution was continuously stirred at 50°C for half an hour. Then the sodium hydroxide (NaOH) solution was slowly added until it reaches the pH is 13. After reaching this pH, the solution was continuously stirred till the black precipitate appears. The precipitate was collected by centrifugation and washed several times to remove the chloride. Then the precipitate is kept in the oven for 1 hr at 50°C. The final product was ground and kept in the muffle furnace for calcined at 800°C for 6 hoursfor removal of organic template. Synthesized perovskite sample were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), compositional analysis (EDAX), Fourier transform infrared (FTIR), UV visible and conductivity study.

Structural analysis

The X-Ray diffraction pattern of LaMn_1-xFexO₃ (0 ≤ x ≤ 0.2) nanoparticle were prepared at 800°C are shown in Figure 1. The XRD pattern reveals that the prepared LaMnO₃ (LMO) nanoparticle and iron doped LaMnO₃ (LMFO) nanoparticle clearly indicates their crystalline nature with the orientation along (1 1 0) and all the other peaks are indexed to the cubic (Pm3m) structure of LMO and LMFO nanoparticles. The prepared samples exactly matched with JCPDS card no 75-0440. From the XRD pattern, the intensity of the peak increases when the iron concentration is increased. The crystallite size was calculated using Debye-Scherrer formula. The crystallite size was found to be in the range 17-20 nm. While increasing the Fe content in the sample, the crystallite size also increases simultaneously [20]. The various lattice parameters like crystallite size, dislocation density and micro strain are calculated from the XRD pattern and its values are shown in the Table 1. From the Table 1, the lattice parameter, dislocation density and strain decreases with increasing the dopant concentration. The decrease in various lattice parameters is due to the substitution of smaller ionic radii (Fe) with the larger ionic radii (La).

Figure 1: XRD pattern of LaMn_1-xFexO₃ (0 ≤ x ≤ 0.2) nanoparticle.

Table 1: Structural parameters of LaMn_1-xFe_xO₃(0 ≤ x ≤ 0.2) nanoparticle.

Morphological analysis

The surface morphology of the prepared sample was analyzed by SEM and the images for sample x=0.0, 0.1 and 0.2 are shown in Figure 2. The SEM images of undoped and 0.1 and 0.2 M of iron doped lanthanum manganite nanoparticle calcined at 800°C. Figure 2 shows the SEM images of pure and iron doped lanthanum manganite nanoparticle. These images depict that the particles possess larger agglomeration leads to the wide distribution of the sample and possibility to confirm the particle which is exhibiting a morphology tending to the spherical shape [21]. The particles of all the prepared samples are seems to be uniformly distributed. Because of the low calcination temperature of the sample, there is porosity found in the sample.

Figure 2: SEM images of LaMn_1-xFe_xO₃ (0 ≤ x ≤ 0.2) nanoparticle.

Compositional analysis

The pure and iron doped lanthanum manganite elemental compositions were carried out with the help of EDAX shown in Figure 3. From the Figure 3a, it shows the expected presence of La, Mn and O in the prepared sample and in Figure 3b and 3c confirms the stoichiometric mixture of La, Fe, Mn and O atoms in iron doped lanthanum manganite (LMFO) samples. There exists an 1% silica found in the spectra. No traces of other element were found in the spectra. The observed composition is almost equal to the initial composition of the sample taken for its preparation [22]. Table 2 represents the composition of the elements present in the samples.

Figure 3: EDAX spectra for LaMn_1-xFe_xO₃ (0 ≤ x ≤ 0.2) nanoparticle.

Table 2: Compositional analysis for LaMn_1-xFe_xO₃(0 ≤ x ≤ 0.2) nanoparticle.

FTIR analysis

FT-IR spectra were recorded in the wavenumber range from 500 to 3500 cm^-1. FT-IR spectra of LaMn_1-xFexO₃ (0 ≤ x ≤ 0.2) nanoparticle are shown in Figure 4. The broad absorption band at 642 cm^-1 in the sample is attributed to the Mn-O vibrations of MnO₆ octahedral in cubic perovskite structure [23]. With the increasing the amounts of Fe, the intensity of absorbance at 642 cm^-1 gradually decreases. The band around 1391 and 1472 cm^-1 are produced by bending vibrations in the bonds N-O. The sample calcined at 800°C, there are still some carbonates because of the presence of the absorption band at 855 cm^-1 [24]. The wavenumber and corresponding assignment of vibrations were shown in Table 3.

Figure 4: FTIR spectra of LaMn_1-xFe_xO₃(0 ≤ x ≤ 0.2) nanoparticle.

Table 3: Functional group of LaMn_1-xFe_xO₃(0 ≤ x ≤ 0.2) nanoparticle.

Optical analysis

The iron doped lanthanum manganite nanoparticle prepared by co-precipitation method whose optical absorption spectra were determined in the wavelength range 200-800 nm. Figure 5a shows the absorption spectra of the prepared sample. The presence of absorption peaks seems to be at 361, 364 and 365 nm of the sample respectively [25]. The absorptions spectra reveal that, while increasing the content of iron in the sample its absorption increases proportionally. Figure 5b the tau c plot i.e., the intercept of the straight line portion of (αhυ)2 versus (hυ) the bandgap has been found for LaMn_1-xFexO₃ (0 ≤ x ≤ 0.2) nanoparticle. The optical bandgap for the prepared samples were calculated and tabulated in Table 4. The values of the bandgap for LaMn_1-xFexO₃ (0 ≤ x ≤ 0.2) nanoparticleis found to be 3.30, 3.25 and 3.24 eV. The value of the bandgap energy decreases with the increasing content of iron in the sample. This is due to the formation of newer energy levels of iron in the lanthanum manganese oxide lattice [26]. The proper replacement of lanthanum by iron sample is the best proof for the decrease in the bandgap energy.

Figure 5a: Absorption spectra of LaMn_1-xFe_xO₃(0 ≤ x ≤ 0.4) nanoparticle (a) x=0.0 (b) x=0.1 (c) x=0.2.

Figure 5b: Plot of (αhυ)² Vs (hυ) for LaMn_1-xFe_xO₃(0 ≤ x ≤ 0.2) nanoparticle (a) x=0.0.

Table 4: Bandgap values of LaMn_1-xFe_xO₃ (0 ≤ x ≤ 0.2) nanoparticle.

Conductivity analysis

The dc electrical conductivity of the pure and iron doped lanthanum manganese oxide nanoparticle was performed by Keithley high resistance electrometer 6517B shown in Figure 6. In this method, initially the current was measured with respect to the applied voltage across the sample at room temperatures [27]. The sample was sandwiched between two copper electrode and annealed in an oven. The conductivity of the sample increases with the dopant concentration because of the mobility of the charge carriers which is depend on hopping mechanism. As the voltage increases, the mobility of the hopping ions increases which in turn increases the conductivity of the material [28]. If increase the iron content in the sample, the oxygen vacancies increased. This results increases the free electrondensity and conductivity of the material.

Figure 6: Conductivity spectra for LaMn_1-xFe_xO₃ (0 ≤ x ≤ 0.2) nanoparticle.

LaMn_1-xFexO₃ perovskite was synthesized by the co-precipitation method. The XRD pattern confirmed the formation of perovskite phase and the lattice parameters are calculated. The SEM micrographs indicated that the porosity due to inter-particle voids and agglomerated crystals. The presence of La was confirmed by EDX analysis. FTIR spectrum determines the various functional groups present in the compound. The optical band gap decrease with increasing the content of iron in the sample. The conductivity study confirmed that the conductivity increases with increasing the content of iron.