Effect of Heat Treatment on Structural, Magnetic and Electrical Properties of La2FeMnO6

In this study, the effect of heat treatment on the structural, magnetic and electrical properties of La2FeMnO6 prepared via the sol–gel and sintering method were investigated. The heat-treatment conditions, i.e., the calcination temperature (1023 K and 1173 K), sintering temperature and time (1273 K for 1 and 3 h) were carried out. X-ray diffraction (XRD) revealed orthorhombic pnma (62) symmetry without any impurity phase for all samples. X-ray photoelectron spectroscopy confirmed the presence of Fe2+–Fe3+–Fe4+ and Mn3+–Mn4+ mixed states, and lanthanum and oxygen vacancies resulting in various magnetic exchange interactions. Furthermore, the magnetisation hysteresis showed enhanced hysteresis loops accompanied by an increase in magnetisation parameters with calcination temperature. The Raman phonon parameters induced a redshift in the phonon modes, alongside an increase in the intensity and compression of the linewidth, reflecting a decrease in lattice distortion, which was confirmed by XRD. The temperature-dependent conductivity showed that the conduction mechanism is dominated by p-type polaron hopping, and the lowest activation energy was approximately 0.237 ± 0.003 eV for the minimum heat-treatment conditions. These results show that varying heat-treatment conditions can significantly affect the structural, magnetic and electrical properties of the La2FeMnO6 system.


Introduction
Double perovskite oxides, A 2 BB O 6 , in which B and B are occupied by different transition metals, have attracted significant attention and have been intensively studied of late [1][2][3][4][5]. Starting from half-metal ferrimagnetic Sr 2 FeMoO 6 (SrFeO 3 -SrMoO 3 ) with a fair Curie temperature (approximately 415 K) and showing potential in spintronic applications, the combination of other perovskites has attracted remarkable research interest [2]. Moreover, the synthesis method affects the structural symmetry, magnetic, optical and electrical transport properties of the synthesised material [3,4].
Among the double perovskites, La 2 FeMnO 6 has gained much interest owing to its structural, magnetic and electrical properties, which are strongly influenced by the preparation conditions and methods [3][4][5][6][7]. Palakkal et al. [3] stated that La 2 FeMnO 6 synthesised via citrate-nitrate gel combustion has an orthorhombic phase with the Pbnm space group, a spin-glass state and various super-exchange interactions due to the mixed Fe 2+ −Fe 3+ and Mn 3+ −Mn 4+ within the system. Dhilip et al. [4] reported that La 2 FeMnO 6 prepared by high-temperature solid-state reactions possesses a cubic symmetry and strong competing antiferromagnetic-ferrimagnetic (AFM-FIM) interactions. Filho et al. [5] investigated La 2 FeMnO 6 nanoparticles synthesised via ionic coordination reactions, which exhibited monoclinic P 21/n symmetry at room temperature, spin-glass behaviour with the transition temperature of approximately 98.9 K and an exchange bias effect below 20 K. Yang et al. [6] reported the occurrence of spin without strain coupling in the paramagnetic state-spin cluster state transition near 270 K for La 2 FeMnO 6 synthesised via conventional solid-state reactions. Li et al. [7] reported that La 2 FeMnO 6 prepared using the citrate-gel method preserves the orthorhombic Pnma symmetry up to 87.8 GPa accompanied by the first-order phase transition in the range of 28-45 GPa.
Most studies have been limited to investigating the low-temperature properties and certain heat preparation conditions. However, there is a need to thoroughly understand the effect of heating conditions on La 2 FeMnO 6 to optimise the material properties, which are crucial for fundamental scientific and electrochemical applications. In this study, the effect of heat treatment on the structural, magnetic and electrical properties of La 2 FeMnO 6 were investigated. The calcination temperature was varied from 1023 K to 1173 K to evaluate its effect on the structural and magnetic properties. The sintering condition was varied between 1 and 3 h at 1273 K to investigate its effect on the structural and electrical properties of La 2 FeMnO 6 . The results of these investigations are discussed below.

Materials and Methods
A double perovskite La 2 FeMnO 6 sample was prepared using the sol-gel and sintering method under the following procedure [7]. Analytical-grade lanthanum (II) oxide (La 2 O 3 , 99.9%), iron (II) nitrate nonahydrate (Fe(NO 3 ) 3 ·9H 2 O, 99.99%) and manganese (II) nitrate tetrahydrate (Mn(NO3) 2 ·4H 2 O, 99.98%) were obtained from Merck. To obtain La 2 FeMnO 6 powder, the precursors were dissolved, based on stoichiometry calculations, into a 0.5 molar concentration of citric acid monohydrate. The solution was continuously stirred at 450 rpm and heated at 120 • C due to the evaporation of the reagents and a gel was formed afterwards. The gel was dried and calcined to obtain the powdered La 2 FeMnO 6 sample. The optimum calcination conditions were varied from 1023 K to 1173 K. The crystal structure and parameters were evaluated by X-ray diffraction (XRD) measurements using X'Pert PRO PANalytical equipment (PANalytical, Almelo, the Netherland) with a Cu Kα (λ = 1.5418 Å) radiation source in the range of 20 • ≤ 2θ ≤ 80 • and a step size of 0.02 • /s. Structural analysis from the XRD pattern was conducted using Fullprof 2k (Institut Laue-Langevin, Grenoble, France) and VESTA software (Version 3.5.7, JP-Minerals, Nakatsugawa, Japan). Furthermore, the chemical states of La, Fe, Mn and O for each condition were investigated by X-ray photoelectron spectroscopy (XPS PHI-5400, Physical Electronics, PHI, Chicago, IL, US). The binding energy was corrected using the C1s peak at 285.0 eV. The XPS spectra were analysed using peakfit software (Version 4.11, Cranes Software International Ltd., Hyderabad, India). The room-temperature magnetic M−H hysteresis loops for calcined samples were measured by VSM (VSM 250, Xiamen Dexing Magnet Tech. Co., Ltd., Xiamen, China) with a sweeping rate of 120 Oe/s.
To form the ceramic sample, the sintering process was conducted under the following steps. First, the powder was pressed into pellets at 3 kN/inch 2 for 3 min. Next, the pellets were sintered at 1273 K. The sintering temperature was varied for 1 and 3 h. Structural analysis was performed by XRD using the X'Pert PRO PANalytical equipment with a Cu Kα (λ = 1.5418 Å) radiation source in the range of 20 • ≤ 2θ ≤ 80 • and a step size of 0.02 • /s. The XRD results were refined using Fullprof 2k and VESTA software. The room-temperature Raman scattering spectra were obtained using a Thermo Scientific DXR2 Raman microscope (Thermo Fisher Scientific Inc., Waltham, MA, US) with a 532-nm laser source in the range of 100-3500 cm −1 . The electrical conductivity was investigated using an IS instrument (RLC meter, FLUKE-PM 6303, Fluke Manufacturing Company, Inc., Everett, WA, US) in the temperature range of 300-400 K and a frequency of 10 kHz. Figure 1 shows the XRD patterns of La 2 FeMnO 6 calcined at 1023 K and 1173 K. For these samples, a series indexed of diffraction peaks at (101), (121), (022), (202), (310), (024), (040) and (240) indicate that the samples are crystallised in orthorhombic Pnma (62) symmetry. No additional diffraction peaks were observed above the background level, indicating a high phase purity for all samples. This result is consistent with that of previous reports for La 2 FeMnO 6 [8,9].  [8,9]. The structural parameters obtained from Rietveld refinement are listed in Table 1. The sample calcined at 1173 K exhibited a larger lattice constant, volume, bond angle and crystallite size than the sample calcined at 1023 K. This is because an increase in calcination temperature promotes the formation of pure units of crystals and a high degree of crystallinity [10]. Conversely, the sample calcined at 1023 K exhibited a larger tilt angle and orthorhombic distortion than that of the sample calcined at 1173 K, indicating that the sample calcined at 1023 K created a higher lattice distortion from the orthorhombic system. Figure 2 shows the XPS spectra of La2FeMnO6 calcined at different temperatures. The spectra show the presence of La 3d, Fe 2p, Mn 2p and O 1s. The entire spectra of each sample showed no additional state or phase. The fitting results for the oxidation states for each sample are shown in Figure 3a-d. All states of La2FeMnO6 at different calcination temperatures agree well with previous reports [3,[11][12][13][14][15]. The structural parameters obtained from Rietveld refinement are listed in Table 2. The sample calcined at 1173 K exhibited a larger lattice constant, volume, bond angle and crystallite size than the sample calcined at 1023 K. This is because an increase in calcination temperature promotes the formation of pure units of crystals and a high degree of crystallinity [10]. Conversely, the sample calcined at 1023 K exhibited a larger tilt angle and orthorhombic distortion than that of the sample calcined at 1173 K, indicating that the sample calcined at 1023 K created a higher lattice distortion from the orthorhombic system. Figure 2 shows the XPS spectra of La 2 FeMnO 6 calcined at different temperatures. The spectra show the presence of La 3d, Fe 2p, Mn 2p and O 1s. The entire spectra of each sample showed no additional state or phase. The fitting results for the oxidation states for each sample are shown in Figure 3a-d. All states of La 2 FeMnO 6 at different calcination temperatures agree well with previous reports [3,[11][12][13][14][15]. Figure 3a shows the main peaks of La 3d 5/2 and La 3d 3/2 , which show a typical La 3+ state. The binding energies of 851.2-852.9 eV and 834.9-836.0 eV are related to the La 3d 3/2 and La 3d 5/2 states, respectively. Another binding energy besides that of the La 3d 3/2 and La 3d 5/2 states was observed. This split is associated with the shake-up state of La 3d caused by the excitation of an electron from the O 2p valence band into the unoccupied La 4f orbital [13]. The La 3d 3/2 −La 3d 5/2 split is in the range of 16.6-16.9 eV, indicating that the La ions are in the La 3+ state.

K Fit
Count/s (arb. units) Binding Energy (eV)    Table 2, the peak position shifted slightly towards a higher binding energy with an increase in calcination temperature, indicating a decrease in O ions in La2FeMnO6. In other words, electrons were transferred from O to La and Fe states [17].

K Fit
The surface atomic composition (%) of La2FeMnO6 at different calcination temperatures and the corresponding atomic and ionic ratios obtained by XPS are listed in Table 3. The sample calcined at 1023 K exhibited the highest Fe/La and Mn/La ratios, indicating more La vacancies in the lattice. This induced the dominance of Fe 4+ −Mn 3+ ions, resulting in oxygen adsorption by Fe/Mn ions and La vacancies due to charge compensation [13]. The OH/O ratio decreased with increasing calcination temperature, which is in agreement with the XRD results (Table 1). With a decrease in the crystallite size, the adsorption sites for oxygen decreased.   Table 1. The peak positions of Fe 2p 3/2 and Fe 2p 1/2 are 710.0-710.6 eV and 723.7-723.8 eV, respectively, which are attributed to the Fe 3+ state. Other Fe states (Fe 2+ and Fe 4+ ) were observed, which are attributed to the generation of oxygen and La vacancies, respectively, as confirmed by the O 1s and La 3d spectra [14,15].  Figure 3c depicts the binding energy of Mn 2p states. The Mn 2p spectra showed two peaks at 641.2-641.4 eV and 653.5-653.7 eV, corresponding to the 2p 3/2 and 2p 1/2 states respectively. The spin-orbit energy spacing is approximately 12.1-12.3 eV, which is typical for the Mn 3+ state [16]. The binding energy of La 3d and Fe 2p were shifted towards higher positions as the calcination temperature increased, suggesting an increase in La and Fe oxidising capability [13,14]. Conversely, as shown in Table 1, the Mn 2p binding energy was shifted towards a lower position with increasing calcination temperature, indicating a decrease in Mn oxidation state [13]. Figure 3d shows the XPS spectra of O 1s states for all samples. The lattice oxygen (OL) is reflected in the binding energy ranging from 529.3 to 529.6 eV, which is related to La-O and Fe/Mn-O binding. The peaks at 529.8-530.0 eV and 531.3-531.7 eV are attributed to oxygen vacancy (OV) and surface adsorbed oxygen (OH), respectively. As shown in Table 1, the peak position shifted slightly towards a higher binding energy with an increase in calcination temperature, indicating a decrease in O ions in La 2 FeMnO 6 . In other words, electrons were transferred from O to La and Fe states [17]. The surface atomic composition (%) of La 2 FeMnO 6 at different calcination temperatures and the corresponding atomic and ionic ratios obtained by XPS are listed in Table 3. The sample calcined at 1023 K exhibited the highest Fe/La and Mn/La ratios, indicating more La vacancies in the lattice. This induced the dominance of Fe 4+ −Mn 3+ ions, resulting in oxygen adsorption by Fe/Mn ions and La vacancies due to charge compensation [13]. The OH/O ratio decreased with increasing calcination temperature, which is in agreement with the XRD results (Table 2). With a decrease in the crystallite size, the adsorption sites for oxygen decreased. Figure 4 shows the hysteresis curve of La 2 FeMnO 6 calcined at 1023 K and 1173 K, and the inset shows the hysteresis curve in the magnetic field until 3 kOe. The magnetic parameters are listed in Table 4

Effect of Calcination and Sintering Conditions on the Structural and Electrical Properties La2FeMnO6
The sintering conditions for each calcination temperature were varied. Conditions were labelled A (c1023 K, s1273 K, 1 h), B (c1023 K, s1273 K, 3 h) and C (c1173 K, s1273 K, 3 h). Figure 5 shows the XRD patterns of La2FeMnO6 at different calcination and sintering conditions.
The   For the sample calcined at 1023 K, the hysteresis shows a small loop at the lower field (<1 kOe) and linear behaviour at the higher field (>1 kOe). According to the XPS results, Mn 4+ and Fe 4+ are dominant states in the system. However, other states, such as Fe 3+ and Mn 3+ are also present. The linear part was subtracted and Ms was approximately 0.59 emu/g.
For the sample calcined at 1173 K, the hysteresis curve shows a larger loop (<3 kOe) than those of the linear part at the higher field (>3 kOe). The Mn 4+ , Fe 4+ , Fe 3+ , Mn 3+ and Fe 2+ states were observed in the XPS analysis with Mn 3+ and Fe 3+ found to be dominant. Ms was approximately 1.09 emu/g.

Effect of Calcination and Sintering Conditions on the Structural and Electrical Properties La 2 FeMnO 6
The sintering conditions for each calcination temperature were varied. Conditions were labelled A (c1023 K, s1273 K, 1 h), B (c1023 K, s1273 K, 3 h) and C (c1173 K, s1273 K, 3 h). Figure 5 shows the XRD patterns of La 2 FeMnO 6 at different calcination and sintering conditions.     Table 5. Sample C showed a higher lattice constant, volume and bond angle, which were attributed to the better formation of crystal units [10]. However, the crystallite size showed a different trend. Sample B showed the highest crystallite size and Sample C showed the lowest crystallite size and the lowest tilt angle and orthorhombic distortion. These results confirm that varying the heat-treatment conditions can significantly affect the structural parameters, crystallinity degree and crystallite size of La 2 FeMnO 6 .  Figure 6 shows the Raman scattering spectra of La 2 FeMnO 6 at different calcination and sintering conditions at room temperature. The Raman phonon modes were fitted by the standard Lorentzian technique. The two main modes (ω 1 and ω 2 ) and the high-frequency mode (ω 3 ) are identified as the characteristic Raman phonon modes of La 2 FeMnO 6 [3,4,11,12]. According to previous reports [3,4,11,12] the ω 1 phonon mode is related to the bending of (Fe/Mn)O 6 octahedra with A1g symmetry, the ω 2 phonon mode corresponds to the stretching of (Fe/Mn)O 6 octahedra with B1g symmetry, and the ω 3 phonon mode is ascribed to second-order processes which are attributed to the strong spin-lattice interactions [11,18]. No additional mode was detected in the Raman spectra indicating a high-purity La 2 FeMnO 6 phase. The phonon parameters including mode position, line width and intensity are shown in Figure 7. The Raman modes were redshifted, and the intensity increased with an increase in the temperature and time of heat treatment. A different trend was observed for the linewidth. The ω and ω modes broadened, whereas ω compressed with an increase in temperature and time of heat treatment. The Raman intensity of all phonon modes was enhanced with the broadening of the line width reflecting a decrease in lattice distortion. This result is consistent with the decrease in orthorhombic distortion revealed by XRD (Table 5). The phonon parameters including mode position, line width and intensity are shown in Figure 7. The Raman modes were redshifted, and the intensity increased with an increase in the temperature and time of heat treatment. A different trend was observed for the linewidth. The ω 1 and ω 3 modes broadened, whereas ω 2 compressed with an increase in temperature and time of heat treatment. The Raman intensity of all phonon modes was enhanced with the broadening of the line width reflecting a decrease in lattice distortion. This result is consistent with the decrease in orthorhombic distortion revealed by XRD (Table 5).  Figure 8 shows the temperature-dependent conductivity (i.e., (a) vs. T, and (b) ln vs. T-1 plots) of La2FeMnO6 under different calcination and sintering conditions. As shown in Figure 8a, (T) increased with an increase in temperature. Further, vs. T was analysed by the small polaron hopping (SPH) model, which is described as follows [3].   Figure 8a, σ(T) increased with an increase in temperature. Further, σ vs. T was analysed by the small polaron hopping (SPH) model, which is described as follows [3].
with σ o as the pre-exponential factor, k B as the Boltzmann constant (8.62 × 10 −5 eV K −1 ) and E a as the activation energy required for conduction. Figure 8b shows the fitting of σ(T) data with the SPH model. There were no changes in the slope of the fitted data in each condition, indicating the existence of a single-conduction mechanism in this temperature range (300-400 K). The SPH model usually holds conduction above charge ordering in the mixed-valence manganite samples, which is a thermally activated mechanism to the weakly localised carriers [3,19]. Herein, E a ranged from 0.20 to 1.0 eV, indicating that the conduction mechanism is dominated by p-type polaron hopping [20]. Furthermore, the lowest E a was obtained in Sample A, followed by Sample B and sample C. This can be explained in terms of the corresponding change in Fe/Mn-O-Fe/Mn bond angles, since the hopping conduction mechanism acts across the Fe/Mn cation sites. As revealed by XRD (Table 5), all samples showed similar Fe/Mn-O-Fe/Mn bonds indicating a similar conduction mechanism. These results showed that the heat treatment affects the conduction mechanism by lowering the activation energy, thereby increasing conductivity. A lowering of activation energy is required to improve the large electrochemical reaction in the system. This may lead to La 2 FeMnO 6 becoming a promising candidate for use as a catalyst material on electrode surfaces in electrochemical reaction applications [20]. Consequently, this study is important for understanding fundamental physical phenomena, and for informing the development of electrodes in electrochemical systems.

σ T = σ T exp E k T
with as the pre-exponential factor, kB as the Boltzmann constant (8.62 × 10 −5 eV K −1 ) and E as the activation energy required for conduction. Figure 8b shows the fitting of (T) data with the SPH model. There were no changes in the slope of the fitted data in each condition, indicating the existence of a single-conduction mechanism in this temperature range (300-400 K). The SPH model usually holds conduction above charge ordering in the mixed-valence manganite samples, which is a thermally activated mechanism to the weakly localised carriers [3,19]. Herein, E ranged from 0.20 to 1.0 eV, indicating that the conduction mechanism is dominated by p-type polaron hopping [20]. Furthermore, the lowest E was obtained in Sample A, followed by Sample B and sample C. This can be explained in terms of the corresponding change in Fe/Mn-O-Fe/Mn bond angles, since the hopping conduction mechanism acts across the Fe/Mn cation sites. As revealed by XRD (Table 5), all samples showed similar Fe/Mn-O-Fe/Mn bonds indicating a similar conduction mechanism. These results showed that the heat treatment affects the conduction mechanism by lowering the activation energy, thereby increasing conductivity. A lowering of activation energy is required to improve the large electrochemical reaction in the system. This may lead to La2FeMnO6 becoming a promising candidate for use as a catalyst material on electrode surfaces in electrochemical reaction applications [20]. Consequently, this study is important for understanding fundamental physical phenomena, and for informing the development of electrodes in electrochemical systems.

Conclusions
La2FeMnO6 was synthesised via the sol-gel method. The sample was calcined at two temperatures and sintered under three conditions. Rietveld refinement analysis of XRD patterns revealed a single phase with an orthorhombic structure and Pnma (62) space group for all samples which was confirmed by XPS and Raman scattering analysis. The lattice constant, crystallite size and magnetisation parameters for the sample increased with an increase in the calcination temperature. The XPS analysis revealed changes in the Mn and Fe ionic states, and in the ionic vacancy (La and O) concentrations with calcination temperature. The electrical conductivity was based on a single-conduction mechanism dominated by p-type polaron hopping. The lowest activation energy is approximately 0.237 ± 0.003 eV at the lowest temperature and time of heat treatment. This study reveals that heating conditions significantly influence the physical properties and material quality of La2FeMnO6.

Conclusions
La 2 FeMnO 6 was synthesised via the sol-gel method. The sample was calcined at two temperatures and sintered under three conditions. Rietveld refinement analysis of XRD patterns revealed a single phase with an orthorhombic structure and Pnma (62) space group for all samples which was confirmed by XPS and Raman scattering analysis. The lattice constant, crystallite size and magnetisation parameters for the sample increased with an increase in the calcination temperature. The XPS analysis revealed changes in the Mn and Fe ionic states, and in the ionic vacancy (La and O) concentrations with calcination temperature. The electrical conductivity was based on a single-conduction mechanism dominated by p-type polaron hopping. The lowest activation energy is approximately 0.237 ± 0.003 eV at the lowest temperature and time of heat treatment. This study reveals that heating conditions significantly influence the physical properties and material quality of La 2 FeMnO 6 .

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

Conflicts of Interest:
The authors declare no conflict of interest.