Optical and Electrical Properties of Fe_2-xSm_xO₃ Compounds (x = 0, 1, 2) at Room Temperature

Due to their great potential for addressing some fundamental scientific issues about low dimensionality and applications, metal oxides nanostructures have attracted extensive interest over the past decade [1]. Among the most important metal oxides for industrial materials it exists iron oxides and samarium oxide. They exhibit various attractive physical and chemical properties due to differences in the valence state of their ions and/or crystal structure [2]. Ferric oxide (Fe₂O₃) has four polymorphs is used as a raw material for magnetic materials and cathodes for lithium ion-batteries is used as a magnetic storage material due to its ferromagnetic properties [3]. While the oxides of rare earths such as samarium has many important applications, including high-efficiency phosphors and catalysts. They show good catalytic properties in several reactions, including the synthesis of ammonia and oxidative coupling of methane [4,5].

The objective of the present work is to synthesize in the first step the compounds Fe₂O₃, Sm₂O₃ and FeSmO3. Then, our samples were characterized by X-ray powder diffraction. FTIR spectroscopy is used to study the vibrational modes and finally the electric and optical properties were studied.

Preparation

The oxide Fe₂O₃, Sm₂O₃ and FeSmO3were prepared by sol-gel method based on the Pechini’s route using stoichiometric amounts of Samarium (III) nitrate hexahydrate Sm₂sub>(NO₃)₃.6H₂O, Iron (III) nitrate nonahydrate Fe₂(NO₃)₃.9H₂O and Ammonium hydroxide NH3.H2O. Firstly, stoichiometric amounts of metal nitrates were dissolved completely in distilled water to obtain a mixed solution. Subsequently, a controlled amount of citric acid were incorporated and dissolved with stirring. The molar ratio was fixed as 1:1 of nitrates to citric acid and pH of the solution was adjusted by adding an amount of ammonia solution. Then the solution heated under stirring to 373K followed by the addition of ethylene glycol as a polymerization agent. Heating, continued until obtaining a gel. The obtained wet gel was dried to obtain a foamy dry, which was grounded in a mortar, followed by heating at 773K for 24h in air, and then heating at 973K for 24h, after grinding, and then samples were heating at 1173K for 24h. In order to obtain the desired crystalline phase, the samples grinding and heating at 1473K during a sufficiently long annealing period 48h. Then, all the results found in the present investigation presented for samples sintered at 1473K. The details of synthesis process of the sample represented in Figure 1.

Figure 1:Schematic of the Pechini method of making metal/organic gels. (The evolution of ‘sol– gel’ chemistry as a technique for materials synthesis) A. E. Danks, S. R. Hall b and Z. Schnepp.

Material characterization/

To ensure the homogeneity and phase purity of synthesized compounds a high precession Xray Powder Diffraction (XRD) was carried out. The measurements were checked by Panalytical X’Pert Prompd operating with copper anticathode radiation CuKα (λ= 1.5060Å) at room temperature in the angular range 20°<2θ<90°. Therefore, in order to study the optical properties of Fe₂O₃, Sm₂O₃ and FeSmO₃ compounds, the UV-Vis spectra were obtained by a UV-VIS spectrophotometer (Shimadzu UV-3101PC)using a source emitting wavelength radiations varying between 200 and 800nm on a pellet of each compound.

The dielectric measurements, which are performed in a wide range of frequency, are deduced from the admittance spectroscopy using an Agilent 4294A analyzer and Janis VPF800 cryostat. Before that, a thin Al layer with a disc shape is deposited on both sides of each sample.

Structural analysis

Crystalline parameters: The structural refinement of the compounds of Fe₂O₃, Sm₂O₃ and FeSmO₃ has been refined using the Rietveld method via the Full Prof software [6]. For the simulation of the peak shapes, we used the pseudo-Voigt function, and the lattice parameters were determined by the Rietveld program, investigated as starting values.

Figures 2a-2c shows X-ray diffractions patterns recorded at room temperature, including the observed, the calculated profile and as well as the difference profile of Fe₂O₃, Sm₂O₃ and FeSmO₃ respectively. All the reflection peaks corresponding to 2θ values are crystallized in the hexagonal system with R-3c space group for Fe₂O₃ compound, in the monoclinic system with C21/mspace group for Sm₂O₃, and orthorhombic system with Pnma, space group with FeSmO₃ compound.

Figure 2:Refined XRPD patterns of Fe₂O₃ (a), Sm₂O₃ (b) and FeSmO3 (c) compounds: The dotted line indicates experimental data and calculated data are represented by black continuous lines. The lowest curve in blue shows the difference between experimental and calculated patterns. The vertical bars in green indicate the Bragg positions..

The obtained reliability factors and lattice parameters of the three compounds given in Table 1 are in good agreement with the published results

Table 1:Crystallographic parameters of Fe₂O₃, Sm₂O₃ and FeSmO₃ compounds refined from Rietveld analysis of XRPD data.

Crystallite size/

The average crystallite size of our samples can be estimated from the X-ray diffraction spectrum based on the width at half height of the main diffraction peaks.

For this we have adopted the following Debye Scherrer formula (equation 1) [7].

Where
D: the average size of the crystallites (nm).
K: Scherrer’s constant: For the full widths of the lines this factor equal to 1 while for the widths at mid-height this factor equal to 0.94.
λ: the X-ray wavelength
β: the FWHM of the diffraction peak
θ: represents the angle of the most intense peaks

The values of crystallite size were found to be 102nm, 58nm and 56nm for SmFeO₃, Fe₂O₃ and Sm₂O₃ respectively.

Structural description: The bibliographic study shows that in the structure of the Fe₂O₃ compound the iron and oxygen atoms are placed in the (0, 0, z) and (x, 0, 1/4) respectively, where z=0.35 and x=0.31 (24,25). Oxygen atoms occupy a Hexagonal Close- Packed (HCP) arrangement with 2/3 of the octahedral positions being occupied by the iron atoms. Along z axis, one empty and two filled octahedrons sharing common face alternate. Due to a mutual repulsion between the neighboring cation layers, cations are shifted from the centers of octahedrons towards the vacant positions (Figure 3); [8,9].

Figure 3:Representation of the structure of Fe₂O₃

In the Sm₂O₃ structure the Sm(1) and Sm(2) atoms are effectively seven-coordinate with the SmO?₇ group derived from an octahedron by adding the seventh oxygen atoms along a three- fold axis [10]; (Figure 4). As is illustrated in Figure 4 this has the effect of displacing the Sm towards one of the octahedral faces. The Sm-O bond distances of the six closest oxygen anions are in the range 2.29-2.55Å for Sm(1) and 2.28-2.48Å for Sm(2) with the longer oxygen; O(3) form Sm(1) at 2.708(10) and Sm(2)-O(5) at 2.755(6) Å. The Sm(3) atom is best described as a distorted octahedron since the “seventh” atom is over 3.11Å away, and the displacement towards the corresponding face is significantly less. The average of the six short Sm-O distances for each of the crystallographic unique Sm atoms falls in a narrow range (2.37-2.40Å). The Bondvalence sums for these are 2.86, 3.17 and 2.97, respectively with the contribution of the seventh atom decreasing from a maximum of 6.6% in Sm(2) to 2% in Sm(3)[11,12].

Figure 4:Representation of the structure of Sm₂O₃.

In these perovskite-like compounds there are four formula units in the unit cell or one in the asymmetric unit. The iron and gadolinium ions have been omitted to give an unobstructed view of the distorted octahedra. In a single ‘level’ of octahedra, the octahedra are tilted toward or away from each other in planes containing neighboring edges. Octahedra in neighboring levels have faces tilted toward or away from each other [13] (Figure 5).

Figure 5:Representation of the structure of SmFeO₃

Infrared absorption spectroscopy analysis

In order to extend our knowledge of the physico-chemical properties of these materials, we report the infrared spectra of the Sm₂O₃., Fe₂O₃ and SmFeO₃ compounds and assign the observed bands. The infrared spectra of the title compound, recorded at room temperature, are shown in Figure 6a-6c. The FTIR spectra of the compounds Sm₂O₃. and Fe₂O₃ each show an intrinsic stretching vibration band of Se-O at 438cm^-1 and Fe-O at 532cm^-1 respectively. On the other hand, the infrared spectrum of SmFeO₃ compound exhibits two intrinsic stretching vibration bands of Se-O at 425cm^-1 and of Fe-O at 528cm-1 [14-17].

Figure 6:Infrared spectrum of Fe₂O₃ (a), Sm₂O₃ (b) and FeSmO₃ (c) compounds.

Optical properties

Figure 7a-c:Optical absorption and reflectance spectra of Fe₂O₃, Sm₂O₃ and FeSmO₃ compounds measured at room temperature respectively

UV-visible study: Optical properties are one of the most important factors to evaluate the light efficiency. To determine the band gap energy of investigated compounds, absorbance spectra of prepared samples were performed in the wavelength range (200-800) nm at room temperature as shown in Figure 7a-7c. The obtained spectra exhibit the nethermost energy absorption peaks at 236nm, 235nm and 213nm, for Fe₂O₃, Sm₂O₃ and FeSmO₃ compounds, respectively. These bands with maximum energies can be assigned to the absorption of the highest energetic level in the conduction band.

To determine the band gap energy, the reflectance spectrum was converted with the Kubelka- Munk equation given by the following form (equation 2) [18]:

Where
F(R): the Kubelka-Munk function
R: the diffuse reflectance

The relation between band gap energy E_g and coefficient absorption a near the band edge was estimated based on the Tauc plot method, expressed as (equation 3) [19]:

Where
α: the absorption coefficient
hν: the incident photon energy
A: constant characteristic of the material
E_g: the optical band
n: the empirical exponent, which characterizes the type of optical transition during the absorption process.

Thus, the value of (n) is equals of 1/2 and 2, corresponding to the allowed direct and indirect transition [20]. The dependence of (αhν)2 versus the photon energy (hν) for each compound is shown in Figure 8a-8c. The energy band gaps E_g can be determined from the intercepts of these plots on the energy axis, since E_g=hν when (αhν)²=0.

Figure 8a-c:Plot of (ahυ)2 versus (hυ) of Fe₂O₃, Sm₂O₃ and FeSmO₃ compounds

For these compounds, the average band gap energy is about 2.85eV for Fe₂O₃, 4.84eV for Sm₂O₃ and 4.27 for FeSmO₃. These compounds can be classified as semiconductors with large gap.

Extinction coefficient: The extinction coefficient represents the amount of attenuation loss when the electromagnetic wave propagates through a medium. It is related to the absorption coefficient and the wavelength using equation 4 [21].

In fact, Figure 9a-9c shows the variation of the extinction index as a function of wavelength for the compounds studied. The extinction coefficient behavior of the Fe₂O₃ and FeSmO₃ samples tends to increase with the wavelength increase while for Sm₂O₃ it decreases with increasing wavelength. Moreover, the fall and rise in the extinction coefficient are due to the variation of the absorbance. In fact, low values of k indicate the region of transparency of each compound [22]. However, the compound Sm₂O₃ transmit almost most of the incident radiation due to their total transparency in the visible area. In fact, when the crystals exhibit awide transmission range in the visible domain, it can be used for non-linear optical applications and anti-reflective layers of solar thermal devices [23].

Figure 9a-c:Dispersion curves of extinction coefficient k for the Fe₂O₃, Sm₂O₃ and FeSmO₃ compounds as a function of wavelength.

Electric properties

Equivalent circuit: The impedance spectroscopy is one of the most adequate techniques to investigate the electrical conductivity and analyze the charge transport processes in the grain; grain boundary and electrode effect in materials for both wide frequency and temperature ranges [24].

In this work, the Nyquist plots (-Z” vs Z’) of Sm₂O₃, Fe₂O₃ and SmFeO3 compounds recorded at room temperature in well-defined range of frequency is illustrates in Figure 10a-10c. From the shape of these plots, this compounds formed by a single semi-circular arc associated the contribution of the grain. The depressed semicircles have their centers on a line below the real axis, which indicates departure from the ideal Debye behavior [25].

Figure 10a-c:Nyquist diagrams of Fe₂O₃, Sm₂O₃ and FeSmO₃ compounds.

Based in the Z-View-software, these spectra were modeled by an equivalent circuit which is collected of a parallel combination of resistance Rg (bulk resistance) and a constant phase element (capacity of the fractal interface CPE).

The impedance of the capacity of the fractal interface CPE is given by the following equation (5):

The fitted values obtained by the equivalent circuit are used to simulate the experimental curve of real and imaginary part of impedance spectroscopy based on the fallowing equations 6 & 7.

Where
ω: the angular frequency
Q: the capacitance value of the CPE element
α: the fractal exponent.

The great conformity of the calculated lines (simulated curve) with experimental data observed in Figure 11a-11c indicates that the chosen equivalent circuit is well described the behavior of these materials. The fitting parameters obtained by the equivalent circuit are listed in Table 2. It is remarkable that the mixed compound SmFeO₃ is less resistive than the other two compounds.

Figure 11a-c:Real and imaginary parts of impedance as a function of frequency for the samples Fe₂O₃, Sm₂O₃ and FeSmO₃.

Table 2:Fitting parameters obtained by the equivalent circuit.

Based on the parameters values extracted from the equivalent circuit using Z-view software, the direct conductivities for the different relaxations are determined at each temperature from equation 8:

Where A: the electrolyte-electrode contact area e: the thickness of the pellet R_g: the bulk resistance value

The real part of impedance decreases with the rise of frequency. This plot shows that conduction increases with the rise of frequency. The imaginary impedance is characterized by the appearance of the maximum value Z”_max at a certain frequency ω_max, to introduce the relaxation time τ from the condition (ω_maxτ=1).

From Table 2, we notice that the values of α are very close to 1 which indicates that CPE behaves like a capacitance or the interactions between the dipoles are small. In addition, the values of grain resistance (R) of sample Sm₂O₃ is greater than that of two other compounds.

AC conductivity study: AC conductivity measurements have been widely used to investigate the nature of defect centers in the disordered systems. Which are supposed to be responsible for this type of conduction. The high-frequency dependence of the AC conductivity is often observed experimentally in most polar semiconductor oxides, and then can be explained in terms of thermally activated hopping of charge carriers between localized sites over the potential barrier.

The value of ac conductivity (σ_ac) of the material is evaluated by an empirical formula (equation 9):

As shown in these spectra (Figure 12a-12c), there are two dominant regions, the first at low frequency where AC conductivity is remains almost constant, the second at higher frequency; the area in which we are in front of a dispersion phenomenon characterized by ωs. This dispersion can be related to the behavior of the space charges that is anticipate, since that their effect disappears with increasing frequencies and temperature [26].

Figure 12a-c:Frequency dependence of AC conductivity for the samples Fe₂O₃, Sm₂O₃ and FeSmO₃.

This is not always the case as shown by the proposed spectra where there are curves for which the rise is not reached showing the existence of space charges even at high frequencies. Those spectra were fitted (solid line) through the expression proposed by Jonsher (equation 10) [27].

Where
σdc: the dc conductivity (a value of conductivity at low frequency)
Bωs: the ac conductivity which consists of all dispersion phenomena
B: the temperature dependent variable
S: the exponent who describes the degree of interaction of
charge carriers with their environment, charges, and impurities.
The value of s varies from 0 to 1 and for ideal Debye-type behavior s=1.

The fitting parameters for the three compounds are collated in Table 3.

Table 3:Fitting parameters obtained by the equivalent circuit.

Figure 12a-12c show that the conductivity is higher for the mixed compound SmFeO₃ compared to the other two compounds. Indeed, the presence of two types of trivalent cations (Sm and Fe) in the SmFeO3 compound increases the disorder in this material and therefore the conductivity.

Dielectric study

Figure 13a-c:dielectric constant ε’ as a function of frequency for Fe₂O₃, Sm₂O₃ and FeSmO₃ compounds..

The frequency dependence of real (ε′) parts of the dielectric constant of Fe₂O₃, Sm₂O₃ are shown in Figure 13a-13c. Based on these figures, it can be noted that the presence of a single low frequency relaxation which corresponds to the space charge for the two compounds Fe₂O₃ and SmFeO₃. On the other hand, the Sm2O3 compound exhibits two peaks (Figure 13(b)), one at low frequency corresponds to the space charge relaxation and the other at high frequency caused by dipole relaxation. At low frequencies, ε′ shows a dispersive behavior. Besides, the increasing value of ε′ with increasing temperature may be attributed to the increase in space charge polarization process [28]. On the other hand, at high frequencies, it approaches a limiting constant value ε′∞ (ω), which can be interpreted as a result of a rapid polarization processes with no ionic motion contribution, because the frequency is too high and the ions can only oscillate without reaching the sample-electrode interface. We can see that dielectric constant ε› of Fe₂O₃, Sm₂O₃ and FeSmO₃ have different values at a maximum of ε’, which is equal 170, 127, 348, respectively. It can be concluded that the mixed compound has a great permittivity compared to the two other compounds.

Figure 14a-14c exhibits the frequency dependence of the imaginary dielectric constant ε» at room temperature for the three compounds. The dielectric loss spectrum, denoting the imaginary part ε′′(ω) of the complex permittivity, reveals the occurring dipolar molecular dynamic processes, with distinct origin processes appearing at diverse characteristic frequency ranges. At low frequency, the dielectric loss is predominantly Joule losses accompanied with charge conduction. The loss spectrum displayed bump-like features, corresponding to distinct reorientational processes.

Figure 14a-c:Variation of dielectric constant ε′′ versus frequency for Fe₂O₃, Sm₂O₃ and FeSmO₃ compounds..

In this work, the Fe₂O₃, Sm₂O₃ and FeSmO₃ were successfully synthesized by sol-gel method based on the Pechini’s route. X-ray diffraction analysis recorded at room temperature for the three compounds revealed that: Fe₂O₃ crystallized in the hexagonal system with R-3C space group, the values of the lattice parameters were determined(a=5.034Å, b=5.034Å and c=13.742Å), the Sm₂O₃ crystallized in the monoclinic system withC21/m space group, the values of the lattice parameters were determined (a=14.180Å, b=3.627Å and c=8.855Å) and the FeSmO3 crystallized in the orthorhombic system with Pnm a space group, the values of the lattice parameters were determined (a=5.596Å, b=7.707Å and c=5.399Å). The vibrational study by FTIR spectroscopy confirms the presence of intrinsic stretching vibration Fe-O in the compound Fe₂O₃, Se-O in the compound Sm₂O₃ and Fe-O and Sm-O in the mixte compound. The variation of ac conductivity as a function of frequency is interpreted using the universal Jonscher’s law. Optical properties of the title compounds are investigated using a UV-Vis spectroscopy to determine some of important parameters such as the band gap energy. Tauc’s model was used to determine the energy of optical gap of the prepared compounds. The analysis of the absorbance data revealed the existence of a direct transition for the three samples. The band gap values were Eg=2.85eV, Eg=4.84eV and Eg=4.27eV for Fe₂O₃, Sm₂O₃ and FeSmO₃ respectively. The compound Sm₂O₃ transmit almost most of the incident radiation due to their total transparency in the visible area, so it can be used for non-linear optical applications and anti-reflective layers of solar thermal devices.

Furthermore, analysis of the real and imaginary parts leads us the presence of only semi- cercle associated to grain. The presence of two types of trivalent cations (Sm and Fe) in the SmFeO₃ compound increases the disorder in this material and therefore the conductivity. The dielectric constant ε’ of Fe₂O₃, Sm₂O₃ and FeSmO₃ have different values at a maximum of έ, which is equal 170, 127, 348, respectively. It can be concluded that the mixed compound has a great permittivity compared to the two other compounds. These studies show that the substitution between Fe and Sm gives a material (FeSmO3) characterized by more important dielectric and optical properties.

We are highly obliged that our university: University of Sfax provided all the facilities and infrastructure. This support and the infrastructure help us to complete the research article.

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