Structure and Physical Properties of the LaBiFe₂O₆ Perovskite Produced by the Modified Pechini Method

Abstract

In this paper the synthesis of the LaBiFe₂O₆ material by the modified Pechini method is reported. Structural, morphologic, magnetic and optic experimental studies were performed. Rietveld refinement of x-ray diffraction patterns revealed that LaBiFe₂O₆ crystallizes in an orthorhombic perovskite structure (space group Pnma, # 62). Scanning electron microscopy images showed the nanometric feature of grains. X-ray dispersive spectroscopy permitted to infer the obtaining of the LaBiFe₂O₆ expected stoichiometry. Results of magnetic susceptibility as a function of temperature and field magnetization evidenced mixed ferromagnetism and superparamagnetism behavior at T=300 K. Mössbauer spectroscopy supported the superparamagnetic and ferromagnetic responses as a result of the nanogranular morphology and anisotropy effects. Spectrum of diffuse reflectance suggest that this material behaves as a semiconductor with energy gap E_g=2.13 eV.

Keywords:
Modified Pechini method; double perovskite; ferromagnetic; semiconductor

1. Introduction

The Pechini method for the synthesis of materials is a relatively new technique that, at first, was implemented for deposition of films base on lead and alkaline earth titanates and niobates applied to the technology of dielectrics and capacitors¹. In the last years this method has enabled synthesis BaTiO₃ nanopowders² and other perovskite-like materials³. The biggest advantage of this method lies in its simplicity and in the almost completely independence of the process conditions of the chemistry of positive ions contained in the final material and a low sintering temperature, resulting in obtaining powders oxides formed by nanometer sized crystals. On the other hand, this method includes some disadvantages such as the use of glycol toxic ethylene and significant volumes of organic reagents of the product, lack of complexes stable citrus elements such as bismuth, as well as partial or complete regeneration of certain components during pyrolysis of polymer gel, such as copper and ruthenium, among others. In order to solve some of the disadvantages of this method in the synthesis of lanthanide oxides, recently it has been applied the so-called modified Pechini method, by which it has achieved a further reduction in the size of the obtained crystals⁴. This method can be applied in the processes of synthesis of simple perovskites based on lanthanum, with crystallographic single phase, in nanometric size grains strongly coupled to form lamellar structures ranging from sizes with tens of nanometers up to several micrometers⁵. So far, there are very few reports of double perovskites produced through this technique^6-7. Double perovskites are materials extensively studied due to the potential that their generic formula AA'MM'O₆, where A- and A'-cation, and M- and M'-cations can be the same or different type, provides to modify the structure and, consequently, the physical properties of this family of compounds, by introducing various elements in sites A and M of that formula^8-9. The novel double perovskite material LaBiFe₂O₆ has recently been reported as a possible biferroic with ferromagnetic and ferroelectric response at room temperature¹⁰. The aim of this work is to study the crystal structure, morphology, composition, and magnetic and optic characterizations of LaBiFe₂O₆ perovskite samples produced by applying the modified Pechini method, and to analyze the effect of the nanometric size grain on the magnetic and electric properties compare with that published for this materials synthesized by the solid state reaction technique.

2. Experimental

LaBiFe₂O₆ nanopowder samples were produced by the modified Pechini method⁴. Aldrich La₂O₃ (99.99%), Bi(NO₂)₂.5H₂O (Aldrich 99.9%) and Fe(NO₂)₂.9H₂O (Aldrich 99.99%) were added separately in hot nitric acid solutions, which was selected as chelating agent (CA). First, the lanthanum oxide was separately mixed with nitric acid and heated up to obtain a clear aqueous solution. This solution was mixed with the Fe and Bi based precursor previously dissolved in water. Then, the nitric acid is added in a 1:1 CA-to-metal molar ratio, and submitted to a strong magnetic stirring for 3 hours. For the polyesterification process, ethylene glycol was used as esterification agent (EA). Ethylene-glycol was added in a 1:2 CA-EA ratio followed by slow evaporation of water at 373 K for 15 hours in constant magnetic stirring. The organic precursor was annealed in two steps, first at 573 K for 3 h and second at 1023 K for 1 h to obtain nanopowder with perovskite structure. The determination of the crystal structure of the LaBiFe₂O₆ samples were carried out by means X-ray diffraction experiments through a PANalytical X'pert PRO diffractometer (λ_CuKα=1.54064 Å). The analysis of the experimental data was performed from Rietveld refinement by using the GSAS+EXPGUI code¹¹, through which it is possible to obtain directly the most relevant structural parameters and the corresponding space group. In order to examine the surface morphology of the samples, Scanning Electron Microscopy (SEM) images were taken by using VEGA 3 TESCAN equipment. A semi-quantitative comparison of the nominal stoichiometry of the samples was established through application of the Energy Dispersive X-ray Spectroscopy (EDS) microprobe coupled to the electron microscope. With the purpose of analyzing the magnetic response of the material, measurements of susceptibility as a function of temperature and magnetization hysteresis curves were performed on Quantum Design MPMS magnetometer. Zero Field Cooling (ZFC) and Field Cooled (FC) procedures were carried out to determine the temperature dependence of the susceptibility on the application magnetic fields H= 500, 2000 and 10000 Oe. Hysteresis loops of magnetization as a function of applied fields were recorded at T=50, 200 and 300 K. This solid-reacted material was previously reported as semiconductor¹⁰, then diffuse reflectance measurements were examined by means a VARIAN Cary 5000 UV-Vis-NIR spectrophotometer. Finally, Mössbauer measurements were performed by means a 100mCi 57Co source in an Rh matrix, which was driven at a constant acceleration in a triangular mode at room temperature. The spectra were recorded in 512 channels and the Lorentzian lines of the folded data were fitted using Recoil software¹². The spectrometer was calibrated with the room temperature spectrum of α-Fe foil. All isomer shift values were expressed with respect to the centroid of this spectrum.

3. Results and Discussion

The structural analysis of the material is made from the predictions of the Structure Prediction Diagnostic Software (SPuDS)¹³, which gives the most probable spatial groups and the respective Wyckoff positions of all the ions for a new material. These predictions are used as input data for the simulation of diffraction patterns in the Powdercell (PCW)¹⁴, in which it is possible to establish all the diffractive planes, with their respective Miller indices and 2θ positions, for each spatial group predicted by the SPuDS. Finally, through the application of the GSAS code and its respective graphic interface EXPGUI¹¹, the simulated patterns are compared with the experimental diffractogram in a refinement process that allows determining to which of the probable spatial groups corresponds the experimental XRD pattern. Likewise, through refinement, all the structural parameters are obtained and all the diffractive planes are indexed, since the code allows the analysis of the shape and structure factors during the refinement process.

The XRD pattern for the LaBiFe₂O₆ samples produced by the modified Pechini method is exemplified in Figure 1, where symbols represents the experimental data, the red line is associated to the simulated diffractogram, the blue line indicates the experimental background and the green line is the difference between the theoretical and experimental patterns. The reliability parameters associated to the refinement quality where χ²=2.81; R(F²)=0.0323; R_p=0.0306 and R_wp=0.0401. This can be said from the analysis that the LaBiFe₂O₆ material crystallizes in an orthorhombic perovskite, Pnma (#62) space group, with lattice parameters a=5.5786(3) Å, b=7.8649(8) Å and c=5.5583(7) Å, and respective angles α=β=γ=90.0°. The appearance of the diffraction peaks (111) and (311) constitutes a strong indication that the La³⁺ and Bi³⁺ cations are placed in an orderly manner into the structure along the crystallographic directions.

Figure 1
Comparison between the refined XRD data and the simulated pattern for the LaBiFe₂O₆ material.

It is expected that the Pnma space group corresponds to simple perovskites as CaTiO₃¹⁵. This orthorhombic structure is characterized by the Glazer notation a^-b⁺a^- , which is characterized by two angles of rotation with quite similar magnitude and opposite directions and corresponds to an increase of $\sqrt{2}$ in the a=b cell parameter and doubling the value of the c parameter when compared with the cubic Pm3m perovskite. The crystallization of double perovskite materials in this structure probably occurs when the ionic radii r_A ≈ r_A' . In our LaBiFe₂O₆ material there is a great similarity between the ionic radii r_La ³⁺=1.22 Å and r_Bi ³⁺=1.24 Å.

In order to properly describe the crystallographic structure the concept of Wyckoff positions is used¹⁶, as presented in Table 1.

Table 1
Relevant lattice parameters for the LaBiFe₂O₆ complex perovskite in the Pnma space group.

	Atomic coordinates
Atom	Wyckoff positions	x	y	z
La, Bi	4c	0.52003	0.25000	0.50333
Fe	4a	0.50000	0.00000	0.00000
O(1)	4c	-0.00344	0.25000	0.44753
O(2)	8d	0.26671	0.04962	0.72421

Eight oxygen atoms are sited in the ac-plane, corresponding to the 8d Wyckoff positions, and four oxygen atoms are along the b-axis, in the 4c Wyckoff positions. This splitting of the oxygen in two different Wyckoff orbits constituted an evidence of the occurrence of a structural transition from a cubic phase Pm3m to the orthorhombic Pnma, since in a cubic one Wyckoff position for the oxygen atoms is sufficient to describe the structure.

It is clear from the Glazer notation a^-b⁺a^- that octahedral distortions are expected with tilting in antiphase along the a-axis and tilting in-phase along the b-axis, where a and b represents the pseudocubic sub-cell lengths¹⁷. These octahedral distortions are easily observable in Figure 2. Figure 2a shows the FeO₆ octahedra in the b-c crystallographic plane of the structure, which it should not be confused with any plane formed by the pseudocubic axes. Furthermore, Figure 2b shows the cell seen from the a-c crystallographic plane. In Figures 2a and 2b the octahedra tilting can be clearly observed.

Figure 2
Structure and octahedral distortions of the LaBiFe₂O₆ material in the Pnma space group.

The irregular shape of the octahedra can be analyzed from Figures 2c and 2d, where it can be determined that the distances between Fe³⁺ cations and O^2- anions adopt different values along the octahedral axes, as follows: 1.987 Å between Fe-O(1), and 1.985 Å and 2.044 Å between Fe-O(2). This octahedral symmetry justifies the difference between the Wyckoff positions of the O(1) and O(2) anions represented in Table 1.

The mean size of the crystalline domains was analyzed by the application of the Scherrer formula, which is given by

(1) ${\mathit{\tau }}_{\mathit{hkl}}=\frac{\varkappa \mathit{\lambda }}{\mathit{\beta }\cos \mathit{\theta }},$

where, τ_hkl is the mean size of the ordered crystalline domains (crystallite), which depends of the hkl direction; κ represents a dimensionless has a typical value close to 0.9, but depends of the crystallite shape; λ=1.54056 Å is the X-ray wavelength, β is the line broadening at half the maximum intensity and θ corresponds to the Bragg angle. The mean size of the crystallite obtained by the Scherrer equation was τ_hkl=44.2 ± 4.1 nm. The crystallite size is calculated by the equation 1 as showed in Figure 3.

Figure 3
Mean crystallite size calculated by the Scherrer equation.

Another way to calculate the average crystallite size is through the equation applied by the GSAS code during the process of Rietveld refinement of XRD experimental data. This method is carried out by considering the Debye-Scherrer cone in the scattering angle 2θ. The applied expression is

(2) $\mathit{\tau }=\frac{18000\varkappa \mathit{\lambda }}{\mathit{\pi }{\mathit{X}}^{\prime }},$

where, κ≈0.9 is the Scherrer constant, λ=1.54056 Å is the X-ray wavelength and X'= X+P_tec is the Lorentzian coefficient of particle size and anisotropy factor for the parallel crystallite size and X for the perpendicular crystallite size. The average crystallite size given by the GSAS refinement was τ=52.2±4.2 nm, which is close to that obtained through the direct application of the Scherrer formula.

The granular surface topology of the samples is exemplified in the micrographs of Figure 4. Perfectly defined polyhedral grains of nanometric dimensions are observed in Figure 4. It is not clear necking between the grains as a result of a possible diffusion, which is explained by the high sintering temperature (1023 K) of the material after the synthesis process by the modified Pechini method.

Figure 4
Granular surface structure of sample in the powder form obtained from SEM images with magnifications (a) 15000X, (b) 32500X and (c) 65000X.

The mean grain size was determined by the Average Grain Intercept (AGI) method, by drawing a set of randomly Positioned line segments on the SEM image, counting the number of times each line segment intersects a grain boundary, and finding the Intercepts ratio of length to line. Thus, the average grain size was estimated at 373 ± 42 nm (Figure 5), which is a much smaller size than the reported value for samples produced by solid state reaction¹⁰.

Figure 5
Determination of the mean grain size by the AGI method from the image of figure 4.

The semiquantitave stoichiometry of samples was analyzed the EDS microprobe coupled to SEM equipment. Figure 6 exemplifies the EDS spectra from electron beams incident on the different surface grains of the samples. From the results it was established that the samples contain the stoichiometry expected from the nominal formula up in 98 %.

Figure 6
EDS spectrum observed for the LaBiFe₂O₆ double perovskite.

The magnetic susceptibility response as a function of temperature on the application of intensity fields H=500 Oe, 2000 Oe and 10000 Oe is presented in Figure 7. For each value of intensity of applied magnetic field H, the sample was first cooled in absence of field. Then H was applied and susceptibility was measured while the temperature was increased in the procedure known as ZFC recipe. In the FC prescription, the sample was cooled and measured under application of H. A strong irreversibility was observed for all values of applied field, which is an indicative of the occurrence of some kind of disorder that facilitates the freezing of the domains of magnetic spins in the ZFC procedure.

Figure 7
Magnetic susceptibility as a function of temperature for the LaBiFe₂O₆ material on the application of H=500 Oe, 2000 Oe and 10000 Oe. The inset shows the temperature derivative of susceptibility for H=500 Oe.

One possible cause of this behavior of magnetically frustrated system is the cationic disorder of La³⁺ and Bi³⁺ atoms at the A and A' sites of the double perovskite. Meanwhile, this possibility is ruled out because the Rietveld refinement permitted to identify the reflection peaks (111) and (311), which constitute an evidence of formation of a FCC type structure that can only take place when the La³⁺ and Bi³⁺ cations are alternately arranged along the A and A' sites of the structure.

The presence of sub-micrometric grain-size can introduce border and surface effects, as well as the possibility of formation of independent magnetic domains which can to give rise relevant contribution to the frustrated nature of the material. For all applied magnetic fields a strong ferromagnetic behavior is observed in the magnetic susceptibility in all temperature regimes studied (50 K<T<325 K), so it is expected that the Curie temperature occurs for values of temperature well above room temperature. An interesting feature involves the no occurrence of anomalies or magnetic pseudo-transitions in the susceptibility for temperature values around T=258 K, as reported for solid reacted samples¹⁰.

In the picture of Figure 8, a clear ferromagnetic behaviour of magnetization measurements as a function of applied field at T=50 K, 200 K and 300 K is presented. It is observed that the lower remnant magnetization and coercive field occur at T=50 K, at T=200 K remnant magnetization and coercive field are higher and then these values decrease considerably for T=300 K, as showed in Figure 8.

Figure 8
Magnetization as a function of applied field measured at the temperatures T=50 K, 200 K and 300 K.

In none of the magnetic hysteresis curves shown in Figure 7 the saturation magnetization (M_S) is reached, but it is possible to infer that most M_S could take place for T=50 K and lower for T=300 K. At temperatures close to room temperature, the behavior of the hysteresis curve (under application of high magnetic fields) reveals a linear trend with no evidence of saturation. Due the nanogranularity of the ferromagnetic material, surface effects gain importance, so that the anisotropy and the magnetic monodomains induced in the surface are more relevant than the exchange mechanisms, resulting in a volumetric ferromagnetic response mixed with superparamagnetic effects, which would justify the linear trend of the hysteresis curve in the regime of high magnetic field strengths for T=300 K. Likewise, this would explain decreasing in values of coercivity field and remnant magnetization when temperature is increased from 200 K to 300 K.

From the curves of susceptibility as a function of temperature (Figure 7), it is expected that at lower temperatures a greater number of magnetic moments align in the direction of the applied field. In Figure 8 it can be observed that indeed the magnetization is greater at low temperatures. Furthermore, in the inset of Figure 7, the curve of temperature derivative of susceptibility shows that there is a change of concavity at T = 165 K. This anomaly may be related with nanoscopic mechanisms that modify the ferromagnetic domains, increasing partially the values of the magnetic parameters (coercive field and remnant magnetization). At higher temperatures, surface effects that give rise to the superparamagnetic nature inhibit the ferromagnetic order, decreasing values of the magnetic parameters.

In order to elucidate the nature of eventual local induced magnetic effects and the vibrational state of Fe³⁺ cations behavior in the structure, measurements of Mössbauer spectroscopy were carried out with the application of a Lorentzian model as showed in Figure 9. The reduced uncertainties calculated from the covariance matrix gives χ² = 0.565.

Figure 9
Mössbauer spectrum for the LaBiFe₂O₆ perovskite and applied fitting model.

The applied model showed the occurrence of a singlet related with a relevant fraction (14%) of superparamagnetic response, which is attributed to the nanometric character of grains observed in Figure 4 and analyzed in Figure 5 ¹⁸. On the other hand, a sextet was clearly identified, related to the Fe³⁺ in the octahedral coordination. The hyperfine parameters from the fits are reported in Table 2.

Table 2
Hyperfine parameters of LaBiFe₂O₆ derived from the fit of room temperature Mössbauer spectra.

δ (mm/s)	ε (mm/s)	Γ (mm/s)	Hf(T)	χ2
		Sextet
0.25±0.01		0.34±0.01
		Singlet
0.38±0.01	-0,06±0,01	0.18±0.01	50.9±0.1	0.5648

From Table 2, it is noted that, the high magnetic field (~51 T) imply that this kind of perovskite has a magnetic behavior at room temperature, which are due to the magnetic interactions of the two different types of magnetic ions: Fe³⁺ and (La/Bi)³⁺, and the competition between Fe-Fe, (La/Bi)-Fe and (La/Bi)-(La/Bi) interactions in different magnetic networks. This means that the interactions between the Fe cations mediated by the oxygen anions are directly affected by the bond lengths and bond angles, which in turn affect the magnitude of the hyperfine field¹⁹. The above results are in good agreement with those observed in the structural and magnetic analysis.

The gap energy was experimentally estimated by the Diffuse Reflectance technique. Results are shown in Figure 10. Three peaks are observed in Figure 10a for wave lengths λ₁=332 nm, λ₂=604 nm and λ₃=799 nm. These peaks correspond to energies E₁=3.73 eV, E₂=2.05 eV and E₃=1.55 eV due to electronic excitations caused by electronic transitions from the valence band to the conduction bands.

Figure 10
Diffuse reflectance measurements in the LaBiFe₂O₆ samples: a) % reflectance as a function of wave length and b) Kubelka-Munk analysis.

In the spectrum for the LaBiFe₂O₆ complex perovskite, these energies are related to the symmetry of the atomic positions in the space group Pnma (#62) by the transitions O_2p-La_5d, O_2p-Fe_3d and O_2p-Bi_6p²⁰ on the irreducible representation of vibrations given by

(3) $\Gamma =5\mathit{Roman}\mathit{Vibration}\mathit{Modes}+7{\mathit{B}}_{1\mathit{u}}+9{\mathit{B}}_{2\mathit{u}}+9{\mathit{B}}_{3\mathit{u}},$

where, the first term correspond to five vibration modes which are observable by Raman spectroscopy and the 7B_1u, 9B_2u and 9B_3u modes appear as three peaks measured through UV-Vis-IR spectroscopy.

For the determination of the energy gap the method of Kubelka-Munk analysis is applied in the way as implemented by V. Kumar et al.²¹.

The absorption coefficient is given by

(4) ${\mathit{\alpha }}_{\mathit{b}}=\frac{\mathit{B}{\left(\mathit{h}\mathit{\nu }-{\mathit{E}}_{\mathit{g}}\right)}^{\mathit{n}}}{\mathit{h}\mathit{\nu }},$

where, hν is the absorbed energy, B is the absorption constant, E_g represents the gap energy and n=1/2 or n=2 if there are direct or indirect semiconductor transitions, respectively. The energy gap is equivalent to the absorbed energy when α_R = ln R, where R is the measured reflectivity relative to the unity. By representing the ${\left[\ln \frac{{\mathit{R}}_{\max }-{\mathit{R}}_{\min }}{\mathit{R}-{\mathit{R}}_{\min }}\right]}^2$ vs hν, it is possible to determine the gap energy as showed in Figure 10b. The results of diffuse reflectance are related to the chemical structure of the material. In the case of Fe³⁺, the valence and conduction bands present a splitting of the 3d-Fe levels in the crystal field as a result of the octahedral coordination of the FeO₆ . From the analysis of the Diffuse Reflectance spectrum, the obtained energy gap was 2.13 eV, which corresponds to semiconductor behaviour. The difference of this value as compared with reports for LaFeO₃ (2.67 eV)²² and BiFeO₃ (2.81 eV)²³ is partially caused by the occurrence of three different Fe_3d-O_2p bonds into the FeO₆ octahedra: 1.987 Å for Fe-O(1), and 1.985 Å and 2.044 Å for Fe-O(2). Likewise, the interatomic distances of the La-O and Bi-O bonds are not symmetrical because of the occurrence of octahedral distortions.

4. Conclusions

The LaBiFe₂O₆ double perovskite has been synthesized by the modified Pechini method. Results of the crystalline characterization through Rietveld analysis of XRD patterns revealed that this material belong to the perovskite family and crystallizes in an orthorhombic structure, Pnma (#62) space group. Despite containing two types of cation (La³⁺ and Bi³⁺) on sites A and A' of the structure, the proximity between values of their ionic radii and their little magnetic character facilitates this material adopts a structure typical of simple perovskites. The crystallite size studied from the analysis of experimental data of XRD and the average grain size analyzed by SEM images revealed the nanometric nature of the particles forming this double perovskite. EDX spectrum showed that the compound has only the elements which were initially established through its nominal stoichiometry. Magnetic susceptibility measurements as a function of the temperature allowed observing the occurrence of magnetic irreversibility between the ZFC and FC procedures, which are attributed to the crystalline anisotropy related to octahedral distortions that are evident in the material structure. This was determined by the results of magnetization as a function of applied magnetic field that the material behaves like a ferromagnetic at room temperature, with a marked tendency superparamagnetic, which linearizes the saturation regime of the hysteresis curve. This interpretation is corroborated by the results of Mössbauer spectroscopy, which reveals a significant fraction of superparamagnetism mixed with the ferromagnetic response. The superparamagnetic feature is attributed to the nanometric nature of the granular morphology. Finally, the diffuse reflectance measurements suggest that this material behaves as a semiconductor energy gap of 2.17 eV. In general, the results obtained for samples produced by the modified Pechini method differ from those solid reacted in its magnetic properties related to the average grain size, because this technique allows the obtaining of grains with sizes of nanometric order, with ferromagnetic and superparamagnetic characteristics, which may have applications in biomedicine²⁴, as sensor in microfluids to facilitate medical diagnoses²⁵ and for water purification^26,27, between others.

5. Acknowledgments

This work was partially financed by DIB (National University of Colombia - Bogotá) and by CODI (Universidad de Antioquia - Medellin, Colombia) through Solid Sate Group-GES Sustainability Strategy 2016-2017.

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Publication Dates

• Publication in this collection
  24 July 2017
• Date of issue
  Sep-Oct 2017

History

• Received
  16 Mar 2017
• Reviewed
  22 Mar 2017
• Accepted
  03 July 2017