The influence of Ca substitution on LaFeO 3 nanoparticles in terms of structural and magnetic properties

Abstract

Background:

The nanocrystalline structure of La_1-xCa _x FeO₃ was prepared by a sol–gel method involving an auto-combustion process. The incorporation of rare-earths in LaFeO₃ induces strain in magnetic properties, especially in terms of the following parameters: replacement amount, oxygen partial pressure, and calcination temperature.

Methods:

To determine the effects of the amount of Ca²⁺ ion doping agent and the calcination temperature on the microstructure, particle morphology, and magnetic properties of LaFeO₃ crystal, we performed the following respective analytical methods: X-ray powder diffraction, Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy, and vibrating sample magnetometer tests.

Results:

The orthorhombic structure of LaFeO₃ perovskite did not change even when it was doped with Ca²⁺ ions, and its space group continued to be Pnma (No.62). FT-IR spectra confirmed that the main band appearing at 568 cm⁻¹ is due to the antisymmetric stretching vibration of Fe–O–Fe bonds in FeO₆. The introduction of Ca inhibits the growth of grains but the morphology of particles is improved. With an increasing concentration of Ca²⁺ ions, magnetic behavior of the samples also witnessed an increasing trend in a proportionate manner. With an increase in calcination temperature, the enclosed area of the magnetic hysteresis curve of the sample reduced remarkably.

Conclusions:

The growth of nanoparticles can be restrained with an increase of Ca content that is used as doping agent. The magnetic behavior of La_1-xCa _x FeO₃ tilts towards G-type antiferromagnetism; the magnetic orientation is achieved from the super exchange interaction of Fe³⁺ ions with oxygen ions.

Introduction

Perovskite oxide is one of the new functional materials which have great potential in the future. ¹ LaFeO₃ is a type of metal oxide with the perovskite structure (ABO₃). When single phase LaFeO₃ and single- or double-doped A/B chemically interact with each other during sol– gel preparation, the resultant crystal structure usually has an orthogonal perovskite structure, with the crystal’s symmetry being the space group Pnma. In this case, the lattice parameters change significantly because different elements are used as dopants during crystal preparation: these variations in lattice parameters have a direct impact on the local environment of Fe ions.^2,3 When LaFeO₃ is modified with A or B site elements that acts as substituents, the general formula of the resultant chemical compounds are La_1-xA _x FeO₃ (A = Ca, Sr, Ba, Ce or other rare earth elements), LaFe_1-xB _x O₃ (B = Mn, Co, Cr, or other transition metals elements), respectively. Because different A- or B-site elements replace LaFeO₃ material, the performance characteristics of the resultant compounds also change, especially in terms of the following parameters: replacement amount, oxygen partial pressure, and calcination temperature. Pecchi et al. conducted a study on the catalytic activity of LaFeO₃ doped with Ca. ² They used the citrate method and co-precipitation method to determine the catalytic activity of the aforementioned LaFeO₃ material. Furthermore, Kong and Shen used the co-precipitation method to determine gas sensitivity of La_1-xCa _x FeO₃. ³ In addition, Ahmed and El-Dek used the conventional solid phase reaction method to determine the magnetic properties of Ca-doped La_1-xCa _x FeO₃ material. ⁴ When the Ca²⁺ concentration was increased steadily in the crystal, the number of Fe⁴⁺ ions increased steadily but the number of Fe³⁺ ions decreased simultaneously. Owing to these changes in the number of Fe³⁺/Fe⁴⁺ ions in the environment, the magnetic susceptibility of the Ca-doped La_1-xCa _x FeO₃ crystal increased markedly. ¹ Abdel-Khalek and Mohamed synthesized La_1-xCa _x FeO₃ by the method of co-precipitation. ⁵ Then they studied its structure and magnetic properties extensively. Based on the results of magnetic measurements, they inferred that the sample has weak ferromagnetic properties and spin (antiferromagnetic) ordering. When the concentration of Ca²⁺ dopants (x ≤ 0.1) is increased continuously in the crystals, there is a corresponding increase in the magnetic parameters of the crystals. The magnetic behavior of crystals may become more pronounced because Fe³⁺ ions are oxidized to Fe⁴⁺ ions, with a steady increase in Ca²⁺ dopants in the crystal environment. ¹ The traditional preparation of LaFeO₃ is done by performing the solid state reaction method under a high-temperature environment (ca. 700°C) to ensure that the reaction reaches completion. The result of the reaction was different when the sintering temperature was higher.^6,7 There are also reports describing how wet chemical methods, such as co-precipitation, and other methods were used for preparing LaFeO₃; however, the sintering temperatures are too high in these methods.^8,9 In different preparation methods, raw materials and the sintering temperature play an important role in the ferroelectric, ferromagnetic, morphology, structure, and other properties of the synthesized samples. ⁷ Presently, few studies have reported about how low-temperature synthesis of Ca²⁺-doped LaFeO₃ nanoparticles can be performed using the citrate sol–gel method. Moreover, the structure, morphology, and magnetic properties of Ca²⁺-doped LaFeO₃ nanoparticles have been studied to a significantly less extent in research studies. In this paper, we studied the effect of Ca²⁺ substitution La_1-xCa _x FeO₃ ferrite and investigate the influence of Ca²⁺ substitution in magnetic properties and microstructure.

Material and methods

Sample synthesis

In this experiment, the citrate sol–gel method of synthesis was used to prepare La_1-xCa _x FeO₃ (x = 0–0.5) perovskite nanoparticles. ¹ The resultant nanoparticles were prepared using different proportions of Ca²⁺ as dopants in the aforementioned nanoparticles. In addition, the effect of varying calcination temperature of the reaction was quantified by determining the structure, morphology, and magnetic properties of as-synthesized samples in the reaction mixture. The starting composition of the principal sample was as follows: La(NO₃)₃·6H₂O, Fe(NO₃)₃·9H₂O, Ca(NO₃)₂·4H₂O, and citric acid. During the synthesis process, citric acid was dissolved in deionized water. To obtain a solution, it was necessary to completely dissolve citric acid in water. Then, a variety of nitrates were dissolved completely in citric acid (solution A) to yield solution B. Thereafter, ammonia was slowly added to solution B to adjust its pH to 7. At this stage, the ammoniacal solution B was transformed into a sol. Then, the resultant sol was placed in a water bath pot and subjected to stirring continuously; the temperature of the water bath was maintained at 60°C. After being kept for about 3 h in the water bath, the sol transforms into a wet gel. At this stage, the stirring of the gel was stopped. The resultant wet gel was aged for about 12 h. Then, it was placed in a blast oven and dried at 130°C for 6 h to yield a xerogel; the accelerant ethanol was added to the dry gel obtained from the spread, and a yellow fluffy powder was obtained subsequently. After polishing the powder, we transferred it into a crucible. Then, the powder was calcined in a muffle furnace, after being calcined at 500°C, 700°C, 800°C for 2 h.

Sample characterization

The integrated technique of thermogravimetric analysis and differential scanning calorimetry (TGA-DSC; SDT Q600, TA Instruments, USA) was performed to analyze the dry gel through thermal decomposition process. In this experiment, X-ray diffraction (XRD; D/max-2500v/pc, Rigaku, Japan) was used to further characterize the structure of the material: the main parameters of the XRD process were as follows: pressure pipe, 40 kV; pipe flow, 20 mA; Cu-K_α target radiation; scanning speed 10°/ min; scanning range 10°–80°(2θ). Furthermore, Fourier-transform infrared (FT-IR) spectrometer analysis (Spectrum Two; Perkin Elmer, USA) was performed to observe functional groups and chemical bonds of the samples, with a spectral resolution of 1 cm⁻¹. Then, IR spectroscopic analysis of the samples was performed in the wavelength range 400–4000 cm⁻¹. Scanning electron microscopy(SEM) analysis (NoVa^TM Nano 430; FEI, USA) was performed to observe the microstructural properties of LaFeO₃ nanomaterial, including particle size, geometrical shape, uniformity, and reunion phenomenon. Vibrating sample magnetometer (VSM) analysis (VSM-100; CCYPCD, China) was performed to observe the magnetic properties. We tested hysteresis loop of samples at room temperature under the following conditions: the maximum magnetic field H = 0.8 T and the highest sensitivity was 5 × 10⁻⁵ emu. With this process, we determined the magnetic parameters of samples, including the saturation magnetization (M_s), remnant magnetization (M_r), and coercive force (H_c) values.

Results and discussion

TGA results

Figure 1 shows that the weight loss of the samples starts from 30°C in the TGA curve. In the differential thermal analysis (DTA) curve, weak endothermic peaks appeared at 47.5°C because of evaporation of water from the wet gel. Furthermore, endothermic peaks appeared at 131.5°C and 169.5°C owing to the expulsion of water that was present hitherto inside the dry gel. For example, adsorption water, water of crystallization, and water vapor generated during the reaction are some types of water molecules entrapped within the dry gel. A sharp exothermic peak appears at 226°C; this peak may correspond to that at about 219°C on the TGA curve, where it primarily represents extensive weight loss caused by the decomposition of organic matter and the combustion of residual organic substances generating CO₂ and N₂ gases. After attaining a temperature of 226°C, the sample does not undergo any significant weight loss. As a result, weak exothermic peaks appear at about 325°C and 420°C in the DTA curve. The temperature 420°C is crystallization temperature of LaFeO₃.^10–12

Figure 1.

TGA and DTA curves of La_0.9Ca_0.1FeO₃ xerogel.

XRD analysis

Figures 2 and 3 display the XRD patterns of La_1-xCa _x FeO₃ (x = 0–0.5). In the presence of the contrast standard JCPDS 37-1493, the orthorhombic structure of LaFeO₃ showed a perovskite structure, and its space group was Pnma.^2,13,14 With an increase in the proportion of Ca substitution in the crystal lattice, the diffraction peaks shifted to 2θ values in a remarkably increasing direction. The shift in diffraction peaks occurred because the ionic radius (0.134 nm) of the Ca²⁺ ion was smaller than the ionic radius (0.136 nm) of the La³⁺ ion. The Ca doping consequently decreased the cell volume of La_1-xCa _x FeO₃, although to slight extent.^15–17 In addition, La³⁺ ion was replaced partially by the lower state Ca²⁺ ion so as to maintain charge neutrality of the entire compound. Subsequently, the perovskite structure either produced an Fe⁴⁺ ion, which is a B-site element having higher valence state, or an oxygen hole.^4,18–20 Because the Fe⁴⁺ ion radius (0.0585 nm) is less than the Fe³⁺ ion radius (0.0645 nm), the cell became still smaller in size. When x = 0.2, the diffraction peaks (141) and (311) appear as a merged peak (141). This phenomenon indicates that the structure of the sample will be distorted when the level of Ca dopants reaches a certain concentration in the reaction mixture. At x = 0.5, the diffraction peak decreased. This indicates that there may be an impurity phase of CaCO₃ in the temperature range 25–35°C.

Figure 2.

XRD diffraction pattern of La_1-xCa _x FeO₃ (x = 0–0.5) without calcination.

Figure 3.

XRD diffraction pattern of La_1-xCa _x FeO₃(x = 0–0.5) through 2 h of calcination at 500°C.

Figure 4 shows the XRD pattern of La_0.6Ca_0.4FeO₃ samples obtained after subjecting them to calcination at different temperatures. It can be seen that all the samples have a single-phase structure, and the shape of the diffraction peak is sharp and well-defined. This indicates that even if calcination temperature is increased steadily, the crystallinity of the crystal continues to be intact. In addition, an impurity phase of CaCO₃ appears in both non-calcined samples and samples calcined at 500°C for 2 h. The appearance of the impurity phase despite calcination indicates that the sample was probably not fully calcined at that temperature.

Figure 4.

XRD diffraction pattern of La_0.6Ca_0.4FeO₃ samples without calcination and with calcination at 500°C or 700°C after 2 h.

Figure 5 illustrates the trends in the average grain size corresponding with the variation in the amount of Ca dopants. The variation in the average grain size was calculated using Jader 5.0 software. Scherrer’s formula was used to do the estimation as follows:

$$D=\frac{K\cdot \gamma }{\beta \cdot \cos \theta }$$

where D is the average grain size, β is full width at half maximum (FWHM), and γ is the wavelength of copper target (0.15405 nm). The Bragg’s diffraction angle is denoted by θ. Based on Figure 5, we infer the following observation: The average grain size of the sample reduces with an increase in the amount of Ca²⁺ dopants. This indicates that that Ca²⁺ ion replaces La³⁺ ion in the crystal lattice and inhibits the growth of particles.

Figure 5.

The average grain size of La_1-xCa _x FeO₃ (x = 0–0.5) sample calcined at 500°C for 2 h.

Infrared analysis

Figure 6 shows the FT-IR spectra of a La_1-xCa _x FeO₃ sample that was calcined at 500°C for 2 h. Using an FT-IR spectrometer with pure KBr as the background, the main band appeared at about 568 cm⁻¹; this band represents the antisymmetric stretching vibration in the Fe–O–Fe bond of FeO₆, a regular octahedron. The main band represents the vibrational stretching of Fe–O–Fe bond in the absorption spectra of samples. According to Abdel-Khalek and Mohamed and Li et al.,^5,21 the aforementioned main band appeared at around 574 cm⁻¹ and 400 cm⁻¹, respectively, in samples of La_1-xCa _x FeO₃. Furthermore, Thirumalairajan et al. found that a small vibration peak appears at about 1058 cm⁻¹; ²² this peak was found to be the main vibration band of the CO₃²⁻ functional group. The appearance of this peak in the absorption spectra indicates that the samples might have been exposed to air, and that La carbonate might be present on the surface of LaFeO₃.

Figure 6.

Infrared spectrum of La_0.9Ca_0.1FeO₃ calcined at 500°C for 2 h.

However, the XRD spectra did not detect this chemical species in the sample. Furthermore, a small vibration speak appears at about 1382 cm⁻¹, which indicates the asymmetric stretching vibration of the metal. The appearance of this band at 1382 cm⁻¹ wavelength is in good agreement with the spectra presented in other studies. ²³ At around 1627 cm⁻¹, a vibrational band appears in the spectrum; this band is attributed to the asymmetric stretching vibration of carboxyl group. In addition, Thirumalairajan et al. reported the appearance of a vibrational band at about 1627 cm⁻¹. ²⁴ A very small absorption band appeared at around 2929 cm⁻¹, which was then attributed to CO₂ species in our sample compound. Moreover, Thirumalairajan et al. reported the appearance of a vibrational band at about 2924 cm⁻¹. ²² At about 3428 cm⁻¹, a broad peak appears due to the symmetric and asymmetric stretching vibration of water molecules absorbed from air; these latent water molecules are present in our sample compound. Furthermore, this broad band extends between 3300 cm⁻¹ and 3600 cm⁻¹, manifesting the correlation between the vibrational levels of various kinds of latent water molecules in the sample.^21,25,26

Structures and grain sizes

Figure 7 shows that the dispersion and morphology have not changed much despite the sample being doped with Ca²⁺ ions. With an increase in the concentration of Ca substitution, the particle size becomes smaller because the Ca²⁺ ion replaces the node position of La³⁺ ion. Consequently, the stacking-fault energy increases in the material, thereby increasing the nucleation node and inhibiting the growth of particles. In summary, the doping impact of Ca²⁺ is change on the performance of the sample. ¹⁵

Figure 7.

SEM images of La_1-xCa _x FeO₃ (x = 0, 0.2, 0.4) calcined at 700°C.

The statistical data was obtained using particle size distribution analysis software (Mac-View ver.4.0; Mountech Co. Ltd, Tokyo, Japan). From Figure 8, which displays the particle size histogram of La_1-xCa _x FeO₃ (x = 0, 0.2, 0.4), we conclude that the particle size of undoped samples was mainly in the range 28–68.8 nm. However, when the content of Ca dopant was 0.2, the particle size was in the range 25–56.2 nm. Furthermore, when the calcium dopant’s content was 0.4, the particle size was in the range 20–53 nm. These results indicate that the particle size decreases with an increase in the amount of Ca²⁺ dopants. The trend of the average grain size is in good agreement with previous studies, as confirmed by statistical data obtained with Jader 5.0 software and distribution analysis software.^27,28

Figure 8.

Particle size histogram of La_1-xCa _x FeO₃ (x = 0, 0.2, 0.4) calcined at 700°C.

Magnetic property of particles

As shown in Figure 9, the hysteresis loop of La_1-xCa _x FeO₃ (x = 0–0.5) samples is not the same as that representing a sample subjected to calcination. In this case, a La_1-xCa _x FeO₃ sample exhibits a tilting toward G-type antiferromagnetism. The magnetic hysteresis (M–H) of La_1-xCa _x FeO₃ compound originated from Fe³⁺ ion tilting toward antiparallel spin; this kind of change in magnetic orientation is achieved by super exchange interaction of Fe³⁺ ions with oxygen ions. ¹ The shape of the hysteresis loop exhibits a weak ferromagnetic behavior, which does not reach the state of saturation. In these conditions, magnetization increases with an increase in the amount of Ca dopants, and the doped crystals exhibit important features of spin antiferromagnetic ordering. In summary, the magnetization level increases in crystals due to a gradual increase in the strength of ferromagnetic interactions and a subsequent decrease in antiferromagnetic interactions in the sample.^4,16–18

Figure 9.

Hysteresis loop of the La_1-xCa _x FeO₃ (x = 0–0.5) samples without calcination.

Figure 10 illustrates the hysteresis loop of La_1-xCa _x FeO₃ (x = 0–0.5) sample compounds subjected to calcination for 2 h at a temperature of 500°C. Because Ca²⁺ ions replace La³⁺ ions in the sample compound, there will be two possible scenarios: when the Ca²⁺ doping amount (x) reaches a certain value, La_1-xCa _x FeO₃ would form two or more phases; with an increase in the value of (x), Fe³⁺ to Fe⁴⁺ transition would progress certainly leading to two phases. ⁴ According to the Figure 10, magnetization had has an increasing trend with an increase in Ca²⁺ concentration (x) as doping agent. Thus, magnetic mutation occurs when x = 0.3. The phenomenon is attributed to the generated Fe₃O₄ impurities, ¹⁶ and it leads to an enhancement in the magnetic behavior of the sample.

Figure 10.

Hysteresis loop of La_1-xCa _x FeO₃ (x = 0–0.5) calcined at 500°C.

Figure 11 shows the hysteresis loop of La_0.8Ca_0.2FeO₃ samples subjected to calcination at different temperatures. Figure 11 clearly suggests that the enclosed area of the M–H curve of the sample area is reduced with an increase in calcination temperature. When the temperature is increased from room temperature to 500°C, the round solid symbol representing the area of the curve moves down but there is no obvious change in hysteresis loop At this stage, the magnetic area of the curve does not exhibit a significant change with changes in temperature; however, saturation magnetization (M_s) of the sample does change with changes in temperature. When the temperature is increased from 500°C to 800°C, the enclosed area of the M–H curve of the sample exhibits mutation, because the spin ferromagnetic ordering weakens gradually and the sample now tilts towards antiferromagnetic ordering. This change in magnetic ordering of the sample has been attributed to the steady increase in sintering temperature. With an increase in the sintering temperature from 500 to 800°C, the iron oxide content decreases in the sample compound. As a result, the magnetic properties of the sample also change dramatically. ¹⁹ In addition, the M–H curve appears horizontal and it seems to have moved from its previous inclination toward vertical direction. This change in direction has been attributed to the ferromagnetic and antiferromagnetic coupling in the sample.

Figure 11.

Hysteresis loop of La_0.8Ca_0.2FeO₃ without calcinations and calcined conditions of 500°C or 800°C for 2 h.

Figure 12 illustrates the variation trend of magnetic parameters with different values of Ca²⁺ doping content (x). The residual magnetization (M_r) and saturation magnetization (M_s) show a tendency to increase with an increase in x values. The coercive force (H_c) exhibits an exactly opposite change trend: with an increase in the x value, there is attenuation in the value of H_c in the sample. Figure 12 also reflects the influence of calcination temperature on the magnetic parameters of the sample; the calcination temperature (T) has the same effect on variation trends of M_r, H_c, and M_s: the values of the aforementioned magnetic parameters decrease as T increases. The coercive force (H_c) of the sample is related to magnetic anisotropy,^13,27,29 indicating that the introduction of Ca²⁺ and high-temperature calcination probably weakens the magnetic anisotropy of the sample, and evidence for this is indicated in experiments using energy-dispersive X-ray spectroscopy or X-ray photoelectron spectroscopy.^24,30,31 As a result, there is a decrease in coercive force (H_c).^32,33

Figure 12.

The diagram depicting the variation trend of magnetic parameters with different values of Ca²⁺ doping content (x) and calcination temperatures.

Conclusion

In this study, we developed an innovative citrate sol–gel method that was easy to implement with precision control. ²⁹ With this novel method, we carried out the synthesis of La_1-xCa _x FeO₃ nanoparticles at a low temperature. Furthermore, the structure, morphology, and magnetic properties of the as-synthesized nanoparticles were also studied in this experiment. The XRD spectra illustrated that La_1-xCa _x FeO₃ nanoparticles have an orthorhombic perovskite structure. With an increase in the proportion of Ca substituents as dopants in the sample, the diffraction peaks of La_1-xCa _x FeO₃ crystals moved remarkably in an increasing direction, leading to a sharp change in Bragg’s angle of diffraction (2θ). SEM showed that the growth of nanoparticles can be restrained with an increase of Ca content that is used as doping agent. This is because the particle size decreases with an increase in the content of Ca²⁺ ions as dopants. Furthermore, La_1-xCa _x FeO₃ exhibits weak ferromagnetism. With an increasing concentration of Ca²⁺ ions, the magnetic behavior of the samples also witnessed an increasing trend in a proportionate manner. With an increase in calcination temperature, the enclosed area of the M–H curve of the sample reduced remarkably.