Tailoring Multiferroic Characteristics in LaFeO3 Nanocrystals via Rare-Earth Pr3+ Doping

Abstract

Multiferroic materials have sparked significant interest in the realm of materials science because of their potential impact on various device applications. This study focuses on the synthesis of nanocrystalline La_1−xPr_xFeO₃ (LPFO) materials, where x can be either 0 or 0.5, using a solid-state technique. The aim is to gain insights into their structural, optical, dielectric, and magnetic properties. To confirm the chemical phase of the synthesized materials, X-ray diffractometer and Raman spectroscopy were employed. The outcome of the Rietveld analysis reveals that the LPFO crystallites exhibit orthorhombic symmetry with a Pbnm space group. The functional groups that were present in the LPFO samples were identified using FT-IR spectroscopic analysis. The morphological studies using scanning electron microscope and transmission electron microscope indicate that the synthesized samples exhibit excellent homogeneity with uniformly distributed grains. In order to investigate the dielectric constant (εʹ) and dielectric loss (tan δ) were examined as functions of temperature and frequency. Pr³⁺ dopants had a notable impact on the dielectric characteristics, particularly within the frequency span of 10 kHz–1 MHz and over the temperature variation of 40–160°C. The ambient temperature magnetic properties of the LPFO ferrite materials displayed antiferromagnetic behavior. Ultimately, this research reveals insightful information on the structural, optical, dielectric, and magnetic properties of the synthesized nanocrystalline LPFO materials, shedding light on their potential applications in the multifunctional devices.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

The perovskite-type ferrite material with the general formula ABO₃, known for its stability, has garnered significant attention from researchers in the field of nanotechnology. Among the various nanosized ferrites, ABO₃-type rare-earth orthoferrites, such as LaFeO₃, stand out as particularly intriguing materials due to their diverse set of properties, including optical, electronic, catalytic, ferroelectric, and magnetic characteristics. These properties make LaFeO₃ highly versatile and suitable for applications in photocatalysts, biosensors, solid oxide fuel cells, photovoltaic cells, and magnetic data storage [1–3]. At room temperature, LaFeO₃ exhibits antiferromagnetic behavior and acts as an insulator with a Neel temperature of 740 K [4]. When scaled down to the nanoscale, LaFeO₃ demonstrates exceptional characteristics, including enhanced sensing properties, a larger surface area, and improved catalytic activity, thanks to the quantum size effect. In the literature, several synthesis methods have been reported for producing nanocrystalline LaFeO₃ materials, including the coprecipitation method [5], microemulsion method [6], electrospinning technique [7], hot soap method [8], hydrothermal method [9], polymer pyrolysis method [10], thermal decomposition method [11], and auto-combustion technique [12]. Each of these methods offers its own set of advantages, disadvantages, and limitations. However, the traditional high-temperature or the approach of solid-state reaction technique has gained significant attention due to its simplicity, high reactivity, and cost-effectiveness in the production of nanocrystalline materials. This method remains a preferred choice for synthesizing LaFeO₃ nanocrystals.

Currently, researchers have become more interested in examining the effects of rare-earth element substitutions in ferrite materials. Remarkably, the perovskite structure of LaFeO₃ demonstrates a remarkable tolerance for a wide range of dopant ions. This flexibility allows researchers to precisely tailor and control the properties of the final product based on the type and concentration of the dopant used. The physicochemical characteristics of LaFeO₃ can be finely tuned by selecting specific synthesis methods and carefully controlling the amount of dopant incorporated. Various dopants have been employed to occupy either the A or B sites within the LaFeO₃ structure, resulting in alterations in the material’s band gaps, morphology, cell characteristics, crystalline size, and grain diameters. These modifications can be harnessed for various applications. In recent studies, conventional solid-state techniques were utilized to synthesize compounds like Sm/Nd-doped LaFeO₃ (La_0.9(Sm/Nd)_0.1FeO₃), Co-doped LaFeO₃ (LaCo_xFe_1−xO₃, where x = 0.05, 0.1, 0.15, 0.2, 0.25), Ti-doped LaFeO₃ (LaTi_xFe_1−xO₃, where x = 0, 0.05, 0.15, 0.25), and Mn-doped LaFeO₃ (LaMn_xFe₁₋O₃, where x = 0.05, 0.1, 0.15, 0.2, 0.25) [13, 14]. These nanocrystalline compounds were successfully synthesized using oxide materials as starting precursors and showed promise in applications such as gas sensing, magnetic memory storage, high-frequency switching, and more. Additionally, compounds like La_0.9M_0.1FeO₃ (M = Ca and Ba) were explored as thermoelectric materials, and have potential uses in power generation, waste heat recovery, and refrigeration [15]. Moreover, recently Kumar et al. [16] have been synthesized using the sol–gel method, multiferroic BiFeO₃─NiO (BFO─NiO) nanocomposites have been created and studied. The stability and physical characteristics of the solid state-synthesized lead-free K, Na, and Nb-doped BiFeO₃ system have been demonstrated by Reygadas et al. [17]. Comparative evaluation of BiFeO₃-modified devices based on “La” for high-conversion efficiency perovskite solar cells have been developed by Raj et al. [18]. The structural, dielectric, transport, and ferroelectric properties of Ca-Modified Bi_0.80La_0.20FeO₃ (BCLFO) polycrystalline samples produced by the solid-state reaction are investigated by Kumar et al. [19]. Using the sol–gel combustion method, nanoparticles (NPs) of (La_1−xSr_x)(Fe_1−xNi_x)O₃, (x = 0.0, 0.1, and 0.2), with an orthorhombic perovskite structure, were successfully prepared by Saad et al. [20]. Redistributing dissolved Cu nanoparticles on LaFeO₃ photocatalyst for adjustable CO₂ reduction through photocatalysis has been demonstrated by Saad et al. [21]. Ni doping’s effect on the physical characteristics of La_0.7Sr_0.3FeO₃ produced using the reverse micelle technique reported by Hassan et al. [22].

Continuing this line of research on LaFeO₃ materials, the current study investigates the incorporation of Pr³⁺ as a dopant to replace rare-earth A-site (La) ions in LaFeO₃. The aim is to explore and understand the intrinsic properties of the resulting materials. As far as we are aware, there have not been any prior investigations into the effects of Pr³⁺ substitution in LaFeO₃ using conventional solid-state techniques. This research seeks to fill this gap in our understanding of these materials.

In this study, our primary objective is to document the synthesis of La_1−xPr_xFeO₃ (LPFO) materials, where x can be either 0 or 0.5, using the conventional solid-state technique. What sets this research apart is its focus on understanding the impact of substituting rare-earth Pr³⁺ ions into the nanocrystalline LaFeO₃ material. Specifically, we aim to investigate how this substitution has an impact on the structural, morphological, dielectric, and magnetic characteristics of nanoscale LaFeO₃. To achieve this goal, we have undertaken a comprehensive characterization of the synthesized nanocrystalline LaFeO₃ materials. Our analytical techniques include X-ray diffractometer (XRD), FT-IR (Fourier Transform-Infrared spectroscopy), the Raman spectroscopy method, scanning electron microscope (SEM) coupled to energy dispersive X-ray spectrometry (EDS), transmission electron microscope (TEM), measurements of the dielectric constant and loss, and magnetic property analysis using a vibrating sample magnetometer (VSM). Through these investigations, we seek to provide insights into how the presence of Pr³⁺ ions influences the intrinsic properties of the nanocrystalline LPFO ferrite material.

2. Experimental Section

2.1. Synthesis Materials and Technique

Praseodymium oxide (Pr₆O₁₁), Lanthanum (III) oxide (La₂O₃), and Iron oxide (Fe₂O₃) precursors with purity of 99.9%, sourced from Sigma–Aldrich, served as the initial materials. The nanocrystalline compounds, denoted as La_1−xPr_xFeO₃ (LPFO) with x values of 0 and 0.5, were fabricated using the traditional high-temperature solid-state reaction technique. To achieve a uniform fine powder, the chosen initial materials were carefully weighed in the required stoichiometric ratio and powdered for ∼1.5 hr using an agate mortar.

Subsequently, the powders underwent calcination at 900°C for a duration of 12 hr, followed by thorough grinding. Following that, they were then placed into an alumina container. In addition, the processing involved sintering the samples at 1100°C for 6 hr, with intermittent crushing steps. The resultant synthesized ferrite powders allowed to achieve homogeneity, the mixture should progressively come down to room temperature at a pace of 5°C/min. At Final, the prepared LPFO powder samples were subjected to a final sintering step at 1100°C for 6 hr to facilitate phase formation. After this, they were well-grounded and ready for various analytical measurements. To facilitate dielectric characteristics, using a hydraulic press, the prepared ferrite powders were pelletized at a pressure of ∼2.5–4.5 tonnes. The schematic diagram depicting the synthesized LPFO ferrite materials schematic diagram is provided in Figure 1.

[figure(s) omitted; refer to PDF]

2.2. Characterization Techniques

The phase and construction of structure composition of the nanocrystalline materials synthesized were investigated using a Bruker D2 phaser XRD, utilizing Cu-Kα (1.5418 Å) radiation. XRD patterns in the 2θ range covering 20–60° were captured. Phase identifications were matched to the PDF-4 database maintained by the International Centre for Diffraction Data (ICDD). Further structural details, including bond angles and bond lengths, were determined through Rietveld refinement using the FULLPROF software package of the recorded XRD profiles. For the additional characterizations, using a Bruker Alpha FTIR Spectrophotometer, FT-IR spectra were acquired in the wavelength spanning 500–1750 cm⁻¹. With the use of a He-Ne laser and a Horiba Jobin-Yvon HR-800 UV Spectrophotometer, room-temperature Raman spectra were obtained that ranged from 100 to 1000 cm⁻¹. A JEOL-ZEM SEM with an energy-dispersive spectrometer (EDX) was used to analyze the surface morphology, elemental distribution mapping, and quantitative assessments of the generated samples. Furthermore, dielectric measurements were performed to assess the temperature and frequency-dependent dielectric properties. These characteristics were measured using a Wayne Kerr—LCR 4275 in the frequency ranges from 10 Hz to 1 MHz. In last, magnetic properties were investigated using a Lakeshore (MODEL-7407) VSM at ambient temperature.

3. Results and Discussion

3.1. Structural Analysis—X-Ray Diffractometer (XRD) Studies

The phase formation of the nanocrystalline LPFO materials that were synthesized was examined using X-ray diffraction. In Figure 2, you can see the XRD pattern obtained through Rietveld refinement of these materials, which were synthesized through the traditional high-temperature solid-state reaction method. This pattern reveals that the diffraction peaks corresponding to the (101), (121), (220), (202), (311), (240), (242), and (204) planes closely matched the ICDD No. 37-1493 database, indicating that the causes of these peaks are the distorted orthorhombic structure of LaFeO₃ with the space group of (Pbnm). The refined crystallographic parameters are provided in Table 1. After substituting Pr³⁺ for La in LaFeO₃, small discrepancies in bond distances and lattice constraints were measured. The lattice expansion or contraction is to blame for this disparity, which occurs owing to differences in the ionic radii of the host and substituent atoms within the crystal structure. Importantly, there were no additional or impurity peaks were detected in the XRD pattern. This suggests the high purity of the nanocrystalline LPFO materials synthesized materials. The broadening of the diffraction peaks in the XRD pattern indicates that the crystallites within the synthesized materials are relatively small in size. Scherrer’s formula was used for estimating the mean size of the crystallite (indicated as “d”) of the nanocrystalline LPFO materials. The result is demonstrated by the equation [23] as follows:

[figure(s) omitted; refer to PDF]

Table 1

The refined structural parameters of the LaFeO₃ and La_0.5Pr_0.5FeO₃ ferrite materials.

Samples/parameters	LaFeO3	La0.5Pr0.5FeO3
a (Å)	5.559 (5)	5.564 (1)
b (Å)	5.569 (4)	5.566 (4)
c (Å)	7.860 (4)	7.859 (7)
Volume (Å3)	242.75 (11)	312.42 (42)
Rwp (%)	3.8	3.6
Rp (%)	2.7	3.2
χ2	2.0	3.4
α (°)	90	90
β (°)	90	90
γ (°)	90	90
Crystallite size in (nm)	88	65
Fe–O bond length in (Å)	2.00	2.12
Minimum Fe–O bond length in (Å)	2.37	2.24
Maximum Fe–O bond length in (Å)	3.27	3.16

$$\begin{array}{}\text{(1)}& d=0.9\lambda /\beta \text{cos}\theta ,\end{array}$$where d-is the mean crystallite size, θ-is the angle of diffraction, λ-is the wavelength of the X-ray beam (0.154 nm), and β-is the full-width at half maximum (FWHM). It is perceived that the mean crystallite size (d) of the produced nanocrystalline LPFO materials was about 88 and 65 nm. The diminution in the mean crystallite size can be attributed to the impediment of diffusivity resulting from the substitution of rare-earth element Pr³⁺ into lanthanum ferrite. The dislocation density (represented as “δ”), which quantifies the length of dislocation lines within a crystal per cubic inch, provides a measure of the number of imperfections present in the crystal structure. Utilizing, the formula dislocation density (δ) was calculated [24] as follows:$$\begin{array}{}\text{(2)}& \delta =1/d^2,\end{array}$$where d-is the mean crystallite size. The computed dislocation density values are 12.92 × 10¹⁵ lines/m² for the synthesized nanocrystalline LaFeO₃ and 23.66 × 10¹⁵ lines/m² for La_0.5Pr_0.5FeO₃ ferrite materials. This increase in δ clearly indicates the successful substitution of Pr³⁺ into the LaFeO₃ ferrite materials.

3.2. Fourier Transform-Infrared Spectroscopic (FT-IR) Studies

In Figure 3, the FT-IR spectra of the synthesized nanocrystalline LPFO materials within the wavelength range of 500–4000 cm⁻¹ are depicted. Several notable features can be observed in these spectra. First, there is a prominent, intense band near 3000 cm⁻¹, which pertains to the stretching vibrational mode of O─H groups. This indicates the presence of hydroxyl (O─H) groups on the material’s surface. Moreover, pristine LFO spectra indicate that the characteristic peaks are present at 1061, 1392, 1591, 2348, 2979, and 3784 cm⁻¹. Additionally, there is another band in the region of 1000–1600 cm⁻¹, which is attributed to the C─H bending vibrations. This indicates the presence of carbon–hydrogen bonds, likely originating from contaminants or impurities.

[figure(s) omitted; refer to PDF]

In the case of perovskite-type LaFeO₃, it adopts an orthorhombic structure wherein the crystal lattice’s tetrahedral (A) and octahedral (B) positions are occupied by La³⁺ ions and Fe³⁺ ions, respectively. Typically, higher frequency bands in the region of 550–600 cm⁻¹ and lower frequency bands in the region of 400–450 cm⁻¹ are indicative of FeO and La─O bonds [25]. In this context, the FT-IR spectra reveal bands at 546 and 559 cm⁻¹, which can be attributed to the intrinsic vibrations of the transition metal (TM) Fe─O complexes in synthesized both LaFeO₃ and La_0.5Pr_0.5FeO₃ materials. However, it is evident that there are slight variations in the intensity and position of these transition metal bands when rare-earth (Pr³⁺) ions are substituted into lanthanum ferrites. These differences may arise from tiny variations in the crystal lattice, as indicated by the powder XRD results. This suggests that the substitution of Pr³⁺ ions influences the material’s vibrational properties, reflecting changes in its crystal structure.

3.3. Raman Spectroscopy Analysis

Raman spectroscopic technique is a resilient tool for analyzing various aspects of materials, including impurity phases, cation distribution, spin–phonon coupling, crystal structure, and dynamical/local lattice distortion. Moreover, in the context of orthorhombic crystal structure, there exists a total of 60 vibrational modes within the Brillouin zone’s center. However, only 24 of these modes (comprising 7A_g, 5B_1g, 7B_2g, and 5B_3g) are Raman active [26]. Figure 4 in the study presents the deconvolution of Raman peaks for La_1−xPr_xFeO₃ (x = 0.0 and 0.5) using a Lorentzian profile fitting. In this figure, the blue lines denote the Lorentzian multipeak matching profile, the solid red curve denotes the sum of all the total intensity curves, and the black open circles depict the experimental data. Table 2 provides information on the various vibrational modes discovered through Raman spectroscopy, along with their probable attributes, based on the study of Watanabe et al. [27] and Sharma et al. [28].

[figure(s) omitted; refer to PDF]

Table 2

Assignment of active Raman modes observed for La_1−xPr_xFeO₃ (x = 0.0 and 0.5).

Raman modes (cm−1)
LaFeO3	La0.5Pr0.5FeO3
154—A1g	161—A1g
283—B1g	287—B1g
434—A2g	345—A2g
627—B2g	417—B2g
709—B3g	557—B3g
775—A3g	624—B4g
817—B4g	769—A3g
936—2A1g	962—2A1g

According to the study, Raman spectroscopy is expected to reveal 24 modes with mixed symmetries in the Raman spectra of La_1−xPr_xFeO₃ (x = 0.0 and 0.5). In the Raman analysis conducted, eight vibrational modes were identified, with the most intense mode appearing at 410–440 cm⁻¹. The table lists lower order frequencies below 900 cm⁻¹. For higher order frequencies, the mode at 930–970 cm⁻¹ is attributed to oxygen stretching with 2A_1g(1) symmetry, while the mode at 936 cm⁻¹ results from FeO₆ octahedra bending with 2A_1g(2) symmetry. Finally, the modes at 936 and 962 cm⁻¹ possess A_1g symmetry and correspond to O₂ “scissors” and stretching movements, respectively. The presence of a few smaller peaks corresponding to B_g phonon modes and a decrease in the intensities of A₁ modes suggest that Pr³⁺ substitution has influenced the La-site in LaFeO₃. Raman scattering spectra are highly sensitive to atomic arrangements, and the observation of Raman active modes in LaFeO₃ ceramics provides valuable insights into the lattice structure alterations induced by Pr³⁺ substitution.

3.4. Morphology Studies

Figure 5 displays SEM and TEM images of the synthesized nanocrystalline LPFO materials. Notably, the particles exhibit a nanoscale dimension, measuring less than 100 nm, and display a spherical shape. Furthermore, there is a tendency for these nanoparticles to cluster together, forming slight agglomerations. This propensity for spherical morphology and agglomeration is attributed to the presence of rare-earth (Pr³⁺) ion substitutions and the synthesis temperature.

[figure(s) omitted; refer to PDF]

3.5. Dielectric Analysis

Figure 6 illustrates the frequency-dependent behavior of the dielectric properties (εʹ and tan δ) of La_1−xPr_xFeO₃ ceramic compounds with different compositions (x = 0.0 and 0.5) at various temperatures. LaFeO₃ is known for its exceptional dielectric characteristics. In the lower frequency range, the dielectric constant exhibited high values, but as the frequency increased, it gradually decreased before stabilizing within the frequency range of 10³–10⁶ Hz. This phenomenon can be attributed to the higher resistivity and greater energy requirements for electron hopping at lower frequencies, resulting in increased energy loss. Conversely, at higher frequencies, conductivity increased, and electron hopping demanded less energy, leading to a reduction in the dielectric loss factor. It is important to remember that variables like composition, structural homogeneity, and sintering temperature affect the dielectric loss factor. The findings revealed that as frequency increased, dielectric loss decreased in both samples [29].

[figure(s) omitted; refer to PDF]

Figure 7 displays the temperature-dependent characteristics of the dielectric properties (εʹ and tan δ) for La_1−xPr_xFeO₃ ceramic compounds with varying compositions (x = 0.0 and 0.5) at different frequencies. In this figure, it is evident that both pure and 0.5% praseodymium-substituted samples exhibit relatively low-dielectric loss factors. Additionally, at low temperatures ranging from 40 to 160°C, the dielectric constant decreases, while at higher temperatures, it increases, particularly at lower frequencies. The greater thermal energy at higher temperatures is blamed for this behavior, which enhances the mobility of charge carriers, leading to a higher hopping rate. Conversely, at low temperatures, thermal energy is insufficient to significantly contribute to charge carrier mobility, resulting in a decrease in the dielectric constant. This observed mechanism results in higher polarization at elevated temperatures, leading to an increase in the dielectric constant. Furthermore, both the εʹ and tan δ demonstrate relaxor behavior as the doping concentration increases. This behavior can be attributed to the chemical pressure induced in La–Fe–O due to the substitution of Pr³⁺ ions. These findings align with our most recent research findings [14].

[figure(s) omitted; refer to PDF]

3.6. Magnetic Analysis

The impact of Pr³⁺ doping on the magnetic characteristics of LaFeO₃ was explored using a vibrating sample magnetometer, as depicted in Figure 8. The magnetization hysteresis loop of the nanocrystalline La_1−xPr_xFeO₃ (x = 0.0 and 0.5) ferrite materials, observed at room temperature, indicates that the prepared LPFO ferrite materials possess a soft nature and exhibit antiferromagnetic behavior since the synthesized materials were not saturated.

[figure(s) omitted; refer to PDF]

From the hysteresis loop, a number of significant parameters were recovered, including coercive field (Hc), magnetization at maximum field (±15 Oe) of the Pr³⁺ content (x = 0.5) in doped LaFeO₃ was exhibits both Ms (ranging from 0.108 to 0.19 emu/g at max. magnetic field) and Mr (varying from 0.01 to 0.027 emu/g) show an increase, while coercivity (decreasing from 1409.1 to 1325.6 Oe) decreases following the introduction of Pr³⁺ doping into LaFeO₃ ferrite materials synthesized via the traditional solid-state method.

This behavior can be elucidated by considering that the incorporation of rare-earth Pr³⁺ ions resulted in a reduction in crystallite size (as indicated in the XRD measurements). Consequently, this reduction disrupts the canted spins without compensation present at the surface and weakens the super–exchange interaction between Pr³⁺ ions and Fe ion (RE─O─RE, RE─O─Fe, and Fe─O─Fe, where RE represents rare-earth). It is worth noting that our results slightly deviate from the literature values, with Ms values reported as 0.6 emu/g for LaFeO₃, 0.5 emu/g for La_0.5Nd_0.5FeO₃, and 0.1 emu/g for La_0.5Sm_0.5FeO₃ [30, 31].

4. Conclusion

In an effort to modify the inherent properties of LaFeO₃ ferrite materials, Pr³⁺ doping (LPFO) was employed using the traditional solid-state method. This doping induced a lattice distortion, thereby allowing for the adjustment of the structural properties. FT-IR was used to analyze the creation of the crystal structure and the subsequent modifications in the bond stretching vibrations of tetrahedral and octahedral metal complexes. The introduction of Pr³⁺ dopants had a notable impact on the dielectric characteristics, particularly within the frequency span of 10 kHz–1 MHz and over the temperature variation of 40–160°C. The surface morphology of the materials was characterized as spherical with slight agglomeration. The room-temperature magnetic properties of the LPFO ferrite materials displayed antiferromagnetic behavior. Finally, it should be noted that the size of the crystallites had a significant impact on these magnetic properties, which resulted in a weakening of the super-exchange contact between Fe and Pr³⁺ ions.

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