La³⁺-Induced Band-Gap Modifications in Barium Hexaferrite: An Investigation of the Structural, Optical, and Dielectric Properties ^†

Abstract

M-type barium hexaferrites, BaFe_(12−x)La_xO₁₉ (x = 0.0, 0.05, 0.1, 0.15, and 0.2), were prepared by a low-cost solid-state reaction method. The specimens crystallized in a non-centrosymmetric hexagonal magnetoplumbite structure that belonged to the P6₃/mmc space group. Morphologically, the samples were dense with hexagonal plate-like grains and size variations of around 1.19–1.70 μm. The optical band gap of the system was reduced in the wide band-gap region from 1.78 to 1.74 eV. The band-gap values would be useful in photocatalysis and photovoltaics. The AC conductivity was enhanced with La³⁺ substitution, following Jonscher’s power law. Maxwell–Wagner-type polarization was observed in the specimen, and tangent loss decreased with La³⁺ substitution. The values of the tangent loss were in the appropriate range for electromagnetic shielding applications.

1. Introduction

Concerns have been growing about the energy crisis and environmental pollution, especially pollutants in water bodies. Various methods have been employed to reduce the pollutants in water and produce ‘clean’ energy by alternative methods. Photocatalysis is used to produce hydrogen, which is regarded as a new energy source. In using photocatalysis, tuning the band gap is a challenging process. Metal oxide semiconductors with energies in the narrow band-gap region are highly stable and show high absorption in the visible range. Also, they have good charge and separation properties [1,2]. Ferrite-based materials are widely known for their excellent photocatalysis properties, relatively low band gaps, thermal and chemical stability, and optical and magnetic properties [3,4].

M-type barium hexaferrite (BaFe₁₂O₁₉) belongs to the ferrite family and shows a hexagonal crystal structure. It is the most common and simple hexaferrite. It shows a high Curie temperature, high saturation magnetization, large magnetocrystalline anisotropy, high chemical stability, high electrical resistance, and a good optical band gap. Each unit cell of barium hexaferrite needs two formula units and comprises ten layers of oxygen anions with four oxygen ions in each layer. Two Ba ions replace one of the oxygen ions in the middle layers. Fe³⁺ ions occupy the interstitial sites created by the oxygen ions. However, the Fe³⁺ ions in the interstitial sites are in five different crystallographic environments, namely, 12k, 4f₁, 4f₂, 2a, and 2b [5,6].

Many research works have been conducted on the optical, electrical, and photocatalytic properties of ferrites [7,8,9,10,11]. The main reason for choosing hexagonal ferrite is the number of oxygens in its lattice (19 per formula unit), as the redox activity depends on the oxygen storage capability of the molecule in its lattice [12]. Also, during sintering, ferrites tend to create oxygen vacancies, which enhances the reduction by fixing oxygen in the existing vacancies and tuning the band gap. Its electrical and optical properties, such as its band gap, are tuned using doping. In summary, these materials are promising for photocatalytic applications. In the present research, La-substituted barium hexaferrites were synthesized using a solid-state reaction method and their structural, morphological, optical, and dielectric properties were studied.

2. Materials and Methods

The La³⁺-substituted barium hexaferrites, BaFe_(12−x)La_xO₁₉ (x = 0−0.20, ∆x = 0.5), were prepared using a solid-state reaction. The precursors were weighed stoichiometrically according to Equation (1).

$${\mathrm{BaCO}}_3+\frac{\mathrm{x}}{2}{\mathrm{La}}_2{\mathrm{O}}_3+(6-\frac{\mathrm{x}}{2}){\mathrm{Fe}}_2{\mathrm{O}}_3\to {\mathrm{BaLa}}_{\mathrm{x}}{\mathrm{Fe}}_{12-\mathrm{x}}{\mathrm{O}}_{19}+{\mathrm{CO}}_2$$

(1)

The weighed precursors were mixed and ground using an agate mortar and pestle for 4 h. These powders were calcined at 1200 °C for 6 h. The calcination was repeated 3 times. The powders were mixed with 1% PVA solution, then pressed using a hydraulic press and sintered at 1200 °C for 6 h. X-ray diffraction measurements were carried out to study the structural properties and phase formation of the prepared BaLa_xFe_12−xO₁₉ using a Brooker (D2 Phaser) instrument in the 2ʘ range of 20–80°. The morphological properties were studied with field emission scanning electron microscopy (FE-SEM) measurements using a CARL ZEISS SIGMA 03-81, and elemental analysis was performed using EDS measurements (OXFORD Instruments, made in Germany). A UV-Vis-NIR spectroscope in diffused reflectance mode was used to study the optical properties, with wavelengths ranging from 180 to 2500 nm.

3. Results and Discussions

3.1. Structure

Powder X-ray diffraction measurements were carried out to study the structures and phases of the prepared samples. Rietveld analysis of the XRD patterns shown in Figure 1 confirmed the single-phase magnetoplumbite structure of barium hexaferrite that crystallized in a hexagonal crystal structure of the P6₃/mmc space group. The values of the lattice parameters, unit cell volumes, average crystallite sizes, and refinement parameters are presented in

Table 1. List of lattice parameters, unit cell volumes, average crystallite sizes, and refinement parameters.

La Content (x)	Crystallite Size D (nm)	a = b (Å)	c (Å)	c/a Ratio	Unit Cell Volume (Å)3	Rp	Rwp	Rexp	χ2
0.0	34.224	5.890	23.215	3.940	697.671	12.1	15.1	10.44	2.10
0.05	44.256	5.896	23.213	3.936	699.019	23.7	23.2	21.35	1.19
0.10	54.947	5.895	23.208	3.936	698.637	14.3	17.6	15.95	1.22
0.15	45.029	5.894	23.202	3.936	698.201	18.3	20.6	15.42	1.79
0.20	46.858	5.895	23.209	3.936	698.687	19.2	21.4	16.13	1.77

. The lattice parameters, ‘a’ and ‘c’, varied slightly, and the unit cell volume, ‘V’, increased after doping of La³⁺ as compared to the pristine sample. These variations, along with the variation in unit cell volume, depended on the ionic radii of the Fe³⁺ and La³⁺ ions. Sherrer’s formula [13] was used to calculate the average crystallite size, ‘D’, given in Equation (2).

$$\mathrm{D}=\frac{\mathrm{k}\mathsf{\lambda }}{\mathsf{\beta }\mathrm{c}\mathrm{o}\mathrm{s}}$$

(2)

where ‘D’ is the average particle size, ‘$\mathsf{\lambda }$’ is the wavelength of the incident beam, ‘$\mathrm{k}$’ is the shape constant, ‘β’ is the full width at half maximum, and ‘θ’ is the angle of diffraction. The average crystal size increased up to x = 0.10 and then slightly decreased.

3.2. Morphological Study Using FE-SEM

FE-SEM was used to study the surface morphology of the prepared samples, as shown in Figure 2. The images were captured at a magnification of 40 kX. Close-packed, hexagonal plate-like grains of different sizes were observed in the images. The average grain size was measured using ‘ImageJ’ software (Figure 3) and found to be in the range of 1.19 to 1.70 μm.

3.3. Optical Properties

The band gaps of the prepared samples were measured to understand the optical properties. A UV-VIS-NIR spectrometer was used in diffused reflectance mode to measure the gaps [14]. The percentage of diffused reflectance (R %) spectra of the BaLa_xFe_12−xO₁₉ (0 ≤ x ≤ 0.2) are shown in Figure 4. The samples absorbed the light in the range of 200–550 nm, and the reflection increased above 550 nm. The reflectance of each sample decreased with the increase in La concentration. The optical absorption coefficient, α, was computed using the Kubelka–Munk function [15].

$$\alpha =\raisebox{1ex}{${(1-R)}^2$}\!\left/ \!\raisebox{-1ex}{$2R$}\right.$$

(3)

where $\alpha$ is the absorption coefficient and ‘R’ is the diffused reflectance. The optical band-gap energy was estimated by the Tauc relation [15].

$$\alpha h\nu =A\left(h\nu -E_g\right)$$

(4)

where ‘$h\nu$’ is the photon energy, ‘$A$’ is a constant, and the absorption characterized index is given by ‘n’. As BaLa_xFe_12−xO₁₉ is considered a direct band-gap material, n was ½. A graph of ($\alpha h\nu$)² v/s Energy $‘h\nu ’$ was plotted. The linear portion of ($\alpha h\nu$)² was extrapolated to calculate the band gap, as shown in Figure 5. The variation in band gap values with La concentration is shown in Figure 6. The band-gap values fell between 1.79 and 1.74 eV in the visible region. The maximum band-gap value was 1.79 eV for x = 0, and it slightly decreased with the concentration of La (x = 0.05, 0.10, 0.15, and 0.20).

As the concentration of La³⁺ increased, a decrease in the bandwidth of the conduction band and valence band was observed, which reduced the band gap significantly. The smaller crystallite size was a reason for the lower-energy band gaps, which were different from the higher band-gap energy values reported around 3.6 and 1.9 eV [16] and 3.92 eV for undoped samples prepared by the sol–gel method [17]. However, band gap tuning of barium hexaferrite with Mn and Mg doping shows 2.94 eV for undoped sample and decreases to 1.90 eV with Mn and Mg doping [9]. The band gap of Sr-Ba hexaferrite with Tm-Tb doping [18] increased from 1.73 to 2.65 eV. Ba_1−xLa_xFe₁₂O₁₉ (x = 0.10–0.04) ferrite nanoparticles [19] showed a band-gap increase from 1.87 to 2.53 eV. In this research, the values were observed in the visible band-gap region, which is promising for photocatalytic applications. The decrease in the band gap with La doping may have been due to the generation of sub-bands. That is, the E_g value decreased with La concentration. A continuous band was developed with the combination of conduction and sub-bands [20]. The decrease in the band gap was caused by the decrease in energy levels within the bands such that the electrons needed greater energy to travel from valence to conduction bands [21].

3.4. Dielectric Spectroscopy

Dielectric spectroscopy was used to explore the frequency of dielectric constant, tanδ, and conductivity. The plots are shown in Figure 7, Figure 8, Figure 9 and Figure 10. The dielectric constant and tanδ decreased with frequency and were independent of frequency at higher frequencies. Such behavior can be explained using Maxwell–Wagner polarization [22,23,24]. According to this theory, heterogeneous materials such as hexaferrites consist of conducting grains separated by insulating grain boundaries. These grain boundaries oppose the electrical conduction and aid in polarization. When an electric field is applied, the charge carriers align at the grain boundaries due to high resistance. This builds up charge carriers near the grain boundaries, which induces large space-charge polarization and a large dielectric constant. In conduction, according to Koops [25], grain boundaries dominate at low frequencies and grains dominate at high frequencies. Thus, as the frequency increases, the grain boundaries lose control and a small space charge builds up, which results in a low value of polarization and decreases the value of the dielectric constant. At low frequencies, grain boundaries become more effective, and charge carriers need more energy to cross the boundaries, causing high loss. Grains are more effective at high frequencies and conduct more charge carriers. This results in loss tangent at high frequencies.

The AC conductivity increased with the log frequency, as the AC conductivity plots show (Figure 9 and Figure 10). AC conductivity was independent of frequency at low frequencies but increased sharply at high frequencies. This behavior followed Jonscher’s power law. When compared with the La concentration, the AC conductivity remained constant at x = 0.0, 0.05, and 0.15, but it increased rapidly for La concentrations of 0.1 and 0.2. This may have been due to electron hopping between Fe³⁺ and Fe²⁺ ions. The hopping of electrons increased as the frequency of the applied field increased. The frequency responses of the dielectric constant and the loss tangent to La concentration are shown in Figure 11 and Figure 12. Electron hopping was caused by the higher electronegativity of La³⁺ (1.1) compared to Ba²⁺ (0.89) but which is lesser than Fe³⁺. La³⁺ donate electrons easily. With the increase in La content, the number of charge carriers increased, which, again, increased the polarization and the dielectric constant. The dielectric constant increased as the frequencies increased, being highest for the sample with x = 0.2. On the other hand, the loss tangent decreased with an increase in frequency, and the loss was high for the pure barium hexaferrite. The loss decreased with an increase in the doping concentration of La (x = 0.05, 0.1, 0.15, and 0.2). The lowest concentration was found for the sample with the La concentration at x = 0.05 because of the smallest grain size of La at x = 0.

4. Conclusions

La-doped barium hexaferrite, BaLa_xFe_12−xO₁₉ (0 ≤ x ≤ 0.2), was prepared using the solid-state reaction method. The X-ray diffraction patterns of the Rietveld refinement confirmed the formation of a single-phase hexagonal magnetoplumbite structure, which belonged to the P6₃/mmc space group. Its crystal size increased with the increase in La concentration because of the lattice distortion caused by the substitution of La³⁺ ions in the Fe³⁺ sites. FE-SEM images showed hexagonal plate-like grains of different sizes. The diffused reflectance spectra revealed a slight decrease in band gap with La content due to a decrease in energy levels within the valence and conduction bands. The dielectric studies showed a decrease in the loss tangent and an increase in the AC conductivity at high frequencies as the concentration of La³⁺ ions in the barium hexaferrite increased. The dielectric constant was found to be inversely proportional to the frequency.