Optical, Dielectric and Magnetic Properties of Cd_xZn_1-xFe₂O₄ Synthesized by Hydrothermal Method Using Uncaria Gambir Extract as Capping Agent

A series of cadmium-substituted zinc spinel ferrite samples with the formula Cd_xZn_1-xFe₂O₄ (x = 0.0, 0.1, 0.15, 0.2, and 0.25) was successfully synthesized via the hydrothermal method, using Uncaria gambir extract as a natural capping agent. X-ray diffraction (XRD) analysis revealed the formation of Cd_xZn_1-xFe₂O₄ with cubic spinel structure in the space group Fd-3m. The crystal structure changed due to Cd doping, with the sample containing 0.25 Cd resulting in the largest crystallite size and the highest lattice constant. The vibrating sample magnetometer (VSM) analysis showed that the prepared materials exhibit paramagnetic behavior, and the saturation magnetization value in range of 1.11 to 4.12 emu/g. Diffuse reflectance spectroscopy (DRS) measurements bandgap energy in range of 1.71 to 1.84 eV in the present of Cd content. All samples exhibited spherical particle shape, with average grain size increasing from 260 to 380 nm as cadmium concentration increased. Furthermore, the LCR meter showed enhanced dielectric behavior, substituting Cd ions and modulation at x= 0.25, which has a maximum dielectric constant (ɛ) of 1834 at a frequency of 50 kHz. The measured results for all samples suggested that Cd_xZn_1-xFe₂O₄ ferrites are recommended for the energy storage application.

Keywords:
Cd-doped; Dielectric properties; Magnetic; Hydrothermal

Visual Abstract

1. Introduction

Spinel-based materials, such as spinel ferrites, have garnered significant interest from scientists due to their unique properties, which include electrical, optical, and magnetic behaviors. These characteristics enable spinel ferrites to serve a wide range of applications, including biomedical¹, energy storage², and photocatalysis³. The spinel structure consists of a primary unit cell composed of eight sub-unit cells arranged in a face-centered cubic (FCC) configuration. Each unit cell contains two types of interstitial sites: tetrahedral (A) sites and octahedral (B) sites. In total, there are 64 tetrahedral and 32 octahedral interstitial sites. The structure features a close-packed arrangement of oxygen atoms, with each unit cell containing 32 oxygen atoms⁴. Spinel ferrites typically follow the formulation (AB₂O₄), where a symbolizes divalent metal ions (Zn²⁺, Ni²⁺, Co²⁺, Mg²⁺, and Mn²⁺) and B signifies trivalent metal cations (Al³⁺, Fe³⁺, and Cr³⁺)⁵. Based on the distribution of cations in octahedral and tetrahedral sites, spinel ferrites are classified into three structures: inverse, normal, and mixed spinel⁶.

Among the various spinel ferrites, ZnFe₂O₄ nanoparticles have garnered significant attention due to their distinctive chemical properties, thermal stability, and magnetic properties that depend on particle size. While most spinel ferrites have a Curie temperature above room temperature, ZnFe₂O₄ does not. This characteristic makes ZnFe₂O₄ suitable for a wide range of applications that require operating temperatures up to 80°C⁷. It has a normal spinel cubic structure, commonly expressed by the formula ZnFe₂O₄. In this structure, oxygen anions form a distorted face-centered cubic (FCC) lattice, with the spaces between the O²⁻ anions being partially filled by iron and zinc cations. Specifically, one-eighth of the tetrahedral sites are occupied by divalent zinc (Zn²⁺) cations, while half of the octahedral sites are filled with trivalent iron (Fe³⁺) cations. Improving the physical properties of ZnFe₂O₄ can be achieved by adding divalent metal dopants (M²⁺), such as Cd²⁺, Cu²⁺, Co²⁺, Ni²⁺, and Mn2+ ⁸-¹⁰. Previous studies on Cd²⁺ doping have shown a decrease in magnetic properties, as indicated by reduced saturation magnetization values. This reduction is associated with enhanced dielectric properties, which are attributed to the lower domain wall energy¹¹. Research conducted by Kogulakrisnan indicates that doping MnFe₂O₄ with Cd²⁺ ions lead to a reduction in its magnetic properties, resulting in a transition from a superparamagnetic to a paramagnetic state. This decrease in magnetic behavior corresponds to a reduction in domain wall energy, which can be beneficial for enhancing dielectric properties. Notably, at a Cd²⁺ doping concentration of x = 0.3, the dielectric constant (ε′) reaches a value of 2000¹².

Various methods have been extensively reported for synthesizing spinel ferrites, including sol-gel auto-combustion, hydrothermal, co-precipitation, solid state reaction, and ultrasonication methods, each with advantages and limitations¹³-¹⁵. Green synthesis of ZnFe₂O₄ using the hydrothermal method has gained popularity due to its mediation by natural substances, straightforward process, high purity of the product, non-toxic nature, feasibility at low temperatures, and the ability to control particle size¹⁶. Previous study successfully utilized plant extracts in the green synthesis of ZnFe₂O₄ to prevent agglomeration and control particle size, such as the leaf extract of Iresine herbstii¹⁷, seed extract of Jatropha curcas¹⁸, flower extract of Nyctanthes arbor-tristis¹⁹, leaf extracts of Ziziphus mauritiana and Salvadora persica²⁰.

This study aims to enhance the properties of Cd-doped ZnFe₂O₄ by employing gambir (Uncaria gambir) leaf extract as a capping agent. This method is intended to regulate particle growth and inhibit agglomeration. Gambir, a plant indigenous to Indonesia, is rich in catechins, which serve as effective capping agents²¹. Photochemical analyses have revealed various secondary metabolites in the leaf extract of gambir, such as flavonoids, terpenoids, saponins, alkaloids, and polyphenols²². These phenolic compounds encapsulate the nanoparticles in an aqueous solution, thereby preventing agglomeration during the precipitation of ions with a base. Numerous studies have reported the successful application of gambir leaf extract as an effective capping agent²³. In particular, there have been no findings regarding the creation of Cd²⁺-doped ZnFe₂O₄ utilizing gambir leaf extract as a capping agent. As a result, this research explores how varying Cd²⁺ levels influence the crystalline structure, magnetic characteristics, surface structure, optical features, and dielectric behavior of ZnFe₂O₄ with the use of gambir leaf extract, applying methods including XRD, VSM, FTIR, SEM-EDX, UV-Vis DRS, and LCR Meter.

2. Materials and Methods

2.1. Materials

The hydrothermal synthesis technique was used to synthesis Cd_xZn_1-xFe₂O₄ (x= 0, 0.1, 0.15, 0.2, and 0.25). All chemicals were purchased from MERCK, including Zn(NO₃)₂·6H₂O (99%), Fe(NO₃)₃·9H₂O (99%), Cd(NO₃)₂·4H₂O (99%), NaOH (99%), and the source of the capping agent was the gambir leaf extract.

2.2. Preparation of gambir leaf extract

The gambir leaf extract was prepared as follows: fresh gambir leaves were dried for a week and ground into a powder. Then, the powder was weighed as much as 10 g and dissolved in distilled water with a g/V ratio of 1:10. The extraction process was carried out for approximately 2 hours at 60°C. After completion, the solution was filtered, and the filtrate was collected. The filtrate was stored in a clean, sealed container. The concentration of Gambir leaf extract used in synthesizing ZnFe₂O₄ and Cd_xZn_1-xFe₂O₄ was 10% (V/V) in 100 mL of distilled water solution.

2.3. Synthesis of Cd_xZn_1-xFe₂O₄ (x= 0, 0.1, 0.15, 0.2, and 0.25)

The synthesis procedure for Cd_xZn_1-xFe₂O₄ (x= 0, 0.1, 0.15, 0.2, and 0.25) using gambir leaf extract as a capping agent. The synthesis was carried out by dissolving Fe(NO₃)₃⋅9H₂O, Zn(NO₃)₂⋅4H₂O, and Cd(NO₃)₂⋅4H₂O in 50 mL of a total mixture of distilled water and gambir leaf extract (the volume ratio of distilled water: gambir leaf extract is 45:5 mL). After that, the mixture was stirred at 500 rpm for 1 hour, and 2M NaOH solution was added until pH 12 was achieved. Then, resulting suspension was transferred to an autoclave and heated at 180°C for 12 hours. Furthermore, the precipitate formed was filtered and washed with distilled water until the pH was neutral. Finally, the precipitate was dried at 110°C for 3 hours and calcined at 600°C for 5 hours.

2.4. Characterization technique

The purity and crystal structure of the Cd_xZn_1-xFe₂O₄ samples (x= 0, 0.1, 0.15, 0.2, and 0.25) were analyzed using X-Ray diffraction (Shimadzu XRD 7000). The XRD characterization employed a Cu Kα radiation source (λ = 1.5418 Å) operating at 45 kV and 40 mA. The 2θ angle range was set between 20° and 80°. Fourier Transform Infrared (FTIR, Perkin Elmer 1600) spectroscopy was used to analyze the stretching or bending vibration modes in the samples. The absorption area and bandgap energy values were measured using Diffuse Reflectance Spectroscopy (SPECORD 210 Plus). Scanning Electron Microscopy with Energy-Dispersive Spectroscopy (SEM-EDS, Hitachi Flex SEM 1000) examined surface morphology and atomic composition. Magnetic properties were assessed using a vibrating sample magnetometer (VSM OXFORD 1.2H), and dielectric properties were measured with an LCR meter in the frequency range of 10 kHz to 300 kHz (BK-Precision).

3. Result and Discussion

3.1. XRD analysis

Figure 1 presents the X-ray diffraction (XRD) patterns of Cd_xZn_1-xFe₂O₄ for values of x = 0, 0.1, 0.15, 0.2, and 0.25. The XRD patterns display diffraction peaks at (222), (311), (400), (422), (333), and (440), which indicate a cubic spinel structure with the space group Fd-3m. The observed peaks were compared with the reference pattern from the Inorganic Crystal Structure Database (ICSD), entry number 158837. Additionally, samples with x values of 0.15, 0.2, and 0.25 exhibited secondary phases, specifically α-Fe₂O₃ and CdO.

Figure 1
XRD diffractogram of Cd_xZn_1-xFe₂O₄ (x=0, 0.1, 0.15, 0.2, and 0.25).

The impurities observed in the samples are attributed to the decomposition of the iron nitrate and cadmium precursors into αFe₂O₃ and CdO before the formation of the spinel ferrite. Yang et al.⁹ also reported the presence of αFe₂O₃ and CdO as impurity phases in Ni-Cu-ZnFe₂O₄ doped with cadmium using the sol-gel method. The samples with x values ranging from 0.1 to 0.25 exhibit a leftward shift in the 2θ peaks. This shift is attributed to the difference in ionic radii between the dopant ion (Cd²⁺), which is larger than that of the Zn²⁺ ion²⁴. The crystallite size was determined using the Debye-Scherrer formula²⁵.

$D=\frac{k\lambda }{\beta \cos \theta }$ (1)

This equation includes four key parameters: the Scherrer constant (k=0.94), the characteristic X-ray wavelength (λ=1.5418 Å), the diffraction peak broadening (β, FWHM), and the angular position of the peak (θ, measured in radians). The average crystallite size is found in Table 1. The lattice constant (a) of all samples is estimated from the standard relation²⁶, hkl is Miller indices, and d is crystal planar distance.

$a=d\sqrt{{\text{h}}^{\text{2}}{\text{+k}}^{\text{2}}{\text{+l}}^{\text{2}}}$ (2)

Table 1
Crystal size and lattice constant of the sample Cd_xZn_1-xFe₂O₄.

Sample names	Crystal size (nm)	Lattice constant (Å)	d-spacing (Å)
x= 0	18.4020	8.4366	1.8864
x= 0.1	18.5060	8.4527	1.8800
x= 0.15	30.5296	8.4664	1.8930
x= 0.2	28.2202	8.4733	1.8948
x= 0.25	33.5332	8.4755	1.8951

The samples with x = 0.0, 0.1, 0.15, 0.2, and 0.25 show crystallite sizes of 18.4020, 18.5060, 30.5296, 28.2202, and 33.5332 nm, respectively (Table 1). The results are attributed to an increase in the crystal size of the samples with higher cadmium (Cd) doping concentrations. An increase in crystal size is associated with the larger ionic radius of Cd compared to zinc (Zn), which contributes to the growth of the crystallite size. Additionally, the variation in cation sites within the ferrite system, the presence of molecular ligands on the crystal surface during crystal growth, and delayed grain growth due to the incomplete occupation of cations at tetrahedral or octahedral sites also influence this increase in crystal size. Consequently, as the Cd composition increases, the additional heat released reduces the concentration of molecules on the crystal surface, thereby inhibiting further crystal growth¹¹.

The relationship between crystal size, lattice constant, and the amount of cadmium doping is illustrated in Figure 2. As the amount of Cd doping increases, the lattice constant also increases (Table 2). The lattice parameter of the sample with x = 0.25 was larger (a = 8.4755 Å) than that of the bulk sample with x = 0.0 (a = 8.4366 Å). This increase is attributed to the substitution of larger Cd²⁺ ions (1.03 Å) for smaller Zn²⁺ ions (0.82 Å), which have different ionic radii. The replacement of the smaller Zn²⁺ ions with the larger Cd²⁺ ions leads to an expansion of the unit cell, resulting in a larger lattice constant. In addition to substitutional effects and differences in ionic radii, impurities can also influence the lattice constant. Samples with x values of 0.15, 0.2, and 0.25, which contain CdO and Fe₂O₃ as secondary phases, exhibit an increase in lattice constant. This observation suggests that impurities may induce local lattice strain, thereby affecting the overall crystal structure²⁷.

Figure 2
Lattice constant and crystal size of Cd_xZn_1-xFe₂O₄ (x=0, 0.1,0.15, 0.2, and 0.25).

Table 2
Absorption bands of all samples.

Sample names	v1 (cm-1)	v2 (cm-1)
x= 0	534	445
x= 0.1	533	447
x= 0.15	532	443
x= 0.2	531	437
x= 0.25	529	446

3.2. FTIR analysis

FTIR spectra for spinel ferrite powder measured within the range of 3000 to 400 cm^-1 are presented in Figure 3. Two major absorption bands are visible in Figure 3 and quantified in Table 2.

Figure 3
FT-IR spectrum of Cd_xZn_1-xFe₂O₄ (x=0, 0.1, 0.15, 0.2, and 0.25) samples.

The low-frequency absorption band is v₂ and the high-frequency absorption band is v₁. The upper absorption band frequency range from 534 cm^-1 to 529 cm^-1 in the FTIR spectrum corresponds to stretching vibrations of metal-oxygen (Cd-O and Zn-O) bonds at tetrahedral. The measured vibration value is slightly lower than those reported in previous studies: Wang et al.²⁸ noted Cd-O vibration at 577 cm⁻¹, and Husain et al.²⁹ noted Zn-O vibration at 556 cm⁻¹ at tetrahedral site²⁸. The lower absorption band frequency range from 437 cm^-1 to 447 cm^-1 reflects the stretching vibrations of iron-oxygen (Fe-O) bonds at octahedral sites³⁰. The vibration at a wavenumber of 534 cm^-1 in sample x= 0 shifts to a lower wavenumber with the increasing composition of Cd to x= 0.25. This shift occurred due to the Cd²⁺ ion having a larger ionic radius than the Fe³⁺ ion about the oxygen ion, resulting in shortening the (Fe³⁺–O²⁻) bond length¹¹.

3.3. DRS UV-Vis analysis

Optical properties analysis using DRS-UV-Vis was conducted to determine the bandgap value obtained from the Cd_xZn_1-xFe₂O₄ (x=0, 0.1, 0.15, 0.2, and 0.25) using the Tauc equation. According to Tauc, the absorption (α) of a material is related to its optical bandgap (Eg) through the following relationship³:

$${\left(\alpha h\nu \right)}^2=B\left(h\nu -Eg\right)$$ (3)

The value of B is a constant independent of temperature, and hν represents the energy of the incoming photon. The optical bandgap of a material is defined as the energy difference between the valence band and the conduction band. Therefore, to determine the optical bandgap value, a plot of (αhν)² and hν has been created for all samples. Figure 4a, the optical bandgap in x= 0 results from the transition from the 3d orbital Fe³⁺ to the 2p orbital O2− ³¹. Figure 4a-e demonstrates that the bandgap of samples decreases as the composition of Cd doping increases from 1.84 eV to 1.71 eV. As the concentration of Cd²⁺ doping increases (from x = 0.1 to x = 0.25), sub-binding energy states may form, leading to a decrease in the optical bandgap. The reduction in bandgap energy following cadmium doping is attributed to the formation of sub-band states within the bandgap, which overlap with the conduction band. This phenomenon produces a continuous band as a result of the substitution mechanism of the Cd²⁺ dopant, which introduces additional electron energy states into the bandgap of ZnFe₂O₄. The decrease in bandgap energy also occurred due to the sp-d hybridization exchange interaction between the stationary d electrons of the cadmium ions and the electron bands of ZnFe₂O₄³². The presence of impurities impacts the band gap because the combined effects of each impurity alter the overall band gap, complicating the analysis of their individual contributions. In the absorption spectrum, multiple absorption peaks are observed, resulting in a spectrum that is not smooth but rather shows several distinct peaks. The decrease in the bandgap was also reported by Chandekar et al.³³, who synthesized NiFe₂O₄ with Co-doping. The reduced Eg value is associated with quantum confinement effects at the nano and micro-strain scales resulting from the increased Co content, as Co²⁺ (72 pm) has a larger ionic radius compared to Ni²⁺ ions (69 pm). The decrease in bandgap energy with increasing dopant composition has also been discussed in studies by Akbar et al.³⁴ and Kardile et al.³⁵, confirming new energy levels localized between electrons and photons. Additionally, the shift at the band edge result sp-d exchange interactions, which are associated with bonding and localized electrons with Zn²⁺ and Cd²⁺ ions.

Figure 4
(a-e) Variation of (αhυ)² with the photon energy (hυ) of samples and (f) Optical absorbance of samples.

3.4. VSM analysis

The magnetic properties of Cd_xZn_1-xFe₂O₄ (where x = 0, 0.1, 0.15, 0.2, and 0.25) were analyzed at room temperature using magnetic hysteresis curves with an applied magnetic field ranging from ±1 T. The hysteresis curves for each sample exhibit distinct magnetic parameters, as illustrated in Figure 5. Key parameters, such as saturation magnetization (Ms), and remanent magnetization (Mr), change with increasing cadmium doping in ZnFe₂O₄.

Figure 5
The hysteresis curves of Cd_xZn_1-xFe₂O₄ (x=0, 0.1,0.15, 0.2, and 0.25).

The changes in magnetization values are attributed to the differences in magnetic moments of the zinc and cadmium cations⁸. Cd²⁺ ions are paramagnetic and tend to occupy tetrahedral coordinated sites. Due to the presence of two ions (Zn and Cd) occupying tetrahedral sites with a significant difference in their ionic radii, there is an increase in lattice distortion and a reduction in the overall magnetic moment of the crystal in Figure 5¹¹. The decrease in magnetic properties of sample x= 0 to x= 0.25 due to the addition of Cd concentration was also reported by Ali et al.²⁷. The non-magnetic Cd²⁺ ions strongly prefer occupying the tetrahedral A positions, while the Zn²⁺ ions are inclined to fill the octahedral B positions. This arrangement may cause Fe³⁺ ions to migrate toward the A position. The shift in cations leads to a decrease in the magnetization of the sample with x = 0.25. Consequently, the magnetic moment at the A position increases due to the influx of more Fe³⁺ ions, while the magnetic moment at the B position decreases because of the presence of Zn²⁺ ions. This complex distribution of cations results in an overall reduction in magnetization, which aligns with Nél's collinear two-sublattice theory, demonstrating the antiparallel orientation of magnetic moments at the A and B positions²⁷. Figure 6 show M_s and M_r variation vs Cd content. The M_s values of samples from x= 0 to x= 0.25 are 4.12, 1.29, 1.52, 0.85, and 1.11 emu/g, respectively. The M_r values of samples from x= 0 to x= 0.25 are 0.06, 0.02, 0.04, 0.05, and 0.03, respectively. The decreased value M_S and M_r of Cd_xZn_1-xFe₂O₄ could be attributed to three factors¹: the enhanced surface-to-volume ratio in nanoparticles, where surface atoms with distinct coordination environments differ from bulk atoms², the weakened A-B site exchange interactions and³ the presence of impurities³⁶. Impurities such as α-Fe₂O₃ (hematite) have very weak magnetic properties, which may also affect the observed reduction in saturation magnetization³⁷.

Figure 6
Magnetization saturation and Magnetization remanence vs Cd content of Cd_xZn_1-xFe₂O₄.

3.5. SEM analysis

The surface morphology of prepared nanoparticles was studied using emission scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) and elemental mapping. The surface morphology of the Cd_xZn_1-xFe₂O₄ (x = 0, 0.1, 0.15, 0.2, and 0.25) samples via SEM is presented in Figure 7a-h. The particle size distribution was evaluated from the SEM images using ImageJ software. The particles exhibit a uniform spherical morphology. Gambir leaf extract acts as a capping agent to prevent agglomeration and control the particle size. The effectiveness of the capping agent in reducing agglomeration was not significant. This agglomeration occurs due to the magnetic properties of the sample, leading to the attraction between ferrite spinel particles³⁸. The particle size distribution is estimated to range from 270 to 360 nm. The sample with x = 0 has an average particle size of 274 nm, as illustrated in Figure 7a and 7b. Upon introducing Cd²⁺ doping into ZnFe₂O₄ (x= 0.1), the average particle size reduces to 263 nm, as depicted in Figure 7c and 7d. For the samples with x=0.15, 0.2, and 0.25, there is a noticeable increase in the average particle size, measuring 325, 353, and 380 nm, respectively, as demonstrated in Figure 7e-j. The increase in average particle size is attributed to the formation of particle agglomeration. Agglomeration leads to an uneven electric field distribution, which reduces polarization efficiency and impacts the dielectric constant²⁴.

Figure 7
SEM image and particle distribution of (a-b) x= 0, (c-d) x= 0.1, (e-f) x= 0.15, (g-h) x= 0.2, and (i-j) x= 0.25.

Energy Dispersive X-ray spectroscopy (EDS) was employed to demonstrate the presence of the corresponding element contained in the synthesized samples. Figure 8a-e shows that all the samples consist of Cd, Zn, Fe, and O elements. An increase in Cd concentration (x) in spinel ferrite from 0.1 to 0.25 results in a gradual decrease in Zn content from 25.94 wt% to 15.95 wt%, while Fe content decreases from 47.08 wt% to 42.73 wt%.

Figure 8
EDX spectra of Cd_xZn_1-xFe₂O₄ compound for a) x= 0, b) x= 0.1, c) x= 0.15, d) x= 0.2, and e) x= 0.25.

3.6. Dielectric properties

3.6.1. Temperature dependence of dielectric constant (ɛ) and dielectric loss (Tan δ)

The relationship between the dielectric constant (Ɛ) and temperature (T in °C) is illustrated in Figure 9. The analysis was performed over a temperature range of 30°C to 250°C at various frequencies: 50 kHz, 100 kHz, 150 kHz, 200 kHz, 250 kHz, and 300 kHz, for all samples. The dielectric constant increases slowly from room temperature (30°C) to 50°C. However, above this temperature, the dielectric constant rises rapidly for all samples, as depicted in Figures 9a-e, and above this temperature, the dielectric constant rises rapidly for all samples, as shown in Figure 9a-e.

Figure 9
Dielectric constant of of Cd_xZn_1-xFe₂O₄ for a) x= 0, b) x= 0.1, c) x= 0.15, d) x= 0.2, and e) x= 0.25.

The thermal energy generated at high temperatures increases the mobility of charge carriers, resulting in an elevated rate of electron hopping. Conversely, lower thermal energy cannot enhance the mobility of charge carriers. Dielectric polarization at higher temperatures will increase, increasing in the dielectric constant (ɛ). The maximum dielectric shift towards higher temperatures is noted with increasing frequency (from 50 kHz to 300 kHz). The rise in frequency causes charge carriers to be unable to align with the rapidly changing field, resulting in a decrease in polarization. Therefore, higher temperatures can provide the energy necessary to restore polarization; in other words, the higher the frequency, the higher the temperature required. Consequently, the maximum dielectric will shift to higher temperatures as the frequency increases³⁹,⁴⁰. The maximum dielectric constant occurred at temperatures approaching the Curie temperature of the ferrite⁴¹.

The reduction in the dielectric constant with increasing frequency happens because the ion and electron exchanges cannot keep pace with the applied field. This leads to diminished polarization and, therefore, a lower dielectric constant⁴². As a result, the maximum dielectric constant value found in the samples is x = 0.25.

Dielectric loss (Tan δ) for sample Cd_xZn_1-xFe₂O₄ (x=0, 0.1, 0.15, 0.2, and 0.25) in Figure 10 shows variation in frequency from 50 kHz to 300 kHz. Figure 10a-e shows the dielectric loss values decrease as the frequency increases. In ferrites, the observed polarization is classified as interfacial polarization, and it operates through the same fundamental mechanism as the conduction process. When electron exchange occurs between Fe²⁺ ions, specifically Fe³⁺ + e⁻ ↔ Fe²⁺, these ions cannot keep up with the external electric field. Consequently, the dielectric constant and dielectric loss have lower values at higher frequencies⁴³. The reduction in tan δ loss with increasing frequency within the specified temperature range suggests a thermally activated relaxation mechanism for charge within the material, where the transport process is governed by charge carrier hopping⁴⁴. In conduction processes that involve hopping phenomena, the loss peak shown in the graph is an important consideration. As illustrated in Figure 10, electricity loss increases with rising temperatures. This trend is expected, as resistivity tends to decrease with increasing temperature.

Figure 10
Dielectric loss of Cd_xZn_1-xFe₂O₄ for a) x= 0, b) x= 0.1, c) x= 0.15, d) x= 0.2, and e) x= 0.25.

The relationship between dielectric loss and temperature reveals two distinct regions. The first zone ranges from room temperature to about 250^oC, the Tan δ remains nearly constant, and the second zone spans from 30^oC to over 250^oC, where Tan δ sharply increases with rising temperature. The increase in Tan δ above 250^oC is due to the higher rate of electron hopping between Fe ions¹¹. The electron exchange between Fe³⁺ and Fe²⁺ cannot match the alternating field. Thus, the maximum dielectric loss occurs when the hopping frequency is nearly equal to the frequency of the applied external electric field⁴⁵. Dielectric loss can also be attributed to the presence of domain wall resonance. At high frequencies, low loss values are observed due to the inhibition of domain walls. An increase in dielectric loss in ferrites occurs when the hopping frequency of localized charge carriers aligns with the applied frequency. This behavior can be explained by the Rezlescu model, which suggests that the collective contributions of electrons and holes to dielectric polarization create a form of dielectric relaxation.

3.6.2. Frequency dependence of dielectric constant (ɛ) and dielectric loss (Tan δ)

Figure 10a and 10bshows the difference in dielectric constant and dielectric loss at frequencies between 10 kHz to 300 kHz at room temperature. The high dielectric constant (ε) values shown in Figure 11a are attributed to the contributions of space charge, interface effects, and ionic polarization at low frequencies. At high frequencies, however, the dielectric constant exhibits a frequency-independent behavior because dipoles cannot keep up with the rapid changes in the applied electric field. The decrease in ε' with increasing frequency can be explained by Koop's and Maxwell–Wagner's theories⁴⁶. The phenomenon of interfacial polarization occurs at grain boundaries in heterogeneous polycrystalline materials due to charge accumulation at the higher resistivity limit, leading to a decrease in polarization (at high frequencies). The decrease in permittivity with increasing frequency indicates a polarization phenomenon that depends on the retardation of ions and dipoles concerning the applied alternating magnetic field due to relaxation time limitations²⁵.

Figure 11
Dielectric properties vs frequency of Cd_xZn_1-xFe₂O₄ a) dielectric constant, b) dielectric loss, and c) AC conductivity.

Figure 11a shows that the dielectric constant increases with increasing Cd dopant content, which the hopping mechanism in spinel ferrite can explain. The exchange interaction between Fe⁺⁺ ions and Fe⁺ions cause local electronic displacement in the direction of the applicable field that determines polarization. Hopping mechanism Fe²⁺ ↔ Fe³⁺ depends on the Fe³⁺ ion concentration at B-sites. Additionally, the concentration of ions located on the B site differs for each metal ion. A hopping mechanism by electron transfer probably causes high-concentration ions. At low frequencies, hopping electrons accumulate, resulting in high surface charge polarization and a high dielectric constant. As frequency increases, the surface charge polarization decreases, and at high frequencies, the hopping mechanism fails, resulting in frequency-independent behavior⁴⁷.

In Figure 11b, the dielectric loss also shows a similar dispersion behavior as the dielectric constant, with the Tan δ value decreasing as the frequency increases. The relaxation occurring at low frequencies is only observed for the x = 0.25 and x = 0.2 samples, which can be explained based on the Debye relaxation theory⁴⁸. For the samples with x = 0 and x = 0.15, the relaxation peak may be lower than the measured frequency range (it is not observed in the investigated frequency range), and the reason for this shift may be attributed to the strengthening of dipole-dipole interactions that hinder the dipole rotation. As a result, the resonance between the rotation of the bipolar and the attached field moves to a lower frequency.

Figure 11c illustrates the relationship between the electrical conductivity of Cd-doped ZnFe₂O₄ at levels x = 0,-0.25, and frequency. At low frequencies, the electrical conductivity gradually increases up to 300 kHz. This behavior observed in disordered materials indicates that a substance's ability to conduct electricity improves with increasing frequency. Additionally, it is possible to differentiate between various frequency ranges within the hopping model¹². According to the Koop model, the dielectric structure consists of two layers. The first layer (the grains) is conductive and is separated by the second layer, which has low conductivity (the grain boundaries), resulting in surface charge polarization. The higher conductivity of the grains is due to the exchange of ions or electrons (Fe²⁺↔Fe³⁺), which causes the material to behave as a slightly ionic dielectric with Fe2+ ⁴⁹.

4. Conclusion

Nanostructure Cd_xZn_1-xFe₂O₄, where x = 0, 0.1, 0.15, 0.2, and 0.25 have been successfully fabricated using hydrothermal method in the present gambir extract as capping agent. X-ray diffraction (XRD) analysis confirmed the formation of the cubic phase of Cd_xZn_1-xFe₂O₄ with crystallite sizes ranging from 18 to 34 nm, and observed slightly impurities from CdO and Fe₂O₃ phases. The lattice constant was found to increase with increasing cadmium doping levels. SEM analysis revealed that all samples exhibited a rounded grain morphology with slight agglomeration. The samples adsorb in the visible light with band gap energies ranging from 1.71 to 1.84 eV. Saturation and remanent magnetizations were observed to decrease with cadmium doping. Dielectric properties showed that the maximum dielectric constant was achieved in the x=0.25 sample. The enhanced dielectric properties suggest that cadmium-doped zinc ferrite has potential as an energy storage material.

5. Acknowledgments

We express our gratitude to the Ministry of Education, Culture, Research, and Technology of the Republic of Indonesia for funding this research under Research Contract Number DRTPM: 041/E5/PG.02.00.PL/2024.

• Data Availability
  All data supporting the findings of this study are included within the manuscript.

6. References

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Edited by

• Associate Editor:
  Jose Eiras.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

All data supporting the findings of this study are included within the manuscript.

Publication Dates

• Publication in this collection
  21 Nov 2025
• Date of issue
  2025

History

• Received
  02 May 2025
• Reviewed
  23 Sept 2025
• Accepted
  28 Sept 2025