Particles in the size range of approximately 1 - 100 nm can display novel optical, electric, magnetic properties because of quantum confinement and surface effects that may find many important technological applications   . Nanotechnology is well-known as a very important key technology in science. In the field of material science, Ferrites are well-known magnetic nanomaterial’s intensively studied as a recording media due to their superior physical properties   . These properties make ferrites an ideal candidate for technical applications such as magnetic resonance imaging enhancement, catalysis, sensors and pigments . Spinel nano ferrites (AB2O4) are materials of today’s research due to their amazing structural, dielectric, electrical and magnetic properties . Such properties are dependent on the nature of cations, their charges and their distribution among tetrahedral (A) and octahedral (B) sites . Various physical and chemical methods of preparation have been developed to achieve nano-sized ferrite particles such as sol-gel , chemical co-precipitation , hydrothermal , and chemical combustion route . Among them, the chemical co-precipitation method seems to be the most convenient method for the synthesis of Zn-Co-Mg ferrites. It is very simple and has better control over the crystalline size and other properties of the materials . In this work, Zn0.5Mg0.5−xLi2xFe2O4 nano ferrite where (x = 0.1, 0.2, 0.3, 0.4 and 0.5) was synthesized using co-precipitation methods. Ultraviolet-visible spectrometer (UV) was used to study the optical, electric and magnetic properties of nanoparticles .
2. Materials and Methods
Zn0.5Mg0.5−xLi2xFe2O4 nanoparticles samples (x = 0.00, 0.10, 0.20, 0.30 and 0.40) were studied by the co-precipitation method . The raw stoichiometric materials are FeCl3, MgCl2∙6H2O, LiCl∙H2O, ZnCl2 with high pure, NaOH was used to found the required solutions with required molarities. Firstly, the solution of MgCl2∙6H2O 0.2 M (25 ml), ZnCl2 and FeCl3 0.4 M (25 ml) were mixed, next, slowly added to NaOH solution with stirring to obtain a mixture of pH 11 - 12. The colloid solution was kept in a water bath at 80˚C for 1 hrs to the removal of NaCl2 and H2O from the powder. The produced powder was washed by deionized water until the filtrate had a pH 7. Then the samples were dried and grinned to absolute powder and annealed to 450˚C for 6 hrs in temperature-controlled muffle furnace Vulcan A-550 at a heating rate 10˚C/min. The absorption of a solution with different concentrations was calculated using UV min 1240 spectrometer Shimadzu.
3. Results and Discussion
UV-VIS absorbance behaviour of curves was found to be the same for five samples of Zn0.5Li2xMg0.5−xFe2O4; as shown in Figure 1. The figure shows the relation between absorbance and wavelengths for five samples of Zn0.5Li2xMg0.5−xFe2O4. It depicts the rapid increase of the absorption at wavelengths 340 nm. It indicated an Lithium index (Li2x) in the absorbance value, as the absorbance value increases with the increase of Lithium index (Li2x).
Absorption coefficient (α): It describes how much light of a given color is absorbed by a material of a given thickness. The absorption coefficient (α) of the five prepared samples (Zn0.5Li2xMg0.5−xFe2O4) was found using the following relation:
where (A) is the absorbance and (t) is the optical length in the samples. Figure 2 shows that the Zn0.5Li2xMg0.5−xFe2O4 plot of (α) with wavelength (λ) was the top among the five samples. The highest value of (α) for all samples was in the U.V region (340 nm). This means that the transition must be corresponding to a direct electronic transition, and the properties of this state are important since they are responsible for electrical conduction. Also, this figure shows that the value of (α) for the five samples increases with the increase of the Lithium index (Li2x), which explains that lithium has a high absorbency  .
Figure 1. The relation between absorbance and wavelengths of Zn0.5Li2xMg0.5−xFe2O4 samples.
Figure 2. The relationship between absorption coefficient (α) and wavelengths of ZnLi2xMg0.5xFe2O4 samples.
Extinction coefficient (K): It is a measure of light lost due to scattering and absorption per unit volume. Extinction coefficient (K) was calculated using the relation The variation at the (K) values as a function of (λ) for five samples of (Zn0.5Li2xMg0.5−xFe2O4) is shown in Figure 3, where it is observed that the spectrum shape of (K) as the same shape of (α). The Extinction coefficient (K) for five samples of Zn0.5Li2xMg0.5−xFe2O4 in Figure 3 indicated that the value of (K) at the wavelength depends on the samples treatment method, where the value of (K) at 338 nm for Zn0.5Mg0.5Fe2O4 sample equals 0.074 while it was 0.207 for the other sample (Zn0.5Li0.1Mg0.8Fe2O4) at the same wavelength. The Extinction coefficient (k) increased with the increase of Lithium index (Li2x) in the Zn0.5Li0.2xMg0.5−xFe2O4 samples.
The optical energy gap (Eg): It is the threshold for photons to be absorbed. The optical energy gap (Eg) has been calculated by the relation (αhυ)2 = C (hυ – Eg) where (C) is constant. By plotting (αhυ)2 vs photon energy (hυ), as shown in Figure 4 for the five prepared samples and by extrapolating the straight thin portion of the curve to intercept the energy axis, the value of the energy gap has been calculated. In this figure, the value of (Eg) for Zn0.5Mg0.5Fe2O4 sample was (3.282) eV while it was (3.121) eV for the Zn0.5Li0.8Mg0.1Fe2O4 sample. The decrease of (Eg) seemed to be associated with the Lithium index (Li2x) for (Zn0.5Li2xMg0.5−xFe2O4) samples as it was observed that the Eg value has decreased with the increase of Lithium index (Li2x) confirming that it is the reason for the bandgap shifts   .
The refractive index (n): It is a dimensionless number that describes how fast light travels through the material. The refractive index (n) is the ratio between the speed of light in a vacuum to its speed in a material which does not absorb this light. The value of n was calculated using the equation Where (R) is the reflectivity. The variation of (n) vs (λ) for the five samples is shown in Figure 5. The figure depicts the relationship of the five prepared samples by Zn0.5Li2xMg0.5−xFe2O4 refractive index (n) spectra. The maximum value of (n) is (2.159) for all samples
Figure 3. The relationship between extinction coefficient (k) and wavelengths of Zn0.5Li2xMg0.5xFe2O4 samples.
Figure 4. The optical energy bandgap of Zn0.5Li2xMg0.5−xFe2O4 samples.
Figure 5. The relationship between refractive index and wavelengths of Zn0.5Li2xMg0.5−xFe2O4 samples.
at the different wavelength which is agreement with (redshift when the Lithium index (Li2x) increased for all samples of (Zn0.5Li2xMg0.5−xFe2O4).
Real dielectric constant (ε1): Figure 6 shows the variation of the real dielectric constant (ε1) with the wavelength of five samples prepared by Zn0.5Li2xMg0.5−xFe2O4 form, which was calculated using the equation ε1 = n2 − k2; where the real the dielectric (ε1) is the normal dielectric constant. From Figure 6; the Real Dielectric Constant (ε1) increased in the region (340 to 432) nm for all samples of (Zn0.5Li2xMg0.5−xFe2O4), where the absorption of the samples at these wavelengths is small, but the polarization was high. The (ε1) reported a maximum value of (4.63) at a wavelength near 360 nm and 390 nm wavelength for the samples (Zn0.5Mg0.5Fe2O4) and (Zn0.5Li0.8Mg0.1Fe2O4), respectively. The effect of treatment by Zn0.5Li2xMg0.5−xFe2O4 on the (ε1) was redshift when the Lithium index (Li2x) increased for all samples of (Zn0.5Li2xMg0.5−xFe2O4).
Figure 6. The relationship between real dielectric constant and wavelengths of Zn0.5Li2xMg0.5−xFe2O4 samples.
The imaginary dielectric constant (ε2): The imaginary dielectric constant (ε2) vs (λ) was shown in Figure 7. This value was calculated using the equation ε2 = 2 nK, where (ε2) represents the absorption associated with free carriers. As shown in Figure 7; the shape of (ε2) is as the same as (ε1). This means that the refractive index was dominated in this behaviour. The maximum values of (ε2) are different according to the treatment operation, so, while the maximum value of (ε1) was (4.63) for (Zn0.5Mg0.5Fe2O4) sample at wavelength near 360 nm, the value of (ε2) for this sample was (0.29) at the same wavelength. Similarly, the maximum value of (ε1) was (4.63) for the (Zn0.5Li0.8Mg0.1Fe2O4) sample at wavelength 390 nm, the value of (ε2) was (0.34) at the same wavelength. This behaviour may be related to the different absorption mechanisms for the free carriers.
Electrical and optical conductivity: The optical conductivity is a measure of the frequency response of material when irradiated with light which is determined using the following relation,
where (c) is the light velocity. The electrical conductivity can be estimated using the equation
The high magnitude of optical conductivity (1.34 × 1012 sec−1) following relation confirms the presence of very high photo-response of the five samples prepared by Zn0.5Li2xMg0.5−xFe2O4 form. The increased optical conductivity at high photon energies is due to the high absorbance of the five samples prepared by Zn0.5Li2xMg0.5−xFe2O4 form and may be due to electron excitation by photon energy as it is shown in Figure 8 and Figure 9. The maximum value of electrical conductivity is 42 (Ω∙cm)−1     .
Figure 7. The relationship between imaginary dielectric constant and wavelengths of Zn0.5Li2xMg0.5−xFe2O4 samples.
Figure 8. The relationship between optical conductivity and wavelengths of Zn0.5Li2xMg0.5−xFe2O4 sample.
Figure 9. The relationship between electrical conductivity and wavelengths of Zn0.5Li2xMg0.5−xFe2O4 samples.
Nanocrystalline, Zn0.5Mg0.5−xLi2xFe2O4 nano ferrites (x = 0.00, 0.10, 0.20, 0.30 and 0.40) samples are successfully prepared by co-precipitation approach. UV-visible spectroscopy showed that the bandgap energy of the samples was computed to be 3.28, 3.24, 3.19, 3.15 and 3.12 eV, for Li1+ concentration increased for the samples, respectively and the physical properties are increased with the lithium concentration and correspond to the Mirghni et al. study. This study was limited to the preparation and study of the composition and photometric properties. It is also recommended to study the rest of the physical and chemical properties to classify these materials and determine their applications.
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