La3+ doped LiCo0.25Zn0.25Fe2O4 spinel ferrite nanocrystals: Insights on structural, optical and magnetic properties
Graphical abstract
The structural, optical, and magnetic properties of La3+ doped LiZn0.25Co0.25Fe2O4 nanocrystal were studied. With further increasing x more and more La3+ will accumulate at the grain boundaries and form a secondary phase of LaFeO3. It is evident from mapping images that the elements Zn, Co, Fe, La, S and O exist, further, those elements are homogeneously distributed. We found that the doped samples exhibit narrow band gaps (2.18–2.47 eV) as well as high porosity and surface area. Overall, the superparamagnetic nature and low values of saturation magnetization and coercivity (130.740–110.630 G) of La3+ doped LiZn0.25Co0.25Fe2O4 samples are suitable to be applied in transformers core.
Introduction
Lithium cobalt zinc ferrite represents an interesting system, which shows anomalous behaviors because it comprises magnetic and non-magnetic elements. In this type of ferrites the magnetic properties originate as a result of the spin coupling of the 3d electrons. Specifically in LiCo0.25Zn0.25Fe2O4 system, both Zn2+ and Li+ neither contribute to the nuclear magnetic field nor to the magnetization of the sublattice i.e., the net magnetic field of this system is mainly due to the Fe3+ ions and Co2+ ions at A and B sites.1, 2, 3 Moreover, cobalt reveals irregular behaviour when incorporated in Li-Zn ferrites. It reduces the efficiency of the system as a result of the spin canting and relaxation of the moments.1 However, as Co2+ replaces Fe3+ the octahedron is expanded, the initial symmetry is retained. According to their concentration, Co2+ ions can occupy both A and B sites. At lower proportions, Co2+ ions settle down in the B sites, while at higher concentrations they favor A sites.4 Studies of the spinel structure show that the size of the cations in the sample has a vital role in determining its site occupancy preference. Moreover, the occupancy of larger ions shifts the oxygen ions diagonally and expands the lattice parameter. The distribution of cations over the sublattices has a significant effect on both chemical and physical properties of the spinel structure and subsequently their applications and performance.5, 6, 7 In our previous work, we synthesized Co-Zn spinel ferrites (ZCFO) NPs via sol-gel method. The ZCFO sample shows a low crystallite size (11.7872 ± 0.2 nm), and high surface area (106.63 m2/g), which made it suitable for environmental applications. So, ZCFO NPs have been used as antimicrobial agents,8 biosensors,9 and as a superior catalyst.10
Interestingly, the structural, dielectric, electrical and magnetic properties of spinel ferrites can be enhanced by substituting proper metal ions. The trivalent ions are supposed to be strong candidates that can effectively replace iron and boost the magnetic properties of spinel ferrites.11 The incorporation of rare-earth ions into the spinel crystal lattice improved the 3d-4f couplings between the transition metal and rare-earth ions. This procedure leads to changes in the electrical and magnetic properties of the ferrites. It is perceived that the occupation of rare-earth ions at B-sites reduces the migration of Fe3+ in the conduction process increasing resistivity.12
Manipulation of the physical properties of Co-Zn spinel ferrites nanoparticles NPs by incorporating larger rare-earth ions into their structure has attracted scientific community attention over years,13,14 for instance, Pawar et al.15 have addressed the induced changes in the optical properties of cobalt-zinc ferrite Co0.7Zn0.3 HoxFe2–xO4 (0 ≤ x ≤ 0.1) due to the insertion of (Ho3+) using a facile sol-gel method. They found that the energy band gap rose from 1.72 to 1.84 eV when the x increased from 0.0 to 0.1. Farid et al.16 have substituted praseodymium (Pr3+) instead of Fe3+ into the system Co0.6Zn0.4PrxFe2-xO4 (x = 0.0, 0.025, 0.05, 0.075, 0.10). They found that the insertion of Pr3+ has brought about ascendant increment in the lattice constant due to the large difference between ionic radii of Pr3+ and Fe3+. Additionally, the resistivity and activation energy increased with the Pr3+ substitution. Nonetheless, more research is still required for the promotion of possible applications of rare-earth (R3+) substituted spinel ferrites. Between rare-earth ions, La3+ is an attractive ion due to the fact that it is lighter than other rare-earth ions and has unique thermal and electrical properties. It is published that substitution of La3+ ions in spinel ferrites, enhances the electrical resistivity of spinel ferrites due to the fact that La3+ ions preferred to occupy B-sites and 3d-4f coupling occurs when rare-earth ions (belongs to 4f elements) partially substituted Fe3+ ions.17 Besides, La3+ ion is a paramagnetic and has a high electrical resistivity at room temperature. Hence, we think that a detailed analysis of the structural, magnetic and optical properties could open a new path to use spinel ferrite in potential applications such as cost-effective catalysis, super capacitors, and gas sensors.11
Herein, we pursue by synthesizing the system LiCo0.25Zn0.25-Fe2O4 using sol-gel method. Subsequently, lanthanum ions (La3+) were inserted into the pristine sample on the sake of Fe3+ ions with different concentrations (LiCo0.25Zn0.25LaxFe2–xO4; x = 0.0–0.1; step = 0.02). The consequences of La3+ incorporation on the structural, optical and magnetic properties of the synthesized samples were conducted using various characterization tools including EDX, XRD, FTIR, SEM, ICP-OES, BET surface area analysis, DRS, and VSM.
Section snippets
Materials and synthesis of nanoferrites
The CH3CH(OH)COOLi (99.95%), La(NO3)3·6H2O (99.99%), Fe(NO3)3·9H2O (98.0%), Co(NO3)2·6H2O (99.5%), ZnSO4·7H2O (98%), C6H8O7 (99.57%) and C2H6O2 (99.8%) were purchased from Sigma Aldrich and used without further purification. The synthesis of LiCo0.25Zn0.25LaxFe2-xO4 ferrites powders was carried out using a facile sol-gel method as described in details elsewhere.8, 9, 10,18,19 In order to get a single crystalline phase nanoparticles with low impurities, the obtained samples were annealed at
Structural studies
The composition of the LiCo0.25Zn0.25LaxFe2-xO4 (x = 0.0, and 0.1) samples was analyzed by EDX (Fig. 1), where the presence of Co, Zn, O, La3+ and Fe was confirmed. Moreover by increasing the La content, lanthanum peaks become more intense at the expense of Fe3+ ions proportion in the LiCo0.25Zn0.25LaxFe2-xO4 samples as presented in Fig. S1 (provided in the supplementary material). It is worthy noting that noted EDX is a rough technique for figuring out the elements in the samples, but it is
Conclusions
In conclusion, a spinel ferrite system of LiCo0.25Zn0.25Fe2O4 (pristine sample) was successfully synthesized using sol-gel method. Subsequently, La3+ ions with different concentrations LiCo0.25Zn0.25Fe2-xLaxO4 with x = 0.00–0.10 and step x = 0.02 were inserted into the core structure of the pristine sample. EDX patterns and elements mapping reveal the stoichiometry as well as the spatial distribution of elements in a sample. XRD investigations combined with SEM images show that all samples
Acknowledgements
The authors thank the Materials Science Unit, Radiation Physics Department, National Center for Radiation Research and Technology, Egypt, for financing and supporting this study under the project Nanostructured Magnetic Materials.
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