La3+ doped LiCo0.25Zn0.25Fe2O4 spinel ferrite nanocrystals: Insights on structural, optical, and magnetic properties

Journal Pre-proof 3+ La doped LiCo0.25Zn0.25Fe2O4 spinel ferrite nanocrystals: Insights on structural, optical, and magnetic properties M.I.A. Abdel Maksoud, Ahmed El-Ghandour, A.H. Ashour, M.M. Atta, Soraya Abdelhaleem, Ahmed H. El-Hanbaly, Ramy Amer Fahim, Said M. Kassem, M.S. Shalaby, A.S. Awed PII:

S1002-0721(19)30765-3

DOI:

https://doi.org/10.1016/j.jre.2019.12.017

Reference:

JRE 678

To appear in:

Journal of Rare Earths

Received Date: 16 September 2019 Revised Date:

11 December 2019

Accepted Date: 31 December 2019

Please cite this article as: Maksoud MIAA, El-Ghandour A, Ashour AH, Atta MM, Abdelhaleem S, El3+ Hanbaly AH, Fahim RA, Kassem SM, Shalaby MS, Awed AS, La doped LiCo0.25Zn0.25Fe2O4 spinel ferrite nanocrystals: Insights on structural, optical, and magnetic properties, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.12.017. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.

La3+ doped LiCo0.25Zn0.25Fe2O4 spinel ferrite nanocrystals: Insights on structural, optical, and magnetic properties 1* M.I.A. Abdel Maksoud , Ahmed El-Ghandour2, A.H. Ashour1, M.M. Atta3, Soraya Abdelhaleem1, Ahmed H. El-Hanbaly4, Ramy Amer Fahim5, Said M. Kassem5, M. S. Shalaby6, A. S. Awed2,7 1

Materials Science Lab., Radiation Physics Department, National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority, Cairo, Egypt. 2 Center Photonics and Smart Materials (CPSM), Zewail City of Science and Technology, October Gardens, 6th of October City, Giza, Egypt. 3 Polymers Physics Lab., Radiation Physics Department, National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority, Cairo, Egypt. 4 Egyptian Nuclear and Radiological Regulatory Authority, Cairo, Egypt. 5 Radiation Protection and Dosimetry Department, National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority, Cairo, Egypt. 6 Magnetic and Thermal lab, Solid State & Accelerators Department, National Center for Radiation Research and Technology, Atomic Energy authority, Nasr City, Cairo, Egypt. 7 Higher Institute for Engineering and Technology, Manzala, Dakahlia, Egypt.

*Correspondence: [email protected] ; [email protected] Tel.: +201016664425

Abstract This paper addresses the manipulation of structural, morphology, optical and magnetic properties of LiCo0.25Zn0.25Fe2O4 ferrite via incorporation of different proportions of 1

La3+ at the expense of iron ions using a sol-gel methodology. The samples are characterized using the X-ray diffraction technique (XRD), Fourier transform infrared (FT-IR) spectroscopy, the energy dispersive X-ray spectra (EDX), inductively coupled plasma optical emission spectrometer (ICP-OES), high resolution scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analyzer, ultravioletdiffuse reflectance spectroscopy (UV-DRS), and vibrating sample magnetometer (VSM) technique. The Rietveld refinements of the samples indicate that at higher concentrations of La3+, nanostructures with dual phase, i.e. cubic spinel and orthorhombic LaFeO3 perovskite with space group (Pbnm) appear. Optical studies show that the energy band gap (Eg) of the bare LiCo0.25Zn0.25Fe2O4 ferrite sample (2.18 eV) reaches up to 2.47 eV at x = 0.06 and after this concentration, it drops sharply to 2.00 eV. Although the saturation magnetization and the coercivity of LiCo0.25Zn0.25LaxFe2– xO4 are lower than that of LiCo0.25Zn0.25Fe2O4 NPs. Overall, the superparamagnetic nature and low values of saturation magnetization and coercivity of LiCo0.25Zn0.25LaxFe2–xO4 NPs are suitable to be applied in transformers core. Keywords: Li-Co-Zn ferrite; superparamagnetic nature; optical properties; magnetic behavior; transformers core; rare earths. 1. 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[1-3]. Moreover, cobalt reveals irregular behaviour when incorporated in LiZn 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 site, 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 [5-7]. 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 a suitable for environmental applications. So, ZCFO NPs have 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 2

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 Co0.7Zn0.3HoxFe2–xO4 (0≤x≤0.1) due to the insertion of (Ho3+) using a facile sol-gel method. They found that the energy band gap rises 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 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 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 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, and magnetic and optical properties could open a new path to use spinel ferrite in potential applications such as costeffective catalysis, super capacitors, and gas sensors[11]. Herein, we pursue by synthesizing the system LiCo0.25Zn0.25Fe2O4 using sol-gel method. Subsequently, lanthanum ions (La3+) have been 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 have been conducted using various characterization tools including EDX, XRD, FTIR, SEM, ICP-OES, BET surface area analyzer, DRS, and VSM. 2. Experimental method and materials 2.1. Materials and synthesis of nanoferrites The CH3CH(OH)COOLi (99.95%), La(NO3)3·6H2O (99.99%), Fe(NO2)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 is carried out using a facile SolGel method as described in details elsewhere [8-10, 18, 19]. In order to get single crystalline phase nanoparticles with low impurities, the obtained samples were annealed at elevated temperature (up to 900 oC) for 5 h to improve their crystallinity. 2.2. Characterization 3

Firstly, the stoichiometry of the pristine LiCo0.25Zn0.25Fe2O4 sample and La3+ substituted LiCo0.25Zn0.25Fe2O4 samples is examined via employing the energy dispersive X-ray spectra (EDX), JEOL JSM-5600 LV, Japan. It is worth noting that well-known EDX is a rough technique for the figure out the elements in the samples, but it is unable to detect light elements such as Li-ion. Also, the ratio of the light element (Li-ion) is divided into the other elements and consequently the ratio of total elements is still 100% [20]. Therefore, the inductively coupled plasma optical emission spectrometer (ICPOES) has been used to evaluate the concentration of the Li ion in the present samples[21]. ICP-OES model Prodigy Prism High Dispersion (Teledyne Leeman ICPOES USA) that has numerous characteristics, as well as, it has a higher optical resolution of 0.0075 nm and high-grade accuracy. The instrument enables the user to obtain all wavelengths from 160 to 900 nm and optical focal length 800 mm with excellent performance in complex matrices. It uses a 40.68 MHz for a free running, water-cooled oscillator that supplies long-term stability. In order to confirm the formation of the spinel ferrite phase, Fourier transform infrared (FT-IR) spectroscopy (NICOLET iS10 model instrument) is conducted over a wide range (350 – 3000 cm–1). The crystal structure of the samples is investigated via x-ray diffraction technique (XRD; Shimadzu XRD-6000). XRD patterns are obtained in the range of 2θ from 17o to 90o at room temperature. Cu Kα is used as a radiation source of wavelength λ = 0.15408 nm, scan rate 0.8(o)/min, operation voltage 50 kV and current 40 mA[22]. Information on the shape and size of the samples' particles are obtained using a high resolution scanning electron microscopy (SEM), JEOL JSM-5600 LV, Japan). Brunauer-EmmettTeller (BET) method [23], most common method used to describe the experimental specific surface area [24]. The BET single point surface areas of LiZn0.25Co0.25Fe2xLaxO4 NPs were measured by a surface area analyzer (Nova 3200 Nitrogen Physisorption Apparatus USA) with liquid N2 as an adsorbate at –196 °C. UV diffusion reflectance spectra study was performed using a Jasco UV-Visible spectrophotometer (V-670 PC). Finally, the magnetic parameters are measured using a vibrating sample magnetometer (MicroSense, Model: EZ-9) at room temperature from –20 to +20 kG. 3. Results and discussion 3.1. 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 is 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 worth noting that noted EDX is a rough technic for the figure out the elements in the samples, but it is unable to detect light elements such as Li (see Table S1 (provided in the supplementary material)[20]. Therefore, the lithium ion was verified by ICP-OES for all the samples as arranged in Table S2 (provided in the supplementary material). It is obvious that the observed ratio of the Li-ion is almost constant and show good agreement with those of desired chemical formula [25]. The presence of sulfur ions (S) is attributed to residuals of sulfate groups that are involved in the synthesis process. In order to further illustrate the 4

structural features of the samples, element mappings have been carried out selectively to the sample LiCo0.25Zn0.25Fe1.9La0.1O4 and the images are depicted in Fig. S2 (provided in the supplementary material). It is evident from these images that the elements Zn, Co, Fe, La, S and O exist, which agreed with preceding EDX results. Further, those elements are homogeneously distributed.

Fig. 1 EDX spectrum of LiCo0.25Zn0.25Fe2–xLaxO4 (x=0.00 and 0.10) NPs. Fig. 2(a) represents the Rietveld refinements room temperature XRD patterns for the LiCo0.25Zn0.25LaxFe2-xO4 samples with x = 0.00 – 0.10; step x = 0.02. It is evident from this figure that the polycrystalline cubic spinel structure with space group Fd3m is dominated and it can perfectly indexed to JCPDS No. 03-0864[26]. Moreover, at higher concentrations of La3+, nanostructures with dual phase, i.e., cubic spinel and orthorhombic LaFeO3 perovskite with space group (Pbnm) appear and it can be indexed to JCPDS No. 74-1900 [27]. We interpret the existence of this phase to the difference in the ionic radius between La3+ (0.106 nm) and the Fe3+ (0.067 nm), where the Fe3+ ions are replaced by La3+ ions until the solubility limit is reached. After this limit, La3+ ions pile up around the grain boundaries and form orthorhombic LaFeO3 perovskite [26, 27].

5

Fig. 2 Rietveld Refinements (a), FTIR spectra (b), the transformed Kubelka-Munk spectra (c), and the M-H loops of LiCo0.25Zn0.25Fe2-xLaxO4 NPs (d) Since the incorporation of rare-earth ions induces structural distortion by enhancing crystal imperfections due to their larger size and creates micro-strain, employing the Williamson-Hall equation to investigate the XRD patterns is more appropriate than using the Scherrer’s equation. The average crystallite size (DW-H) and the microstrain (ε) are two meaningful structure parameters that can be calculated using Williamson-Hall (W-H) method [28]. cos

=

.

+ 4 sin

(1)

where λ is the wavelength of the X-ray beam, β the full width at half maximum and θ the Bragg’s angle. A plot is drawn with 4 sin along the x-axis and cos along the y-axis for the studied samples as presented in Fig. S3 (provided in the supplementary material). From the linear fit to the data, the average crystalline size was determined from the y-intercept, and the strain, from the slope of the fit [18, 19]. On the other hand, D decreases sharply from 44 nm at x = 0.00 to reach 23.74 nm at x = 0.04 and beyond this concentration the situation is turned upside down as illustrated in Table 1. It is seen that the growth of the LiCo0.25Zn0.25Fe2O4 is limited by the incorporation of La3+ ions, leading to a relatively small crystallite size compared with a pure LiCo0.25Zn0.25Fe2O4 6

ferrite. This behavior attributed to the higher bond energy of La3+–O2–, as contrasted with that of Fe3+–O2–, it is clear that more energy is needed to substitute La3+ ions into the B-sites. The energy demanded this process is satisfied at the expense of crystallization and consequently limits the growth of the crystallites and a smaller crystallite size is hence seen for the La3+ substituted samples. It is striking that for higher substitution amounts (x > 0.04) of La3+, the size of the crystallites slightly expand to 40 nm at x = 0.1. The increment in crystallite size is attributed to the replacement of the smaller ionic crystal radius of Fe3+ (0.064 nm) by the larger La3+ (0.106 nm) ions. With further increasing of x more and more La3+ will accumulate at the grain boundaries and forms a secondary phase of LaFeO3 [26, 27]. It is concluded from this investigation that the amount of energy demanded the incorporation of La3+ ions into B-sites is now being appropriated for the growth of the crystallites[26]. Furthermore, the samples reveal a negative microstrains which indicate that they undergo compressive strains [18, 19]. It is fascinating to see that the LiCo0.25Zn0.25Fe1.94La0.02O4 and LiCo0.25Zn0.25Fe1.94La0.08O4 samples have a positive strain. This means that the substitution of larger sized La3+ ions is expanding the crystal lattices by shifting the strain from compressive to tensile[26]. ………………………………. ….Table 1…………………………… Fig. S4 [A] (provided in the supplementary material) gives the peak position for the preferred plane (311) where the peak positions are shifted to a lower angle side with substitution of La3+, recommending increasing of the distortion in the lattice of pure cubic structure[29]. In other words, this shift in terms of the expansion of the unit cell and the increase of the lattice constant [18] as depicted in Table 1. This distortion is may be due to La3+ substitution which occupies interstices of the ferrite lattice, the increased amount of La3+ substitution may create a new phase of perovskite which arises concurrently with the cubic phase of ferrite. Fig. S4 [B] (provided in the supplementary material) illustrates the variations of the lattice constant (aexp) and average crystallite size (DW-H) of the system LiCo0.25Zn0.25Fe2-xLaxO4 as functions of La3+ concentration x. Overall, aexp increases gradually with increasing La3+ concentration up to x = 0.04 (0.83827 nm). The lattice parameters increase with the increasing La content; which is due to the difference between the ionic radii of both La3+ (0.106 nm) and Fe3+ (0.067 nm), this increase is also a good indication of the insertion of La3+ cations into the LiCo0.25Zn0.25Fe2O4 spinel structure. Consequently, La3+ has a strong site preference for the octahedral site which results in the expansion of unit cell[30]. Similar results are reported [31, 32]. Furthermore, it is observed that the value of the lattice constant is smaller for LiCo0.25Zn0.25Fe1.94La0.06O4 (0.83814 nm) LiCo0.25Zn0.25Fe1.90La0.10O4 (0.83883 nm) compared with LiCo0.25Zn0.25Fe1.92La0.08O4 (0.83894 nm). It is obvious from the Rietveld refinement patterns that the cell volume of the second phase for LiCo0.25Zn0.25Fe1.92La0.08O4 sample (0.2432579 nm3) is higher than that of LiCo0.25Zn0.25Fe1.94La0.06O4 (0.2418027 nm3) and 3 3+ LiCo0.25Zn0.25Fe1.90La0.10O4 (0.2420242 nm ). The substituted content of the La ions does not enter into the LiCo0.25Zn0.25Fe2O4 sub-lattices and form more of the secondary LaFeO3 phase, and therefore a reduced value of the lattice constant of spinel ferrite is 7

seen[26]. Besides, the LiCo0.25Zn0.25Fe1.92La0.08O4 sample has a tensile strain in contrast to the other two samples that have a compressive strain as observed in Table 1. The phase analysis and structural parameters such as lattice constant obtained from Rietveld refinement of LiCo0.25Zn0.25Fe2-xLaxO4 are outlined in Table 1. The relative percentages of the orthorhombic LaFeO3 perovskite phase with space group (Pbnm) and the spinel phase with space group Fd3m and the variation of lattice constant “a” for spinel phase and “a”, “b”, and “c” for the phase of perovskite have been listed in Table 1. It is clear from this table that the relative percentages of the orthorhombic LaFeO3 phase increased as La ion content increased in the samples and reached to 15.37% for LiCo0.25Zn0.25Fe1.90La0.1O4 NPs[29]. The allowed Braggs positions for the Fd3m space group are marked as vertical lines. All the observed peaks are allowed Bragg 2 positions. The quality of profile fit was assessed by the value of reliability factors in Rietveld refinement (Table 1). The low values of R-factors suggest the excellent quality of profile fit[33]. The hopping distance of magnetic ions between the A and B sites is calculated as follows[30, 34]: −

=

√

−

=

√!

−

=

√""

(2) (3) (4)

#

It is seen from Table S3 (provided in the supplementary material) that the hopping distance expands slightly as the La3+ content increases, which is consistent with the lattice constant increment as mentioned earlier and elsewhere[30, 34]. The FTIR spectra of the pure and La3+ doped LiCo0.25Zn0.25Fe2O4 ferrites are illustrated in Fig. 2[B]. The positions of the vibrational bands are obtained and listed in Table S3 (provided in the supplementary material). In general, spinel ferrites show two essential vibrational bands, namely υ1 and υ2 and are corresponding to the stretching vibration of tetrahedral groups (A-site) and the stretching vibration of octahedral groups (B-site), respectively [18, 19]. Clearly, from Fig. 2[B], the insertion of La3+ ions into the structure of LiCo0.25Zn0.25Fe2O4, shifts the bands of the tetrahedral and octahedral sites towards the lower frequency side. The substitution of La3+ ions into the B-site results in a migration of an equal number of Li+ and Co2+ ions to the A-sites, consequently an equal number of Fe3+ ions also migrate from A-sites to B-sites to ease the strain[26]. The ionic radii of the Co2+ (0.078 nm) and Li+ (0.074 nm) are higher than that of the Fe3+ ion (0.067 nm), which increases the ionic radii of the A sites. Likewise, the ionic radii of the B sites increase due to La3+ ions settlement. This augmentation in the ionic radii of the A and B sites reduces the fundamental frequency[26]. The bands appeared at 3400 and 1600 cm–1 are due to the presence of hydroxyl groups in the samples where the peaks 1118 cm–1 are related to the bond formation [8-10, 18, 19].

8

The force constant for the tetrahedral site (Ft) and octahedral site (Fo) was determined using the position of υ1 and υ2 bands and listed in Table S3 (provided in the supplementary material). Their values fluctuated with increasing the La3+ content in the samples, which may be due to the solubility limits and the induced stresses[35]. The Debye temperature ΘD is the temperature correlates the elastic properties with the thermodynamic properties such as thermal expansion, thermal conductivity, specific heat, and lattice enthalpy. The value of Debye temperature as a function of La3+ is illustrated in Table S3 (provided in the supplementary material). It is obvious that the Debye temperature decreases with increasing La3+ content, which outweighs a rigidity reduction as reported earlier in [36]. 3.2. Morphological study Fig.S5 (provided in the supplementary material) presents SEM micrographs of LiCo0.25Zn0.25Fe2-xLaxO4 NPs. Overall, the micrographs depict inhomogeneous distribution of nanoparticles (nanopillar arrays) over the total area of the image. At higher magnifications, the samples obviously comprise nanopillars. These nanopillars are lined up in a high density phase forming many randomly orientated pores. Table S4 (provided in the supplementary material) illustrated that the BET single surface area increases from 7.48 m2/g for the pristine LiCo0.25Zn0.25Fe2O4 NPs to 18.70 m2/g at x = 0.02 and then decreased to 5.61 m2/g at x = 0.10. The specific surface area is behaves opposite trend to the cell volume of the spinel ferrite. So, it is expected that the LiCo0.25Zn0.25Fe1.98La0.02O4 sample possesses large surface area as presented in Table S2. Similar behavior is detected in literature [18, 19] 3.3. Optical Properties Absorbance and band gap are determined based on ultraviolet visible and diffuse reflectance spectroscopies as shown in Fig. S6 (A) and Fig. 6(B) (provided in the supplementary material), respectively[37, 38]. The optical absorption coefficient (α) is calculated using reflectance data according to the Kubelka-Munk equation [39]: $ %&' =

[ " )% ']+ ! )% '

,

(5)

where R(λ) is the percentage of reflected light. The incident photon energy (hν) and the optical band gap energy (Eg) are related to the transformed Kubelka-Munk function[40]: F(R) hυ =A(hυ‒Eg) r , (6) where F(R) is absorption coefficient, hυ is the photon energy, A is a constant that depends on the transition probability and the exponent r depends on the nature of the transition ( r = 2 or 3 for indirect allowed and forbidden transitions, respectively, and r = 1/2 or 3/2 for direct allowed and forbidden transitions, respectively)[41]. Our calculations outweigh the indirect transition and the band gap is determined by plotting ($%&'ℎ-)0.5 as a function of photon energy (ℎν) as demonstrated in Fig.2(c). Eg of the bare LiCo0.25Zn0.25Fe2O4 sample equals to 1.94 eV, which increases gradually until it reaches a maximum value with 2.05 eV at x = 0.06 and after this concentration it drops sharply to 1.59 eV. We interpret this trend to the agglomeration of La3+ around the grain boundaries at higher content x > 0.06. 9

Furthermore, the difference in the bond energy between La3+ and O2– and that of Fe3+ and O2–, where more energy is required to combine La3+ ions into lattice in contrast to Fe3+ and O2–. 3.4. Magnetic properties Fig.2(d) exhibits the magnetic hysteresis loops for LiCo0.25Zn0.25Fe2-xLaxO4. Narrow hysteresis loops are observed, which is a sign for superparamagnetic nature. The fundamental magnetic parameters such as the saturation magnetization (Ms), remanence (Mr), coercivity (Hc), and squareness (Mr/Ms) are obtained from the curves and listed in Table S5 (provided in the supplementary material). From this table, Ms drops with increasing La3+ content in the sample. This can be clarified firstly in the light of diamagnetic nature of La3+ ion. Where La3+ has zero electrons in the 4f level (zero magnetic moment) and subsequently it has zero contribution to the exchange interactions with its neighboring ions[42], which reduces the magnetic superexchange interactions between the cations in the A and B sites. Secondly, structural parameters such as the reduction of the crystallite size and the increment of the surface area give rise to notable distortion in the surface. Atoms at the surface expose to strains, which results in vacancies, difference of interatomic spacing and low coordination numbers[43]. These interactions are a sort of spin disorder, which leads to the reduction of saturation magnetization [43]. The coercivity Hc behaves in contrast to the average particle size DW-H. The behavior of Hc is significantly dependent on the average crystallite size, thus when DW-H decreases from 48 nm (x = 0.00) to 27 nm (x = 0.04), the Hc increases from 90 to 132 G. Likewise, as DW-H increases from 37 nm (x = 0.06) to 42 nm (x = 0.10), the Hc decreases from 132 to 110 G. The situation mentioned above can explained in terms of the coercivity variation with the crystallite size [42]: 01 = 2 +

3

(7)

where e and f are constants. The values of squareness ratio (Mr/Ms) are calculated to probe the effect of nano sized domains. We found that the values of the magnetic squareness under 0.5 refer to the magnetic multi-domains while beyond 0.5 the single magnetic domains are dominant. These values confirm the superparamagnetic behavior of La3+ substituted LiCo0.25Zn0.25Fe2O4 [44]. The magnetic moment per formula unit (ηB in Bohr magneton) has been calculated from the values of saturation magnetization MS using the follow relation [44]: 45 =

67 68 ×"

:

(8)

;< =>

where, Mw is the molecular weight of the sample (mol/g), MS is the saturation magnetization (A·m2/kg), Na is the Avogadro's number (NA= 6.02×1023 mol–1), μ magnetic moment of electron ( μ =9.27×10–24A·m2). The value of ηB reduced with rising in the La3+ ions content, which dependent on the obtained Ms values according to Neel's model for ferromagnetism[45]. Where, in Neel's 10

model, the net magnetic moment in spinel ferrites is the resultant magnetic moment due to the antiferromagnetic ordering between A-site and B-site magnetic moments. The origin of disordered surface spins is attributed to broken exchange bonds, lack of long‐ range ordering and high surface anisotropy for ferrite nanocrystals. It is noted that the presence of a magnetically disordered surface layer demands more field energy to saturate. The reduction of the Bohr magneton ηB and the magnetization as the increase of La content can be interpreted based on nonlinear or canted spin ordering[46]. The produced microstrain in the tiny crystal due to radii mismatch leads to a noncollinear arrangement of surface spins in the nano range, which also produces a notable supplying in decreasing saturation magnetization due to the migration of Li+ and Co2+ ions to the A-sites leading to the reduction of Bohr's magneton. According to random canting model, the substitution of diamagnetic cations (such as La ions) in one sublattice of ferrimagnet leads to spin canting in the other sublattice resulting in a reduction in total magnetization per formula unit @ A in terms of Bohr's magneton of spinel ferrite and other symbols have their usual meaning which is presented by[46]: @ A = @ %cos < CA >' − @

(9)

Where @ and @ are magnetic moments of A and B sites and CA is canting angle. The presence of canted surface spins also affects the magnetic properties of nanosized spinel ferrites. In other words, the incorporation of La ions into the B-site of the LiCo0.25Zn0.25Fe2O4 spinel ferrites leads to spin canting in A-site due to the migration of Li+ and Co2+ ions to the A-sites. This behavior resulting in a reduction of total magnetization per formula unit and consequently reduction of the Bohr's magneton[47]. The anisotropy constant values K for the present samples were determined through Brown’s relation[48]: D=

68 EF

(10)

. #

It is evident from Table S5 (provided in the supplementary material) that the anisotropy constant increases, K, with the increasing La3+ content in the sample due to its high anisotropy constant value as compared to this of Fe3+ ions. The increase in K means the increase in domain wall energy[48]. The reduction of magnetic moment (@ B) with La3+ content may be described by the lattice defects and lower magnetic superexchange interactions between A and B sites in the spinel ferrites. As La3+ ions have larger ionic radii than Fe3+, the La3+ replacement may deform the lattice and reduces the homogeneous composition, so it produces the deterioration in the magnetic moment. Moreover, due to their large ionic radii, La3+ ions may substitute Fe3+ on B-site [26], therefore, decreasing the magnetic moment as well as reducing the strong Fe3+–Fe3+ negative interaction rising from La3+ substituting due to antiferromagnetic coupling [26]. The disturbance befell in the spinel ferrite lattice, by substitution of magnetic Fe3+ ions with nonmagnetic La3+ ions could be expected because the La3+ ions reduce the ferromagnetic regions at the expense of the nonmagnetic one[34]. The variation in coercivity, with La3+ content, maybe due to the 11

stronger L–S coupling and weaker crystal field due to La3+ which induces the stronger magnetocrystalline anisotropy. However, it is also observed that the coercivity values decrease with the presence of the secondary LaFeO3 phase for (x =0.08). Besides, the reduction in coercivity may be due to the reduction of Fe3+ at B-sites and the migration of Li+ and Co2+ ions to the A-sites. Furthermore, it is thought to initiate the pinning of domain walls of the antiferromagnetic phase along with the existing ferrimagnetic phase to control the coercivity[34]. The domain wall motion of a ferrimagnetic phase into an antiferromagnetic phase is usually complex and in the existing case, this mechanism probably goes together to reduce Hc (x = 0.08) as presented in Table S5 (provided in the supplementary material). The anisotropy constant (K) determined shows similar behavior to that of seen for coercivity variation with the La content. The reduction in K value for La3+ concentration (x = 0.08) in LiCo0.25Zn0.25Fe2-xLaxO4 is ascribed to the non-incorporation of the amount of La3+ into the LiCo0.25Zn0.25Fe2O4 lattices and also due to the migration of Li+ and Co2+ ions from the B-site to A-site. Kakade et al. [34] have independently reported the similar results on the anisotropy constant and coercivity variation with the rare earth content. The observed coercivity for Co1.1Fe1.93+ xErxO4 NPs, increases from 999 Oe (x = 0.00) to 1984 Oe (x = 0.10) with Er concentration and then decreases for higher Er3+ content (x = 0.15). Also, the anisotropy constant (K) calculated shows the similar behavior to that of observed for coercivity variation with the erbium content. The results reported in this study in the regard of the pure LiCo0.25Zn0.25Fe2O4 system and La3+ doped LiCo0.25Zn0.25Fe2O4 samples are in a good agreement with [44]. Lately, Srinivasamurthy et al.[45] have synthesized Ni2+ doped cobalt ferrite nanoparticles. They found that the saturation magnetization in the range (41–76) emu/g. Also, the coercivity decreased from 1049.6 to 162.5 Oe with increasing the Ni content. They assumed that the small values of coercivity with moderate saturation magnetization indicate that the samples can be used as promising materials to achieve low core loss in the transformer. Also, Akhtar et al. [44] have studied the influence of Cu substitution on the structural and morphological characteristics of Ni–Zn ferrites. The saturation magnetization in the range of 14–102 emu/g and the values of coercivity in the range of 14–50 Oe and the sample show soft nature. These properties made of Cu substituted Ni–Zn ferrite promising materials for many industrial and domestic applications such as components of transformers core, and switching. Finally, Junaid et al. [49]have synthesized Tb and Dy doped Li-Ni nano-sized ferrites by micro-emulsion technique. The saturation magnetization, and the coercivity are calculated and these lie in the range (54–27) emu/g, and (120–156) Oe. The smaller magnetic suggested the possible utility of these nano-materials in switching and high frequency applications. Overall, we assume the superparamagnetic nature and low values of saturation magnetization and coercivity of La3+ doped LiCo0.25Zn0.25Fe2O4 samples may be promising for variety of applications such switching, and transformers' cores applications.

12

4. Conclusions In conclusion, a spinel ferrite system of LiCo0.25Zn0.25Fe2O4 (pristine sample) is successfully synthesized using sol-gel method. Subsequently, La3+ ions with different concentrations LiCo0.25Zn0.25Fe2-xLaxO4 with x = 0.00 – 0.10; step x = 0.02 are 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 comprises particles in the nano-scale. These particles decline from 48 nm at x = 0.00 to 27 nm at x = 0.04, beyond this concentration the situation is turned upside down due to the ionic radii difference between iron and lanthanum ions. The coercivity Hc behaves in contrast to the average particle size (D). when D decreases from 48 nm (x = 0.00) to 27 nm (x = 0.04), the Hc increases from 90 to 132 G. Likewise, as D increases from 37 nm (x = 0.06) to 42 nm (x = 0.10), the Hc decreases from 132 to 110 G. Further, low coercivity (130. 740 – 110. 630) of the doped samples makes them potential candidates for transformers' cores. 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.

Graphical Abstract

13

The structure, optical, and magnetic properties of La3+ doped LiZn0.25Co0.25Fe2O4 nanocrystal have studies. With further increasing of x more and more La3+ will accumulate at the grain boundaries and forms 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.

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17

Table1: The Rietveld refinement phase analysis, lattice constants, reliability factors, the crystallite sizes and the strain of LiCo0.25Zn0.25Fe2-xLaxO4 NPs

Crystal structure

Lattice parameters,

Reliability factors

Cubic spinel (Fd3m) (100%) Orthorhombic ( Pbnm ) (0%) Cubic spinel ( Fd3m ) (98.01%) Orthorhombic ( Pbnm ) (1.99%)

a = b = c = 0.83787 nm Cell Volume = 0.5882051 nm3 -

RB = 5.88, RF = 6.27 -

a = b = c = 0.83725 nm Cell volume = 0.5869031 nm3 a = 0.55403 nm, b =0.55899 nm, c = 0.78231 nm Cell volume = 0.2422766 nm3 a = b = c = 8.3819 nm Cell volume = 0.5888824 nm3 a = 5.5494 nm, b = 5.5574 nm, c = 7.8432 nm Cell volume = 0.2418849 nm3 a = b = c = 8.3812 nm Cell volume = 0.5887317 nm3 a = 5.5389 nm, b = 5.5661 nm, c = 7.8432 nm

RB = 2.44 RF = 3.69 RB = 22.6 RF = 18.4

La3+ conte nt (x)

0.02

0.04

0.06

Cubic spinel (Fd3m) (96.37%) Orthorhombic ( Pbnm ) (3.63%) Cubic spinel ( Fd3m ) (94.29%) Orthorhombic ( Pbnm ) (5.71%)

18

RB = 2.60 RF = 2.94 RB = 7.03 RF = 5.59 RB = 4.36 RF = 4.04 RB = 7.27 RF = 6.33

Crystallite size, DW-H (nm )

Strain G

44.00

– 1.859 7

36.13

3.433 0

23.74

– 4.516 9

34.89

– 2.207 4

0.08

0.10

Cubic spinel ( Fd3m ) (91.19%) Orthorhombic ( Pbnm ) (8.81%) Cubic spinel (Fd3m) (84.63%) Orthorhombic (Pbnm) (15.37%)

Cell volume = 0.2418027 nm3 a = b = c = 8.3894 nm Cell volume = 0.5905322 nm3 a = 5.5393 nm, b = 5.6038 nm, c = 7.8367 nm Cell volume = 0.2432579 nm3 a = b = c = 8.3874 nm Cell volume = 0.5900504 nm3 a = 5.5492 nm, b = 5.5596 nm, c = 7.8448 nm Cell volume = 0.2420242 nm3

19

RB = 3.14 RF = 3.01 RB = 7.64 RF = 7.11

35.02

1.368 3

40.26

– 2.128 5

RB = 3.84 RF = 4.38 RB = 18.7 RF = 15.8

Highlights: • La3+ substituted Li-Co-Zn ferrite are successfully synthesized via facile sol-gel methodology. • Narrow band gap and the high porosity of LiCo0.25Zn0.25LaxFe2-xO4 are suitable to be applied in the catalysis applications. • Low coercivity values of LiCo0.25Zn0.25LaxFe2-xO4 reveal their potential for transformers core applications.

Conflict of Interest Form All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version. This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue. The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript

La3+ Doped LiCo0.25Zn0.25Fe2O4 Spinel Ferrite Nanocrystals: Insights on Structural, Optical, and Magnetic Properties M.I.A. Abdel Maksoud1*, Ahmed El-Ghandour2, A.H. Ashour1, M.M. Atta3, Soraya Abdelhaleem1, Ahmed H. El-Hanbaly4, Ramy Amer Fahim5, Said M. Kassem5, M. S. Shalaby6, A. S. Awed7 1

Materials Science Lab., Radiation Physics Department, National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority, Cairo, Egypt. 2 MSc in Experimental Physics, Damietta University, New Damietta, Egypt. 3 Polymers Physics Lab., Radiation Physics Department, National Center for Radiation Research and Technology(NCRRT), Atomic Energy Authority, Cairo, Egypt. 4 Egyptian Nuclear and Radiological Regulatory Authority, Cairo, Egypt. 5 Radiation Protection and Dosimetry Department, National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority, Cairo, Egypt. 6 Magnetic and Thermal lab, Solid State & Accelerators Department, National Center for Radiation Research and Technology, Atomic Energy authority, Nasr City, Cairo, Egypt. 7 Higher Institute for Engineering and Technology, Manzala, Dakahlia, Egypt.

The funding sources: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for- profit sectors. *Correspondence: [email protected] ; [email protected] Tel.: +201016664425

1

Abstract This paper addresses the manipulation of structural, morphology, optical and magnetic properties of LiCo0.25Zn0.25Fe2O4 ferrite via incorporation of different proportions of La3+ at the expense of iron ions using a sol-gel methodology. The samples are characterized using the x-ray diffraction technique (XRD), Fourier transform infrared (FT-IR) spectroscopy, the energy dispersive X-ray spectra (EDX), inductively coupled plasma optical emission spectrometer (ICP-OES), high resolution scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analyzer, ultravioletdiffuse reflectance spectroscopy (UV-DRS), and vibrating sample magnetometer (VSM) technique. The Rietveld refinements of the samples indicate that at higher concentrations of La3+, nanostructures with dual phase, i.e. cubic spinel and orthorhombic LaFeO3 perovskite with space group (Pbnm) appear. Optical studies show that the energy band gap (Eg) of the bare LiCo0.25Zn0.25Fe2O4 ferrite sample (1.94 eV) reaches up to 2.05 eV at x = 0.06 and after this concentration, it drops sharply to 1.59 eV. Although the saturation magnetization and the coercivity of LiCo0.25Zn0.25LaxFe2xO4 are lower than that of LiCo0.25Zn0.25Fe2O4 NPs. Overall, the superparamagnetic nature and low values of saturation magnetization and coercivity of LiCo0.25Zn0.25LaxFe2-xO4 NPs are suitable to be applied in transformers core. Keywords: Li-Co-Zn ferrite; superparamagnetic nature; optical properties; magnetic behavior; transformers core. 1. 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[1-3]. Moreover, cobalt reveals irregular behaviour when incorporated in LiZn 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 site, 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 [5-7]. 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 a suitable for environmental applications. So, ZCFO NPs have used as antimicrobial agents[8], biosensors[9], and as a superior catalyst[10]. 2

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 Co0.7Zn0.3HoxFe2-xO4 (0≤x≤0.1) due to the insertion of (Ho3+) using a facile sol-gel method. They found that the energy bandgap rise 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 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 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 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, and magnetic and optical properties could open a new path to use spinel ferrite in potential applications such as cost-effective catalysis, supercapacitors, and gas sensors[11]. Herein, we pursue by synthesizing the system LiCo0.25Zn0.25Fe2O4 using sol-gel method. Subsequently, lanthanum ions (La3+) have been 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 have been conducted using various characterization tools including EDX, XRD, FTIR, SEM, ICP-OES, BET surface area analyzer, DRS, and VSM. 2. Experimental method and materials 2.1. Materials and Synthesis of Nanoferrites The (CH3CH(OH)COOLi, 99.95%), (La(NO3)3.6H2O, 99.99%), (Fe(NO2)3·9H2O, 98.0%), (Co(NO3)2·6H2O, 99.5%), ZnSO4.7H2O, 98%), (C6H8O7, 99.57%), and (C2H6O2 ,99.8%) are purchased from Sigma Aldrich and used without further purification. The synthesis of LiCo0.25Zn0.25LaxFe2-xO4 ferrites powders is carried out using a facile Sol-Gel method as described in details elsewhere [8-10, 18, 19]. In order 3

to get single crystalline phase nanoparticles with low impurities, the obtained samples were annealed at elevated temperature (up to 900 oC) for 5 h to improve their crystallinity. 2.2. Characterization Firstly, the stoichiometry of the pristine LiCo0.25Zn0.25Fe2O4 sample and La3+ substituted LiCo0.25Zn0.25Fe2O4 samples is examined via employing the energy dispersive X-ray spectra (EDX), JEOL JSM-5600 LV, Japan. It is worth noting that well-known EDX is a rough technique for the figure out the elements in the samples, but it is unable to detect light elements such as Li-ion. Also, the ratio of the light element (Li-ion) is divided into the other elements and consequently the ratio of total elements is still 100% [20]. Therefore, the inductively coupled plasma optical emission spectrometer (ICPOES) has been used to evaluate the concentration of the Li-ion in the present samples[21]. ICP-OES model Prodigy Prism High Dispersion (Teledyne Leeman ICPOES USA) that has numerous characteristics, as well as, it has a higher optical resolution of 0.0075 nm and high-grade accuracy. The instrument enables the user to obtain all wavelengths from 160 to 900 nm and optical focal length 800mm with excellent performance in complex matrices. It uses a 40.68 MHz for a free running, water-cooled oscillator that supplies long-term stability. In order to confirm the formation of the spinel ferrite phase, Fourier transform infrared (FT-IR) spectroscopy (NICOLET iS10 model instrument) is conducted over a wide range (350 - 3000 cm-1). The crystal structure of the samples is investigated via x-ray diffraction technique (XRD; Shimadzu XRD-6000). XRD patterns are obtained in the range of 2θ from 17o to 90o at room temperature. Cu Kα is used as a radiation source of wavelength λ = 0.15408 nm, scan rate 0.8o/min, operation voltage 50 kV and current 40 mA[22]. Information on the shape and size of the samples' particles are obtained using a high resolution scanning electron microscopy (SEM), JEOL JSM-5600 LV, Japan). Brunauer–Emmett– Teller (BET) method [23], most common method used to describe the experimental specific surface area [24]. The BET single point surface areas of LiZn0.25Co0.25Fe2xLaxO4 NPs were measured by a surface area analyzer (Nova 3200 Nitrogen Physisorption Apparatus USA) with liquid N2 as an adsorbate at -196 °C. UV diffusion reflectance spectra study was performed using a Jasco UV–Visible Spectrophotometer (V-670 PC). Finally, the magnetic parameters are measured using a vibrating sample magnetometer (MicroSense, Model: EZ-9) at room temperature from -20 kG to +20 kG. 3. Results and discussion: 3.1. Structural Studies: The composition of the LiCo0.25Zn0.25LaxFe2-xO4 (x = 0.0, and 0.1) samples is analyzed by EDX (Fig.1), where the presence of Co, Zn, O, La3+ and Fe is 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 worth noting that noted EDX is a rough technic for the figure out the elements in the samples, but it is unable to detect light elements such as Li (see Table S1 (provided in the supplementary material)[20]. 4

Therefore, the lithium ion was verified by ICP-OES for all the samples as arranged in Table S2 (provided in the supplementary material). It is obvious that the observed ratio of the Li-ion is almost constant and show good agreement with those of desired chemical formula [25]. The presence of sulfur ions (S) is attributed to residuals of sulfate groups that are involved in the synthesis process. In order to further illustrate the structural features of the samples, element mappings have been carried out selectively to the sample LiCo0.25Zn0.25Fe1.9La0.1O4 and the images are depicted in Fig. S2 (provided in the supplementary material). It is evident from these images that the elements Zn, Co, Fe, La, S and O exist, which agreed with preceding EDX results. Further, those elements are homogeneously distributed. ...………………………………. .Fig. 1…………………………… Fig. 2[A] represents the Rietveld refinements room temperature XRD patterns for the LiCo0.25Zn0.25LaxFe2-xO4 samples with x = 0.00 - 0.10; step x = 0.02. It is evident from this figure that the polycrystalline cubic spinel structure with space group Fd3m is dominated and it can perfectly indexed to JCPDS no. 03-0864[26]. Moreover, at higher concentrations of La3+, nanostructures with dual phase, i.e. cubic spinel and orthorhombic LaFeO3 perovskite with space group (Pbnm) appear and it can be indexed to JCPDS no. (74-1900) [27]. We interpret the existence of this phase to the difference in the ionic radius between La3+ (1.06 Å) and the Fe3+ (0.67 Å), where the Fe3+ ions are replaced by La3+ ions until the solubility limit is reached. After this limit, La3+ ions pile up around the grain boundaries and form orthorhombic LaFeO3 perovskite [26, 27]. …………………………………. Fig. 2[A -D]…………………………… Since the incorporation of rare-earth ions induces structural distortion by enhancing crystal imperfections due to their larger size and creates micro-strain, employing the Williamson–Hall equation to investigate the XRD patterns is more appropriate than using the Scherrer’s equation. The average crystallite size (DW-H) and the microstrain (ε) are two meaningful structure parameters that can be calculated using Williamson-Hall (W-H) method [28]. cos

=

.

+ 4 sin

(1)

where λ is the wavelength of the X-ray beam, β the full width at half maximum and θ the Bragg’s angle. A plot is drawn with 4 sin along the x-axis and cos along the y-axis for the studied samples as presented in Fig. S3 (provided in the supplementary material). From the linear fit to the data, the average crystalline size was determined from the y-intercept, and the strain, from the slope of the fit [18, 19]. On the other hand, D decreases sharply from 44 nm at x = 0.00 to reach 23.74 nm at x = 0.04 and beyond this concentration the situation is turned upside down as illustrated in Table 1. It is seen that the growth of the LiCo0.25Zn0.25Fe2O4 is limited by the incorporation of La3+ ions, leading to a relatively small crystallite size compared with a pure LiCo0.25Zn0.25Fe2O4 ferrite. This behavior attributed to the higher bond energy of La3+-O2-, as contrasted 5

with that of Fe3+-O2-, it is clear that more energy is needed to substitute La3+ ions into the B-sites. The energy demanded this process is satisfied at the expense of crystallization and consequently limits the growth of the crystallites and a smaller crystallite size is hence seen for the La3+ substituted samples. It is striking that for higher substitution amounts (x > 0.04) of La3+, the size of the crystallites slightly expand to 40 nm at x = 0.1. The increment in crystallite size is attributed to the replacement of the smaller ionic crystal radius of Fe3+ (0.064 nm) by the larger La3+ (0.106 nm) ions. With further increasing of x more and more La3+ will accumulate at the grain boundaries and forms a secondary phase of LaFeO3 [26, 27]. It is concluded from this investigation that the amount of energy demanded the incorporation of La3+ ions into B-sites is now being appropriated for the growth of the crystallites[26]. Furthermore, the samples reveal a negative microstrains which indicate that they undergo compressive strains [18, 19]. It is fascinating to see that the LiCo0.25Zn0.25Fe1.94La0.02O4 and LiCo0.25Zn0.25Fe1.94La0.08O4 samples have a positive strain. This means that the substitution of larger sized La3+ ions is expanding the crystal lattices by shifting the strain from compressive to tensile[26]. ………………………………. ….Table 1…………………………… Fig. S4 [A] (provided in the supplementary material) gives the peak position for the preferred plane (311) where the peak positions are shifted to a lower angle side with substitution of La3+, recommending increasing of the distortion in the lattice of pure cubic structure[29]. In other words, this shift in terms of the expansion of the unit cell and the increase of the lattice constant [18] as depicted in Table 1. This distortion is may be due to La3+ substitution which occupies interstices of the ferrite lattice, the increased amount of La3+ substitution may create a new phase of perovskite which arises concurrently with the cubic phase of ferrite. Fig. S4 [B] (provided in the supplementary material) illustrates the variations of the lattice constant (aexp) and average crystallite size (DW-H) of the system LiCo0.25Zn0.25Fe2-xLaxO4 as functions of La3+ concentration x. Overall, aexp increases gradually with increasing La3+ concentration up to x = 0.04 (8.3827 Å). The lattice parameters increase with the increasing La content; which is due to the difference between the ionic radii of both La3+ (1.06 Å) and Fe3+ (0.67 Å), this increase is also a good indication of the insertion of La3+ cations into the LiCo0.25Zn0.25Fe2O4 spinel structure. Consequently, La3+ has a strong site preference for the octahedral site which results in the expansion of unit cell[30]. Similar results are reported [31, 32]. Furthermore, it is observed that the value of the lattice constant is smaller for LiCo0.25Zn0.25Fe1.94La0.06O4 (8.3814 Å) LiCo0.25Zn0.25Fe1.90La0.10O4 (8.3883 Å) compared with LiCo0.25Zn0.25Fe1.92La0.08O4 (8.3894 Å). It is obvious from the Rietveld refinement patterns that the cell volume of the second phase for LiCo0.25Zn0.25Fe1.92La0.08O4 sample (243.2579 Å3) is higher than that of LiCo0.25Zn0.25Fe1.94La0.06O4 (241.8027 Å3) and LiCo0.25Zn0.25Fe1.90La0.10O4 (242.0242 Å3). The substituted content of the La3+ ions does not enter into the LiCo0.25Zn0.25Fe2O4 sub-lattices and form more of the secondary LaFeO3 phase, and therefore a reduced value of the lattice constant of spinel ferrite is seen[26]. Besides, the

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LiCo0.25Zn0.25Fe1.92La0.08O4 sample has a tensile strain in contrast to the other two samples that have a compressive strain as observed in Table 1. The phase analysis and structural parameters such as lattice constant obtained from Rietveld refinement of LiCo0.25Zn0.25Fe2-xLaxO4 are outlined in Table 1. The relative percentages of the orthorhombic LaFeO3 perovskite phase with space group (Pbnm) and the spinel phase with space group Fd3m and the variation of lattice constant “a” for spinel phase and “a”, “b”, and “c” for the phase of perovskite have been listed in Table 1. It is clear from this table that the relative percentages of the orthorhombic LaFeO3 phase increased as La ion content increased in the samples and reached to 15.37% for LiCo0.25Zn0.25Fe1.90La0.1O4 NPs[29]. The allowed Braggs positions for the Fd3m space group are marked as vertical lines. All the observed peaks are allowed Bragg 2 positions. The quality of profile fit was assessed by the value of reliability factors in Rietveld refinement (Table 1). The low values of R-factors suggest the excellent quality of profile fit[33]. The hopping distance of magnetic ions between the A and B sites is calculated as follows[30, 34]: −

=

√

−

=

√!

−

=

√""

(2) (3) (4)

#

It is seen from Table S3 (provided in the supplementary material) that the hopping distance expands slightly as the La3+ content increases, which is consistent with the lattice constant increment as mentioned earlier and elsewhere[30, 34]. The FTIR spectra of the pure and La3+ doped LiCo0.25Zn0.25Fe2O4 ferrites are illustrated in Fig. 2[B]. The positions of the vibrational bands are obtained and listed in Table S3 (provided in the supplementary material). In general, spinel ferrites show two essential vibrational bands, namely υ1 and υ2 and are corresponding to the stretching vibration of tetrahedral groups (A-site) and the stretching vibration of octahedral groups (B-site), respectively [18, 19]. Clearly, from Fig. 2[B], the insertion of La3+ ions into the structure of LiCo0.25Zn0.25Fe2O4, shifts the bands of the tetrahedral and octahedral sites towards the lower frequency side. The substitution of La3+ ions into the B-site results in a migration of an equal number of Li+ and Co2+ ions to the A-sites, consequently an equal number of Fe3+ ions also migrate from A-sites to B-sites to ease the strain[26]. The ionic radii of the Co2+ (0.78 Å) and Li+ (0.74 Å) are higher than that of the Fe3+ ion (0.67 Å), which increases the ionic radii of the A sites. Likewise, the ionic radii of the B sites increase due to La3+ ions settlement. This augmentation in the ionic radii of the A and B sites reduces the fundamental frequency[26]. The bands appeared at 3400 cm-1 and 1600 cm-1 are due to the presence of hydroxyl groups in the samples where the peaks 1118 cm-1 are related to the bond formation [8-10, 18, 19].

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The force constant for the tetrahedral site (Ft) and octahedral site (Fo) was determined using the position of υ1 and υ2 bands and listed in Table S3 (provided in the supplementary material). Their values fluctuated with increasing the La3+ content in the samples, which may be due to the solubility limits and the induced stresses[35]. The Debye temperature ΘD is the temperature correlates the elastic properties with the thermodynamic properties such as thermal expansion, thermal conductivity, specific heat, and lattice enthalpy. The value of Debye temperature as a function of La3+ is illustrated in Table S3 (provided in the supplementary material). It is obvious that the Debye temperature decreases with increasing La3+ content, which outweighs a rigidity reduction as reported earlier in [36]. 3.2. Morphological study Fig.S5 (provided in the supplementary material) presents SEM micrographs of LiCo0.25Zn0.25Fe2-xLaxO4 NPs. Overall, the micrographs depict inhomogeneous distribution of nanoparticles (nanopillar arrays) over the total area of the image. At higher magnifications, the samples obviously comprise nanopillars. These nanopillars are lined up in a high density phase forming many randomly orientated pores. Table S4 (provided in the supplementary material) illustrated that the BET single surface area increases from 7.48 m2.g-1 for the pristine LiCo0.25Zn0.25Fe2O4 NPs to 18.70 m2.g-1 at x = 0.02 and then decreased to 5.61 m2.g-1 at x = 0.10. The specific surface area is behaves opposite trend to the cell volume of the spinel ferrite. So, it is expected that the LiCo0.25Zn0.25Fe1.98La0.02O4 sample possesses large surface area as presented in Table S2. Similar behavior is detected in literature [18, 19] 3.3. Optical Properties Absorbance and band gap are determined based on ultraviolet visible and diffuse reflectance spectroscopies as shown in Fig. S6 (A) and Fig. S6 (B) (provided in the supplementary material), respectively[37, 38]. The optical absorption coefficient (α) is calculated using reflectance data according to the Kubelka-Munk equation [39]: $ %&' =

[ " )% ']+ ! )% '

,

(5)

where R(λ ) is the percentage of reflected light. The incident photon energy (hν) and the optical band gap energy (Eg) are related to the transformed Kubelka-Munk function[40]: F(R) hυ =A(hυ‒Eg) r , (6) where F(R) is absorption coefficient, hυ is the photon energy, A is a constant that depends on the transition probability and the exponent r depends on the nature of the transition ( r = 2 or 3 for indirect allowed and forbidden transitions, respectively, and r = 1/2 or 3/2 for direct allowed and forbidden transitions, respectively)[41]. Our calculations outweigh the indirect transition and the band gap is determined by plotting (F%R'hν)0.5 as a function of photon energy (ℎν) as demonstrated in Fig.2 (C). Eg of the bare LiCo0.25Zn0.25Fe2O4 sample equals to 1.94 eV, which increases gradually until it reaches a maximum value with 2.05 eV at x = 0.06 and after this concentration it drops sharply to 1.59 eV. We interpret this trend to the agglomeration of La3+ around the grain boundaries at higher content x > 0.06. 8

Furthermore, the difference in the bond energy between La3+ and O2- and that of Fe3+ and O2-, where more energy is required to combine La3+ ions into lattice in contrast to Fe3+ and O2-. 3.4. Magnetic Properties Fig.2D exhibits the magnetic hysteresis loops for LiCo0.25Zn0.25Fe2-xLaxO4. Narrow hysteresis loops are observed, which is a sign for superparamagnetic nature. The fundamental magnetic parameters such as the saturation magnetization (Mr), remanence (Mr), coercivity (Hc), and squareness (Mr/Ms) are obtained from the curves and listed in Table S5 (provided in the supplementary material). From this table, Ms drops with increasing La3+ content in the sample. This can be clarified firstly in the light of diamagnetic nature of La3+ ion. Where La3+ has zero electrons in the 4f level (zero magnetic moment) and subsequently it has zero contribution to the exchange interactions with its neighboring ions[42], which reduces the magnetic superexchange interactions between the cations in the A and B sites. Secondly, structural parameters such as the reduction of the crystallite size and the increment of the surface area give rise to notable distortion in the surface. Atoms at the surface expose to strains, which results in vacancies, difference of interatomic spacing and low coordination numbers[43]. These interactions are a sort of spin disorder, which leads to the reduction of saturation magnetization [43]. The coercivity Hc behaves in contrast to the average particle size DW-H. The behavior of Hc is significantly dependent on the average crystallite size, thus when DW-H decreases from 48 nm (x = 0.00) to 27 nm (x = 0.04), the Hc increases from 90 G to 132 G. Likewise, as DW-H increases from 37 nm (x = 0.06) to 42 nm (x = 0.10), the Hc decreases from 132 G to 110 G. The situation mentioned above can explained in terms of the coercivity variation with the crystallite size [42]: 23 = 4 +

5

(7)

where e and f are constants. The values of squareness ratio (Mr/Ms) are calculated to probe the effect of nano sized domains. We found that the values of the magnetic squareness under 0.5 refer to the magnetic multi-domains while beyond 0.5 the single magnetic domains are dominant. These values confirm the superparamagnetic behavior of La3+ substituted LiCo0.25Zn0.25Fe2O4 [44]. The magnetic moment per formula unit (ηB in Bohr magneton) has been calculated from the values of saturation magnetization MS using the follow relation [44]: η =

78 79 :"

;

(8)

<= >?

where, Mw is the molecular weight of the sample (mole/g), MS is the saturation magnetization (A.m2.kg-1), Na is the Avogadro's number (NA= 6.02x1023 mol-1), @ magnetic moment of electron ( @ =9.27x10-24A.m2). The value of ηB reduced with rising in the La3+ ions content, which dependent on the obtained Ms values according to Neel's model for ferromagnetism[45]. Where, in Neel's 9

model, the net magnetic moment in spinel ferrites is the resultant magnetic moment due to the antiferromagnetic ordering between A-site and B- site magnetic moments. The origin of disordered surface spins is attributed to broken exchange bonds, lack of long‐ range ordering and high surface anisotropy for ferrite nanocrystals. It is noted that the presence of a magnetically disordered surface layer demands more field energy to saturate. The reduction of the Bohr magneton ηB and the magnetization as the increase of La content can be interpreted based on nonlinear or canted spin ordering[46]. The produced microstrain in the tiny crystal due to radii mismatch leads to a noncollinear arrangement of surface spins in the nano range, which also produces a notable supplying in decreasing saturation magnetization due to the migration of Li+ and Co2+ ions to the A-sites leading to the reduction of Bohr's magneton. According to random canting model, the substitution of diamagnetic cations (such as La ions) in one sublattice of ferrimagnet leads to spin canting in the other sublattice resulting in a reduction in total magnetization per formula unit @ A in terms of Bohr's magneton of spinel ferrite and other symbols have their usual meaning which is presented by[46]: @ A = @ %cos < CA >' − @

(9)

Where @ and @ are magnetic moments of A and B sites and CA is canting angle. The presence of canted surface spins also affects the magnetic properties of nanosized spinel ferrites. In other words, the incorporation of La ions into the B-site of the LiCo0.25Zn0.25Fe2O4 spinel ferrites leads to spin canting in A-site due to the migration of Li+ and Co2+ ions to the A-sites. This behavior resulting in a reduction of total magnetization per formula unit and consequently reduction of the Bohr's magneton[47]. The anisotropy constant values K for the present samples were determined through Brown’s relation[48]: D=

EF GH

(10)

. #

It is evident from Table S5 (provided in the supplementary material) that the anisotropy constant increases, K, with the increasing La3+ content in the sample due to its high anisotropy constant value as compared to this of Fe3+ ions. The increase in K means the increase in domain wall energy[48]. The reduction of magnetic moment (@ B) with La3+ content may be described by the lattice defects and lower magnetic superexchange interactions between A and B sites in the spinel ferrites. As La3+ ions have larger ionic radii than Fe3+, the La3+ replacement may deform the lattice and reduces the homogeneous composition, so it produces the deterioration in the magnetic moment. Moreover, due to their large ionic radii, La3+ ions may substitute Fe3+ on B-site [26], therefore, decreasing the magnetic moment as well as reducing the strong Fe3+–Fe3+ negative interaction rising from La3+ substituting due to antiferromagnetic coupling [26]. The disturbance befell in the spinel ferrite lattice, by substitution of magnetic Fe3+ ions with nonmagnetic La3+ ions could be expected because the La3+ ions reduce the ferromagnetic regions at the expense of the nonmagnetic one[34]. The variation in coercivity, with La3+ content, maybe due to the stronger L–S coupling and weaker crystal field due to La3+ which induces the stronger 10

magnetocrystalline anisotropy. However, it is also observed that the coercivity values decrease with the presence of the secondary LaFeO3 phase for (x =0.08). Besides, the reduction in coercivity may be due to the reduction of Fe3+ at B-sites and the migration of Li+ and Co2+ ions to the A-sites. Furthermore, it is thought to initiate the pinning of domain walls of the antiferromagnetic phase along with the existing ferrimagnetic phase to control the coercivity[34]. The domain wall motion of a ferrimagnetic phase into an antiferromagnetic phase is usually complex and in the existing case, this mechanism probably goes together to reduce Hc (x = 0.08) as presented in Table S5 (provided in the supplementary material). The anisotropy constant (K) determined shows similar behavior to that of seen for coercivity variation with the La content. The reduction in K value for La3+ concentration (x = 0.08) in LiCo0.25Zn0.25Fe2-xLaxO4 is ascribed to the non-incorporation of the amount of La3+ into the LiCo0.25Zn0.25Fe2O4 lattices and also due to the migration of Li+ and Co2+ ions from the B- site to A-site. S. G. Kakade et al [34] have independently reported the similar results on the anisotropy constant and coercivity variation with the rare earth content. The observed coercivity for Co1.1Fe1.93+ xErxO4 NPs, increases from 999 Oe (x = 0.00) to 1984 Oe (x = 0.10) with Er concentration and then decreases for higher Er3+ content (x = 0.15). Also, the anisotropy constant (K) calculated shows the similar behavior to that of observed for coercivity variation with the erbium content. The results reported in this study in the regard of the pure LiCo0.25Zn0.25Fe2O4 system and La3+ doped LiCo0.25Zn0.25Fe2O4 samples are in a good agreement with [44]. Lately, K.M. Srinivasamurthy et al.,[45] have synthesized Ni2+ doped cobalt ferrite nanoparticles. They found that the saturation magnetization in the range (41-76) emu.g1 . Also, the coercivity decreased from 1049.6 to 162.5 Oe with increasing the Ni content. They assumed that the small values of coercivity with moderate saturation magnetization indicate that the samples can be used as promising materials to achieve low core loss in the transformer. Also, Majid Niaz Akhtar et al., [44] have studied the influence of Cu substitution on the structural and morphological characteristics of Ni– Zn ferrites. The saturation magnetization in the range (14-102) emu.g-1 and the values of coercivity in the range (14-50) Oe and the sample show soft nature. These properties made of Cu substituted Ni–Zn ferrite promising materials for many industrial and domestic applications such as components of transformers core, and switching. Finally, Muhammad Junaid et al., [49]have synthezed Tb and Dy doped Li-Ni nano-sized ferrites by micro-emulsion technique. The saturation magnetization, and the coercivity are calculated and these lie in the range (54–27) emu/g, and (120–156) Oe. The smaller magnetic suggested the possible utility of these nano-materials in switching and high frequency applications. Overall, we assume the superparamagnetic nature and low values of saturation magnetization and coercivity of La3+ doped LiCo0.25Zn0.25Fe2O4 samples may be promising for variety of applications such switching, and transformers' cores applications.

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4. Conclusion In conclusion, a spinel ferrite system of LiCo0.25Zn0.25Fe2O4 (pristine sample) is successfully synthesized using sol-gel method. Subsequently, La3+ ions with different concentrations LiCo0.25Zn0.25Fe2-xLaxO4 with x = 0.00 - 0.10; step x = 0.02 are 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 comprises particles in the nano-scale. These particles decline from 48 nm at x = 0.00 to 27 nm at x = 0.04, beyond this concentration the situation is turned upside down due to the ionic radii difference between iron and lanthanum ions. The coercivity Hc behaves in contrast to the average particle size (D). when D decreases from 48 nm (x = 0.00) to 27 nm (x = 0.04), the Hc increases from 90 G to 132 G. Likewise, as D increases from 37 nm (x = 0.06) to 42 nm (x = 0.10), the Hc decreases from 132 G to 110 G. Further, low coercivity (130. 740 – 110. 630) of the doped samples makes them potential candidates for transformers' cores. 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. Declaration of Competing Interest There are no conflicts to declare.

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Graphical Abstract

The structure, optical, and magnetic properties of La3+ doped LiZn0.25Co0.25Fe2O4 nanocrystal have studies. With further increasing of x more and more La3+ will accumulate at the grain boundaries and forms 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 G – 110. 630 G) of La3+ doped LiZn0.25Co0.25Fe2O4 samples are suitable to be applied in transformers core.

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