Tuning the Structural and Magnetic Properties of the Stuffed Framework Structures MeFe2O4 (Me = Ni, Ca, and Sr)

Spinel ferrite nanoparticles (NPs), have received a lot of attention in medical applications. Therefore, facile synthesis of ferrite NPs of numerous shapes and sizes using the citrate autocombustion technique was utilized in this article. A series of ferrite with the general formula MeFe2O4 [Me = nickle (Ni), calcium (Ca), and strontium (Sr)] are synthesized with varying average ionic radii and cation disorder on the A-site. The structural and morphological characterization of the prepared samples was performed using XRD, HRTEM, FESEM, EDAX, XPS, and Raman analyses. The phase transformation from cubic (Ni) to orthorhombic (Ca) to monoclinic (Sr) was also revealed by XRD. Accordingly, HRTEM images demonstrated nanoparticles in orthorhombic and monoclinic shapes, which are inconsistent with XRD analyses. The coercive field HC for monoclinic SrFe2O4 is ≈ 42 times larger than the Hc for NiFe2O4 with a cubic structure. This deviation in HC compared to the cubic shape particles can be coupled to the shape anisotropy present in SrFe2O4 and refers to the presence of a preferred magnetization direction within the material. The use of monoclinic SrFe2O4 NPs as antifungal activity agents is noteworthy due to their advantages in terms of surface area, efficacy, and biodegradability.


Introduction
In recent eras, the increase in microorganisms resistance to antibiotic treatments has become a concern because of the potentially harmful effects on human health. Among the fungal pathogens, Candida albicans and Aspergillus niger are the most prevalent microorganisms involved in fungal infection [1,2]. The two pathogens cause numerous infections in the mouth, skin, blood, throat, ear, heart valves, and severe lung disease. Additionally, some fruits and vegetables have also been affected [3,4]. Therefore, it is crucial to develop novel alternatives to prevent the abovementioned adverse effects. One of the novel approaches proposed is the use of nanomaterials to inhibit fungal growth.
Meanwhile, the physical properties of nanomaterials depend upon their precise composition, shape, and size. Therefore, numerous shapes and types of nanoparticles based on different sorts of materials have been reported [5,6]. As illustrative examples, MgFe 2 O 4 , SrFe 2 O 4 , and BaFe 2 O 4 have cubic [7], hexagonal [8] and orthorhombic [9] structures, respectively. Additionally, barium ferrite can also be synthesized using the hydrothermal technique. It formed in a monoclinic system with the space group C2/m [10]. Among the different nanostructures, spinel ferrite nanoparticles have a distinctive morphology and desirable features. The combination of their distinct characteristics and the abovementioned fact has caused an enormous surge in the number of inquiries and investigations into their innovative applications in environmental pollution monitoring and biomedical applications. However, various difficulties in analyzing the merits and drawbacks of spinel ferrite NPs and issues particular to each application were highlighted due to insufficient in-depth understanding of spinel ferrite characteristics [11,12]. Spinel ferrite nanoparticles have an intriguing chemical composition and architecture. The complex oxide of spinel ferrite crystal structure is face-centered cubic, and the unit formula is MeFe 2 O 4 . Several studies coupled a trivalent cation (Fe 3+ ) with a transition divalent cation (Me = Mn, Co, Ni, Zn) [13][14][15]. However, few studies have been conducted on alkali and alkaline divalent cations. Based on the crystallographic structures, the ferrite NPs 1 3 can be categorized as orthorhombic, hexagonal, and cubic [16][17][18]. The cubic MeFe 2 O 4 (Me = Ni 2+ , Fe 3+ , Co 2+ , Mg 2+ ) and orthorhombic MeFe 2 O 4 (Me = Ba 2+ or Ca 2+ ) magnetic materials are promising for various applications [19][20][21][22][23]. Cubic ferrite NPs such as NiFe 2 O 4 have a spinel structure and exhibit magnetically soft behavior [24]. In order to customize the physical properties under optimum environmental circumstances, researchers are continuously attempting to implement a simple and effective technique for producing nanocrystalline ferrites. Various approaches have been used to synthesize nanostructured ferrite materials, such as citrate auto-combustion [25,26], sol-gel [27], co-precipitation [28], electrospinning technique [29], high-energy milling [30], hydrothermal precursor [31], and microwave combustion method [32]. Nevertheless, the aforementioned approaches have complicated handling reaction processes, are expensive, time-consuming, and have low yield. The citrate gel method stands out as one of the best and simplest among them owing to its low cost, straightforward synthesis procedure, homogeneous mixing, highly crystalline output powder, non-toxicity, eco-friendly, high purity, and higher reaction efficiency compared with the other physicochemical methods [33]. The architecture adopted by the various MeFe 2 O 4 compounds actually depends on the size of the Me cation, cation charges, the electrostatic contribution to the lattice energy, and crystal field effects. Therefore, producing homogenous ferrite NPs with various ionic radii, different magnetic properties, and numerous crystallographic structures is a great challenge. Accordingly, an interstitial doping mechanism in the spinel structure is demonstrated for the three investigated cations under study (Ni 2+ , Ca 2+ , and Sr 2+ ) with specific characteristics involving a shift away from the spinel (Ni 2+ ) to orthorhombic(Ca 2+ ) and monoclinic (Sr 2+ ). Furthermore, the second challenge is investigating the synthesized samples' application potential.

Samples Preparation
As reported previously, MeFe 2 O 4 nanoparticles were synthesized using a simple citrate auto combustion procedure with slight modifications to that previously reported, as illustrated in Fig. 1 [34]. First, stoichiometric amounts of high purity (99.9%, Fisher chemicals) iron, nickel, calcium, strontium nitrates, and citric acid were weighed and dissolved in distilled water and stirred well using a magnetic stirrer. After that, the pH of the solution was adjusted and stirred for approximately 1 h at 80 °C followed by drying at 200 °C. The citric acid [C (OH) (COOH) (CH 2 COOH) (2H 2 O)] was used to improve homogeneous mixing with 1:1 molar ratio to total metal nitrates. Then the powders were grinded and heated at 800 °C, 950 °C, and 950 °C for Ni, Ca, and Sr, respectively, with a heating rate of 4 °C/min.

Characterizations and Analyses
The structures were tested by an X-ray diffractometer (XRD) using Diano Corporation of target Cu-Ka (k = 1.5418 Å). The morphology and nanostructure of the samples were investigated by field emission scanning electron microscopy (FESEM) using SEM Model Quanta 250 FEG attached with an EDAX unit (energy dispersive X-ray analysis) and highresolution transmission electron microscopy (HRTEM). Additionally, the Raman spectra (RS) were measured in the wavenumber range of 100-800 cm −1 at room temperature (25 °C) using Raman spectrometry (Horiba LabRam HR Evolution, Horiba, Ltd, UK). Furthermore, the chemical states of various elements in the produced samples were evaluated using X-ray photoelectron spectroscopy (XPS) with monochromatic X-ray Al K-alpha radiation. At room temperature (300 K), the magnetization (emu/g) was measured using a vibrating sample magnetometer (VSM) model, Lake Shore 7410.

Antifungal Activity Test
The effects of antibiotic nanosamples on Aspergillus Niger and Candida albicans growth were assessed using the Sabouraud dextrose agar medium and the well diffusion method  [35]. Nystatin which had an excellent effect against fungi strains in 12 of 16 cases, was used as a standard drug for the fungal strains. Additionally, it is a polyene macrolide antibiotic that is sensitive to many molds and yeasts, including the Candida species. A major advantage of nystatin is that it is not absorbed by the intact skin [36]. DMSO was also utilized as a solvent (negative) control. Furthermore, these plates were incubated at 37 °C for 24 h in closed conditions and the inhibition zone was measured.

XRD Analysis
The crystal structure is designated by giving the values of both lattice constants LCs (a, b, and c) and the angles between the axes. Additionally, the atomic positions can be expressed in units of the (LCs) and site occupancy by definite atoms corresponding to the chemical formula of prepared samples. Therefore, the repetition of the unit cell in three directions can generate a complete image of the entire crystal lattice by knowing the symmetry properties of each atomic position.
Generally  [37][38][39]. This constitutes a very distinctive network similar to that found in perovskite-related compounds. Figure 2c shows that SrFe 2 O 4 crystallizes in a monoclinic architecture with space group P1121/n which has a three-dimensional connected FeO 4 -tetrahedra framework structure. The anionic [Fe 2 O 4 ] 2framework and the irregular seven-fold coordinated strontium cations residing in the voids balance the framework's charge.
All atoms in SrFe 2 O 4 occupy general positions, with each of the four crystallographically non-equivalent Fe atoms positioned in the center of a distorted tetrahedron designed by oxygen atoms (Fig. 2f). It also appears that the Ca with an orthorhombic phase accepts less Fe substitution compared to Ni. This will be noticeable in EDAX analysis with a low reduction in the unit cell volume. Additionally, the lower affinity of Fe for the orthophase is coupled with a greater affinity for Sr with the monoclinic structure. Table 1 shows approximately the same basal lattice contraction (a, b) when inserting either Ca 2+ or Sr 2+ , but obviously shows a significant increase in the "c" LC in the case of Sr 2+ insertion.
The average crystallite sizes D of the samples are detected from the full width at half the maximum β of the mean diffraction peak using Scherrer's equation [11]. The calculated D is presented in Table1.
Moreover, since the peak broadening is influenced by the crystallite size (L) and lattice strain (ε), the Williamson-Hall W-H method is employed for additional examination using the following relation [40]: From the intercept and slope of the W-H plot in Fig. 2g-I, the L and ε are determined and presented in the Table. As shown in the table, a slight deviation between the calculated crystallite size using Scherrer's equation and that from the W-H plot exists, and this variation is due to the effect of lattice strain. Additionally, the "D" decreases with the increasing ionic radii of Me cations, as shown in the table. This designates the creation of "ε" inside the lattice owing to the ionic size mismatch between Ca 2+ and Sr 2+ compared to Fe 3+ . Therefore, this leads to local structural disorder and inhibits the nucleation rate as a result of decreasing the crystallite size.
The samples' quality is detected by evaluating the dislocation density (δ). The dislocation (the length of dislocated lines per unit volume of the crystal lattice) is calculated using the following equation [41]: As shown in the Table δ increases for the samples with large ionic radii (Ca, Sr) samples compared to the NiFe 2 O 4 sample. This increase designates a reduction in crystallite and crystallinity. Simultaneously, it provides more energy-friendly locations for nucleation, causing an increase in the number of nucleation sites. Therefore, as the rises, the crystallinity deteriorates and the crystal grains become finer. Finally, from the previous discussion, one can conclude that a crystallographic description of the investigated samples indicates that the Fe cation is located at the A&B sites for Ni with a lower ionic radius, and is shifted to the BO6 site in the case of Ca. This shift implies an increase in the Fe 3+ coordination CN from fourfold coordination (four oxygen atoms from the A-site) to sixfold CN (six oxygen atoms from B sites BO6) and a third oxygen atom from the SrFe 2 O 4 .
Additionally, there are two types of defect structures: (i) oxygen deficient (or excess metal), (ii) metal deficient (or excess oxygen). In the present case, metal deficiency is predominant. This deficiency (generated here during the transformation from cubic NiFe 2 O 4 to orthorhombic CaFeO 3 ) should be compensated through the formation of either metal vacancies or oxygen interstitials (excess oxygen).

FESEM Micrographs
The FESEM micrographs display the samples structure, morphologies, grain size, and porosity (Fig. 3a-c). In the case of the NiFe 2 O 4 sample, some irregularly cubic particles are observed (Fig. 3a) with high agglomeration, possibly attributed to the Ni 2+ ions' magnetic nature.
The CaFe 2 O 4 sintered at 950 °C is susceptible to yielding the NPs with homogeneous particle size and regular shape. The porous architecture of the interconnected grains is observed (Fig. 3b). Whereas the grains keep their anisotropic  shape. Moreover, the morphology of each grain has a regular shape with homogenous particle size, and most of the particles agglomerate together at the same time. However, the SrFe 2 O 4 sintered at 950 °C has NPs with inhomogeneous particle size and an irregular shape. The presence of crystallized dense grains is the key feature of the SrFe 2 O 4 sample (Fig. 3c). Some grains are necked in shape; necking is owing to the particles´ diffusion during sintering.

EDAX Analysis and Mapping Images
EDAX analyses and the corresponding elemental mapping images were employed to study the composition, stoichiometry, and homogeneity of the prepared compounds (Fig. 4a-c). Intense O, Ni, Ca, Sr, and Fe peaks are detected, and no impurity were found in the EDAX spectrum, confirming the purity of synthesized samples.
Additionally, as shown in the mapping images, the elements are uniformly distributed, indicating that the samples are successfully prepared. Figure 5a, b displays the HRTEM images of the CaFe 2 O 4 ferrite nanoparticles. The images show irregular orthorhombic shape with little agglomeration. Similar to the FESEM image, a continuous connection between the particles is detected (Fig. 3b). Furthermore, the enlarged view presented in Fig. 5b confirmed the formation of an orthorhombic structure, as we can observe along the a-axis. The SAED pattern (Fig. 5c) taken on an individual CaFe 2 O 4 represents the lattice fringes of CaFe 2 O 4 belonging to the crystalline planes of orthorhombic CaFe 2 O 4 , which is consistent with the obtained XRD pattern. In contrast, a highly agglomerated dense particles with irregular shapes are observed in the SrFe 2 O 4 sample (Fig. 5d, e). Further, the high agglomeration of the particles is attributed to the magnetic nature of the sample, as we can see later.

HRTEM Images
The enlarged view in Fig. 5e signifies the monoclinic structure of SrFe 2 O 4 nanoparticles along the c-axis. The selected area diffraction data in Fig. 5f, indicates the lattice planes of the P1121/n monoclinic phase, which can be correlated with the results obtained from the XRD pattern.

Raman Analysis
To further probe any subtle changes in the structure as a consequence of changes in the composition, we obtained Raman spectra of the testified samples. Figure 6a-c represents the room temperature Raman spectra of the prepared ferrite nanostructures. As shown, all the samples reveal sharp and intense Raman bands demonstrating their high crystallinity. The Raman spectrum of NiFe 2 O 4 (Fig. 6a) reveals the presence of five first-order Raman active modes (1A 1g + 1E g + 3T 2g ) corresponding to space group O7 h (Fd-3 m). Notably, there are two well-defined intense bands near 478.84 and 694.22 cm −1 and three less intense bands near 203.37, 327.12, and 566.12 cm −1 . Additionally, the higher energy band at 694.22 cm −1 corresponds to the fundamental A1g modes concerning the symmetric stretching of an oxygen atom around iron in a tetrahedral void [42]. Furthermore, the bands appear around 327.12, 203.37, 478.84, and 566.12 cm −1 owing to the symmetric and/or antisymmetric bending of oxygen atoms in the Ni/ Fe − O bond at octahedral voids. These bands correspond to the E g and 3T 2g modes, respectively [43]. The experimental Raman data for CaFe 2 O 4 (Fig. 6b) consist of broad bands between 200-750 cm −1 . The lines of A g and B 2g symmetry are easily identified by their appearance in the The low energy bands for CaFe 2 O 4 are revealed up to 800 cm −1 , labelled according to the A g and B 2g symmetry of the Pbnm a space group. As shown in the figure, Ca vibrations create modes below 200 cm −1 , denoted as (A g ). Oxygen octahedral tilt modes were between 200 and 300 cm −1 , oxygen octahedral bending vibration (B.V.) ranged between 400 and 450 (B 2g ) cm −1 , and oxygen stretching vibrations were above 500 cm −1 (S.V.) [44]. Figure 6c presents the results of the RS for SrFe 2 O 4 . As mentioned earlier in the explanation of XRD, the SrFe 2 O 4 has a monoclinic structure with space group P1121/n. Therefore, it is predicted to have 12 sets of Raman active vibrational modes at Γ points as shown in the figure. Γ = 6 A g + 6B 2g . Accordingly, the SrFe 2 O 4 phase are located in the position of 178,183, 248, 287, 319,376,475, 502, 517,549, 610, 626, and 745 cm −1 as observed.
Group theory calculations anticipate the activity of 24 modes, twelve of which with A g symmetry and twelve with 7A g + 5B 1g + 7B 2g + 5B 3g . B g symmetry, when focusing solely on Raman-active modes and assuming a P1121/n crystal structure with the same volume as the primitive unit cell. Several strong Raman peaks are associated with the fingerprint position of the monoclinic SrFe 2 O 4 phase.  (Fig. 7b, f). The Fe 2p 3/2 peak can be deconvolved into two bands consistent with the presence of Fe 3+ /Fe 2+ oxidation states [45]. Besides, the existence of Fe 2+ is recognized by feature peaks detected for CaFe 2 O 4 and SrFe 2 O 4 at 715.99 and 715.44 eV respectively [46][47][48]. Furthermore, two peaks were observed in the Ca 2p spectrum (Fig. 7c): Ca 2p 3/2 and Ca 2p 1/2 orbitals [49]. The binding energy values indicated that the Ca element existed as a Ca 2+ cation [50]. Figure 7g displays the spectrum of the Sr 3d orbital. Two spin-orbit doublet peaks were designated to the Sr 3d 3/2 and Sr 3d 5/2 orbitals [51]. According to the binding energy values, the Sr element existed as an Sr 2+ ‫‬ cation [52]. The oxygen (1s) core-level spectrum component for all samples (Fig. 7d, h) is split into multiple peaks. The low binding energy < 530 eV was owing to the Fe/ Ca-O and Fe/Sr-O bonds [53]. Higher binding energy peaks > 530 eV can be attributed to the surface oxygen species of lattice defects [53,54]. According to ZHAO Kun et al. [55], the observed peaks can be due to the adsorbed oxygen (O 2 2− , O 2 1− ), which is interrelated to the oxygen vacancies.

Magnetic Measurements
Calcium nanoferrite CaFe 2 O 4 has an orthorhombic structure where the Fe 3+ is bonded octahedrally with O 2− ions. CaFe 2 O 4 is less popular compared to spinel ferrite. In fact, SrFe 2 O 4 exhibits numerous properties compared to NiFe 2 O 4 and CaFe 2 O 4 , such as the high capacity of magnetization, highly stable, and high coercivity. Figure 8a-c depicts the hysteresis loops of the investigated samples at room temperature RT, and Table 2 summarizes the obtained magnetic parameters such as saturation magnetization (M s ), retentivity (M r ), coercivity (H c ), anisotropy constant (K), hysteresis loss, and magnetic moment. Figure 8a represents a traditional hysteresis loop for soft magnetic NiFe 2 O 4 with an "S" shape [56]. In this structure, the A site is entirely occupied by Fe 3+ ions while Ni 2+ and Fe 3+ ions occupy the B site. It demonstrates ferrimagnetism originating from antiparallel spins between Fe 3+ (at the A site), and Fe 3+ and Ni 2+ (at the B site) see inset of Fig. 8a.
The magnetization curve for the CaFe 2 O 4 sample (Fig. 8b) demonstrates antiferromagnetic behavior, likely explained based on the orthorhombic structure with the presence of corner-and edge-sharing distorted FeO6 octahedra. Therefore, the d orbital configuration of the B-site atoms in the double chain is projected to orientate toward neighboring B-site atoms and overlap, implying that the direct charge transfer through the d (e g , t 2g ) orbitals will result in a strong exchange interaction.
The anti-ferromagnetic ordering arises from the indirect exchange interaction between Fe 3+ ions through O 2− ions. Additionally, due to the Dzyaloshinky-Moriya-interaction, a weak ferromagnetic moment is exhibited in the prepared CaFe 2 O 4 . The hysteresis plot approves weak ferromagnetism The measured M-H loop for SrFe 2 O 4 , shown in Fig. 6c, indicates that the sample possesses a hard-ferromagnetic nature. This behavior can be explained based on the crystal structure of SrFe 2 O 4 where the Fe atom is placed in four non-equivalent tetrahedral sites. Accordingly, the four different spin configurations are the reason for the ferrimagnetic state. SrFe 2 O 4 's polyhedral connectivity forms a three-dimensional network of six-, four-, and eight-membered rings. The arrangement of up (U) and down (D) pointing apices within a single ring is unequally UDDU, considering the four-membered rings, as predicted by Z Javdani et al. [57]. The effects of magnetic anisotropies or longrange interactions oppose frustration. Furthermore, the sample has a ferrimagnetic interaction between Fe 3+ and Fe 2+ .  The change of A cations' ionic radii can directly affect the anisotropy by modifying structural factors. The substitution with cations having different ionic radii changes the crystal field of ligands of ionic radii on the crystal field in spinel ferrite [58]. The small squareness values (M r /M s < 0.5) are also recognized for the surface spin disorder effects. This is owing to the canted spin on the surface of nanoparticles. The variety of H c values (130-5396) depends on numerous factors such as morphology, magneto crystalline anisotropy, size distributions, and the exchange coupling. Therefore, a coercive field H C of 130 Oe is observed in NiFe 2 O 4 cubic spinel NPs and a maximum coercivity of 5396 Oe is observed in SrFe 2 O 4 . The H c for monoclinic SrFe 2 O 4 is ≈ 42 times larger than the that for NiFe 2 O 4 with a cubic structure. This deviation in H C compared to the cubic shape particles can be coupled to the shape anisotropy in SrFe 2 O 4 and refers to the presence of a preferred magnetization direction within the material, which ultimately determines the magnetic properties of the sample. The observed variation in H C although the size of the cubic structure is smaller due to the coherent rotation of spins in the entire stack of nanodiscs. Furthermore, the saturation magnetization decreases as the size of the samples decreases because of an increase in NPs surface/volume ratio and absorption of ligands at the surface of the nanocrystals, which affects the electronic and magnetic structures at the surface and consequently impacts the magnetic properties of the system [59][60][61].

Antibacterial Activities
The two pathogen, Aspergillus niger and Candida albicans (CA) are utilized to observe the antifungal activity of the prepared samples. The results revealed that the CaFe 2 O 4 and SrFe 2 O 4 nanoparticles had better antifungal activity against CA. However, no such kind of microbial growth was detected against Aspergillus niger. Fig. 9 presents the results in the bar diagram (the photo regarding the antifungal effect). The obtained results indicated that active oxygen species generated in the distorted CaFe 2 O 4 and SrFe 2 O 4 ferrite structures possess more potential to penetrate the cell membrane and kill the fungi.
The percentage of viable cells is calculated using the following equation [62]: The SrFe 2 O 4 nanoferrite sample showed 60% inhibition (%) compared to CaFe 2 O 4 (45%). Alternatively, SrFe 2 O 4 is biodegradable compared to CaFe 2 O 4 , and can be elucidated based on creation of reactive oxygen species such as hydrogen peroxide (H 2 O 2 ), superoxide anion (O 2 − ), hydroxyl radical, and hydroxyl ion (OH − ) [63,64]. Furthermore, the size of nanocrystalline SrFe 2 O 4 is smaller than that of CaFe 2 O 4 . Thus, the nanocrystalline SrFe 2 O 4 possesses a large surface/volume ratio, which plays a crucial role in its strong antifungal activity.  Finally, these substances can be further optimized in the future to develop more potent and selective antimicrobial agents.

Conclusion
The ferrite magnetic nanoparticles of NiFe 2 O 4 , CaFe 2 O 4 , and SrFe 2 O 4 have been successfully synthesized by a citrate auto combustion technique. X-ray diffraction patterns revealed the phase transition from cubic (Ni) to orthorhombic (Ca) to monoclinic (Sr). The different crystal structures of the samples mainly depend on the difference between the ionic radii of A and B cations. Specifically, large deformation and less symmetric structures are achieved when this difference is large. Furthermore, HRTEM images presented nanoparticles in different shapes, inconsistent with the different structures obtained from XRD analyses. The different structures of the samples, including their morphology, agglomeration, and porosity, are also evident from FESEM images of the samples. Besides, the variation in the characteristic Raman bands confirms the crystal structure of synthesized ferrites. The lattice parameters of the NiFe 2 O 4 , CaFe 2 O 4 , and SrFe 2 O 4 systems are estimated considering the structural types belonging to the space groups Fd3m, Pbnm, and P1121/n, respectively. Furthermore, the M-H hysteresis loops of the investigated samples revealed that the magnetic properties of the Ni, Ca, and Sr samples are soft ferromagnetic, antiferromagnetic, and hard ferromagnetic, respectively. Additionally, the H c for monoclinic SrFe 2 O 4 is ≈ 42 times larger than that for NiFe 2 O 4 with a cubic structure. Finally, the experimental results demonstrate that the particle size, morphology, and magnetic properties can be manipulated by the variation of the ionic radii of the A cations. Moreover, the SrFe 2 O 4 sample is considered a suitable candidate for its application as an antifungal agent.