Structural and Mössbauer Spectral Studies of Nanosized Aluminum Doped Manganese Zinc Ferrites

Abstract

Nanoparticles of aluminum-substituted manganese zinc ferrites, Mn_1.05Zn_0.05Al_𝑥Fe_1.9−𝑥O₄ with 0.4≤𝑥≤1.0 were synthesized by solid-state reaction route and characterized by XRD, TEM, and Mössbauer spectroscopy measurements. The particle size is found to very from 46 to 28 nm with increase of aluminum ions concentration. The unit cell parameter ‘‘𝑎_𝑂’’ is found to decrease linearly with aluminum ions concentration due to smaller ionic radius of aluminum. The cation distributions were estimated from X-ray diffraction intensities of various planes. The theoretical lattice parameter, bulk density, porosity, X-ray density, oxygen positional parameter, ionic radii, jump length, as well as bonds and edges lengths of the tetrahedral (A-) and octahedral (B-) sites were determined. ⁵⁷Fe Mössbauer spectra recorded at room temperature were fitted with two sextets corresponding to Fe³⁺ ions at tetrahedral (A-) and octahedral (B-) sites. The magnetic hyperfine fields at A- and B-sites show a gradual decrease with increase of Al³⁺ ion concentration, which has been explained on the basis of supertransferred hyperfine field. The cation distribution obtained from X-ray intensities and Mössbauer data indicates a decrease in Fe³⁺(B)/Fe³⁺(A) ratio with increasing aluminum concentration thereby suggesting a decrease in ferrimagnetic behavior.

1. Introduction

Nanoparticles of manganese zinc ferrites exhibit some unusual properties, for example, enhanced coercivity, modified saturation magnetization, super paramagnetism, and metastable cation distributions [1]. These nanoparticles have wide applications in heat transfer devices, drug delivery systems, and medical diagnostics, that is, especially in cancer treatment, and so forth [2, 3]. Mn-Zn ferrites possess a spinel structure in which Mn²⁺, Zn²⁺, and Fe³⁺ cations are distributed among two interstitial tetrahedral (A-) and octahedral (B-) sites. In general, spinels are classified as normal, inverse, and mixed. In most cases, Mn²⁺ ions tend to occupy the octahedral B-sites and produce an inverse spinel structure. In normal spinel structure, Zn²⁺ ions prefer to hold the tetrahedral A-sites position. Although the Mn-Zn ferrites have mixed spinel structure, the magnetic and electronic properties of these ferrites are dependent of their chemical composition, method of preparation, grain size, and distribution of cation between two interstitial sites [4–6]. The structural and the magnetic environments of these two sites are quite different. The substitutions of nonmagnetic ions, for example, Al³⁺ ions in simple and mixed ferrites have received a great deal of attention over the past few years [7–9]. It has been reported that the presence of nonmagnetic ions reduces the magnetic interactions between the two interstitial sites, causes decrease in hyperfine magnetic fields, and hence changes the magnetic and electronic properties. These effects are particularly apparent in the Mössbauer studies [10, 11]. In recent years, researches on spinel ferrites have received great interest due to availability of new and sophisticated techniques for the synthesis and characterization of nanoparticles. Presently, the ferrites nanoparticles of average diameter 7 nm and 20 nm were synthesized by doping titanium and gallium ions, respectively, using solid-state reaction technique [12, 13]. In the present work, aluminum substituted manganese zinc ferrites nanoparticles have been synthesized via solid-state reaction route. In order to determine the cation distribution in the ferrite system understudy XRD, Mössbauer and TEM measurements have been carried out.

2. Experimental

Samples of the mixed spinel ferrites Mn_1.05Zn_0.5Al_𝑥Fe_1.9−𝑥O₄ for 𝑥=0.4,0.6,0.8, and 1.0 were synthesized by using standard solid-state reaction technique. The required compositions were weighed and mixed in a mortar and pestle for 2 hours with acetone. The mixed powders were calcinated in air by raising temperature up to 200^∘C at the rate of about 5^∘C/min and then up to 600^∘C in 60 minutes and allowed to cool to room temperature; finally the temperature was raised to 1000^∘C in 10 hours and slowly cooled to room temperature at a rate of 2^∘C/min. The reacted materials were well grounded in an agate mortar and pestled by adding a few drops of acetone again to reduce the particle size. To ensure their single-phase nature, the powder X-ray diffraction studies were made on Rigaku X-ray diffractometer by using Cu-K_𝛼 radiation of 1.54060Å. The particle morphology was examined by transmission electron microscope model Hitachi H-7500. ⁵⁷Fe Mössbauer absorption spectra were recorded in transmission geometry at room temperature using a multichannel analyzer with a drive in constant acceleration mode. A ⁵⁷Co(Rh) source with initial activity of 20 mCi was used. The spectrometer was periodically calibrated using a natural iron foil as a standard.

3. Results and Discussion

3.1. XRD and Tem Analysis

The X-ray diffraction patterns of mixed spinel ferrites (Mn_1.05Zn_0.5Al_𝑥Fe_1.9−𝑥O₄ for 𝑥=0.4,0.6,0.8, and 1.0) are displayed in Figure 1. The indexing of diffraction lines from various planes was assigned using JCPDS file no. 742 435. All the samples were found to be face-centered cubic (FCC) with Fd-3m space group. The values of the lattice constant ‘‘𝑎_𝑂’’ for all the samples determined from X-ray data are listed in Table 1. The lattice constant ‘‘𝑎_𝑂’’ is found to decrease linearly with aluminum concentration (𝑥) as shown in Figure 2(a), thereby obeying Vegard’s law [14]. The decrease in lattice constant is attributed to the replacement of Fe³⁺ ions of larger ionic radius (0.64Å) by smaller Al³⁺ ions (0.50Å), which causes shrinking in the unit cell dimensions. The decrease in ‘‘𝑎_𝑂’’ and the shift of reflections toward higher angle with the increasing aluminum concentration show that aluminum atoms have been incorporated into the spinel structure [15]. The bulk density of each sample was determined by Archimedes’ principle and is listed in Table 1. The X-ray density ‘‘𝑑_𝑥’’ of each sample was calculated from the values of the lattice constant using the formula 𝑑_𝑥=𝑍𝑀𝑁𝑎_𝑂³, (1) where ‘‘𝑍’’ represents the number of molecules in a unit cell of the spinel lattice, 𝑀 the molecular weight of the ferrite samples, 𝑁 the Avogadro’s number, and 𝑎_𝑂 the lattice constant of the ferrite. The X-ray density decreases with increasing aluminum concentration (𝑥) and hence follows similar trend as that of lattice constant ‘‘𝑎_𝑂’’ as shown in Figure 2(b). This is due to the fact that the decrease in mass (due to replacement of iron by aluminum) overtakes the decrease in volume of the unit cell. The percentage porosity of each sample was calculated using the relation: 𝑑Percentageporosity(𝑝%)=[1−𝑑_𝑥]×100, (2) where notations have their usual meanings. The calculated values of the bulk density, X-ray density, and the percentage porosity of the present ferrite system are listed in Table 1. The calculated values of the porosity are in good agreement with the values generally observed in ferrites prepared by solid-state reaction route [12, 16].

Table 1 

Lattice constant, density, porosity, and grain size for Mn_1.05Zn_0.05Al_𝑥Fe_1.9−𝑥O₄ system.

Figure 1 

X-ray diffraction pattern of Mn1.05Zn0.05Al𝑥Fe1.9−𝑥O4 system.

Figure 2 

Variation of (a) lattice constant and (b) density with aluminum concentration.

The average grain size of all the samples is evaluated from the line broadening of the (311) peaks using Debye-Scherrer equation 𝐷₍₃₁₁₎=0.9𝜆𝛽_1/2cos𝜃, (3) where ‘‘𝐷’’ is the average particle size of the crystallites, 𝜆 is the wavelength of the incident X-ray, 𝜃 is the corresponding Bragg angle, and 𝛽_1/2 is the full width at the half maximum (FWHM) of the (311) peak. The uncertainties in the particle size determination were estimated from the errors in the fitting procedures while fitting the 311 peak, which lie in the range ±1 nm. The average particle sizes of all the samples are listed in Table 1. The transmission electron micrographs of the ferrite nanoparticles are displayed in Figure 3. It has been observed that the particles prepared with smaller aluminum substitution displayed in Figure 3(a) are somewhat agglomerated in nature. The degree of agglomeration decreases with increase of aluminum substitution, the particles prepared from higher aluminum concentration; 𝑥=1.0 are discrete and somewhat spherical as shown in Figure 3(b). The decrease in agglomeration is due to the replacement of Fe³⁺ ions by Al³⁺ ions that leads to decrease in cation vacancies, which in turn decreases the grain size. The average particle sizes estimated from the TEM studies listed in Table 1 are in good agreement with those values obtained from XRD analysis using Debye-Scherrer equation.

(a)

(b)

Figure 3 

Transmission micrographs for Mn1.05Zn0.05Al𝑥Fe1.9−𝑥O4 (a) for 𝑥=0.4 and (b) for 𝑥=1.0.

The cation distribution in the various spinel ferrite systems has been estimated from X-ray diffraction, Mössbauer and Magnetization measurements [17–19]. It has been reported that the intensities of (220), (422), and (400) planes are sensitive to cations on tetrahedral (A-) and octahedral (B-) sites [20, 21]. Therefore, we have used the intensities of (220), (422), and (400) planes to determine the cation distribution. The relative X-ray diffraction intensity ‘‘𝐼_ℎ𝑘𝑙’’ of a given diffraction line was calculated using the formula suggested by Buerger [22] 𝐼_ℎ𝑘𝑙=|𝐹_ℎ𝑘𝑙|²⋅𝑃⋅𝐿_𝑃, (4) where 𝐼_ℎ𝑘𝑙 is the relative integral intensity, 𝐹_ℎ𝑘𝑙 the structure factor, 𝑃 the multiplicity factor, 𝐿_𝑃 the Lorentz factor. The intensity ratios (𝐼₂₂₀/𝐼₄₀₀ and 𝐼₄₂₂/𝐼₄₀₀) have been considered to be more sensitive to the cation distribution [23]. It is known that Fe³⁺ and Al³⁺ ions distribute over A- and B-sites [24], while Mn²⁺ and Zn²⁺ ions have strong preference for A-site [25–27]. The intensities corresponding to (220) and (422) reflections are most sensitive to cations on tetrahedral sites [20, 21], while those of (222) reflection to cation on octahedral sites [20]. Figure 1 shows that the intensities of (220), (422), and (222) reflections decrease with substitution of aluminum ions indicating that Al³⁺ ions enter in both octahedral (B-) and tetrahedral (A-) sites. Based on the above, intensity ratios: 𝐼₂₂₀/𝐼₄₄₀, 𝐼₄₂₂/𝐼₄₄₀, and 𝐼₄₄₀/𝐼₄₀₀ for all the samples were calculated for various cation distributions. The calculated values, those closet to the experimental data, are given in Table 2. On the basis of estimated cation distribution (Table 2), the value of theoretical lattice constant ‘‘𝑎_𝑡’’ for various compositions has been determined using the relation [28, 29] 𝑎_𝑡=83√3(𝑟_A+𝑟_𝑂)+√3(𝑟_B+𝑟_𝑂), (5) where 𝑟_A,𝑟_B are the radii of tetrahedral (A-) and octahedral (B-) sites, respectively, and 𝑟_𝑂 is the radius of the oxygen ion O²⁻ (1.48Å). The calculated values ‘‘𝑎_𝑡’’ are nearly equal to the experimental observed values ‘‘𝑎_𝑂’’ and hence confirming that the estimated cation distribution is correct. The site radii 𝑟_A and 𝑟_B used above were determined from the following equation: 𝑟_A=(0.627−0.23𝑥)𝑟_tetFe³⁺+0.05𝑟_tetZn²⁺+0.323𝑟_tetMn²⁺+0.23𝑥𝑟_tetAl³⁺,𝑟_B=(1.273−0.77𝑥)𝑟_octFe³⁺+0.727𝑟_octMn²⁺+0.77𝑥𝑟_octAl³⁺. (6) The calculated values of 𝑟_A and 𝑟_B are listed in Table 3. The value of 𝑟_A decreases slowly; however the value of 𝑟_B decreases noticeably with increasing aluminum concentration. This is due to the substitution process and the distribution of cations amongst the tetrahedral (A-) and octahedral (B-) sites. The value of the oxygen positional parameter ‘‘𝑢’’ is calculated by using the following relation [28, 29]: 𝑟_A=𝑎_𝑂√3(𝑢−0.25)−𝑟_𝑂. (7) The calculated values of ‘‘𝑢’’ are listed in Table 3. The values of the tetrahedral (𝑑_AL), octahedral bond lengths (𝑑_BL), tetrahedral edge length (𝑑_AE), shared (𝑑_BE) and unshared octahedral edge lengths (𝑑_BEU) are calculated by using the experimental values of lattice constant ‘‘𝑎_𝑂’’ and oxygen positional parameter ‘‘𝑢’’ from the following equations [28, 29]: 𝑑_AL=𝑎_𝑂√𝑑3(𝑢−0.25),_BL=𝑎_𝑂(3𝑢²−114𝑢+43𝑑64),_AE=𝑎_𝑂√𝑑2(2𝑢−0.5),_BE=𝑎_𝑂√𝑑2(1−2𝑢),_BEU=𝑎_𝑂(4𝑢²−3𝑢+1116). (8) The calculated values of the above X-ray parameters are listed in Table 3. It is observed that 𝑑_AL, 𝑑_BL, 𝑑_AE, 𝑑_BE, and 𝑑_BEU decrease with increasing aluminum concentration (𝑥). This is attributed to the substitution process and the cation distribution, especially the replacement of Fe³⁺ larger ions at B- and A-sites with smaller Al³⁺ ions. These results are in good agreement with the results reported earlier [30]. It has been reported that the jump length ‘‘𝐿’’ (the distance between the magnetic ions) of electrons influences the physical properties of the ferrite system [31]. Electrons that are hopping between B- and A-sites are less probable compared to those between B- and B-sites because the distance between the two metal ions placed at B-sites is smaller than the distance if they are placed one at B-sites and the other at A-sites [32, 33]. The jump length ‘‘𝐿’’ of the tetrahedral (A-) and octahedral (B-) sites is determined from the following relation [34, 35]: 𝐿_A=𝑎_𝑂√34,𝐿_B=𝑎_𝑂√24. (9) The calculated values of jump length ‘‘𝐿’’ of A- and B-sites are shown in Table 3. It is evident from Table 3 that the jump length ‘‘𝐿’’ decreases with increasing Al³⁺ concentration (x) and behaves similarly as that of lattice constant. The decrease in jump length is due to the replacement of Fe³⁺ ions of larger ionic radius with Al³⁺ ions of smaller ionic radius from A- and B-sites, causing a decrease in distance between the magnetic ions. Pandit et al. [23] and Amer [30] have reported similar results that the introduction of smaller ions on the A- and B-sites decreases the distance between the magnetic ions.

Table 2 

Cation distribution data calculated from XRD pattern of the Mn_1.05Zn_0.05Al_𝑥Fe_1.9−𝑥O₄ system.

Table 3 

X-ray parameters (errorbar±0.002Å) tetrahedral and octahedral bond lengths (𝑑_AL and 𝑑_BL), jump lengths (𝐿_A and 𝐿_B), tetrahedral edge 𝑑_AL, and shared and unshared octahedral edge lengths (𝑑_BE and 𝑑_BEU).

3.2. Mössbauer Measurements

⁵⁷Fe Mössbauer absorption spectra of mixed spinel ferrite system, Mn_1.05Zn_0.05Al_𝑥Fe_1.9−𝑥O₄; 𝑥=0.4,0.6,0.8, and 1.0, recorded at room temperature are displayed in Figure 4. The experimental data were fitted using NORMOS/SITE program [36]. Each spectrum exhibits a superposition of two Zeeman sextets. Outer sextet corresponding to a higher magnetic field is attributed to Fe³⁺ ions on the octahedral B-site and the inner sextet corresponding to lower magnetic field is attributed to Fe³⁺ ions on the tetrahedral A-site. The computed values of the hyperfine parameters are listed in Table 4. In the present ferrite system, the value of isomer shift (𝛿) of B-site is greater than A-site and is in agreement with the reported data. This is due to the large band separation of Fe³⁺-O²⁻ for the B-site ions compared to A-site ions. It is observed that on increasing the Al³⁺ ions concentration (𝑥), the value of isomer shift of the A- and B-sites do not show any appreciable change, indicating that the s-electron distribution of Fe³⁺ ions is hardly influenced by Al³⁺ substitution. Furthermore, the observed values of isomer shift (𝛿) are significantly less than the expected value, 0.5 mm/s for the Fe²⁺ ions [17]. Hence the presence of Fe²⁺ ions in the present ferrite system is ruled out. Thus the electron exchange interaction (Fe²⁺↔Fe³⁺+e⁻) does not occur and hence the oxidation state of Fe³⁺ remains unchanged during synthesis process. The hyperfine field 𝐻_hfs values at B- and A-sites show a gradual monotonic decrease with increasing aluminum ion concentration (𝑥). This is because of supertransferred hyperfine field at the central cation that originates from the magnetic moments of the nearest-neighbor cations, that is, from the intrasublattice contributions ℎ_AA and ℎ_BB and the inter sublattice contributions ℎ_AB and ℎ_BA. In the present ferrite system, under study, the intersulattice contributions ℎ_AB and ℎ_BA are predominant. The introduction of Al³⁺ ions replaces Fe³⁺ ions from the A- and B-sites and decreases inter sublattice contributions, which in turn decreases the hyperfine field 𝐻_hfs at both A- and B-sites. As nonmagnetic Al³⁺ ions replaces Fe³⁺ ions, the correct amount of Fe³⁺ present at A- and B-sites is estimated by determining the area under the Mössbauer absorption. The correct amount of Fe³⁺ ions occupied by octahedral (B-) and tetrahedral (A-) sites obtained from the Mössbauer absorption spectra and those calculated on the basis of cation distribution are in good agreement. The Fe³⁺(B)/Fe³⁺(A) ratio obtained from the Mössbauer spectra and X-ray intensity calculations decreases with increasing aluminum concentration suggesting the decrease in ferrimagnetic behavior. With increase of Al³⁺ ions concentration, intensity and the line width of the Mössbauer spectra increases indicating a reduction in particle size and is in accordance with TEM measurements. These results are in consistent with the results reported earlier [13].

Table 4 

Room temperature Mössbauer effect parameters for Mn_1.05Zn_0.05Al_𝑥Fe_1.9−𝑥O₄ system as a function of 𝑥.

Figure 4 

Room temperature Mössbauer absorption spectra of Mn1.05Zn0.05Al𝑥Fe1.9−𝑥O4 for 𝑥=0.4,0.6,0.8, and 1.0.

4. Conclusion

The substitution of Al³⁺ ions in Mn_1.05Zn_0.5Al_𝑥Fe_1.9−𝑥O₄ system leads to the formation of nanosized mixed spinel ferrites with particle size 46 nm to 28 nm. The unit cell parameter decreases linearly with the increase of aluminum concentration due to its small ionic radius. The cation distribution has been determined from X-ray intensity ratios of various planes and hence verified by comparing the theoretical lattice parameter with experimental observed lattice parameter. It is observed that the correct amount of Fe³⁺ ions occupied by B- and A-sites obtained from Mössbauer spectra are in good agreement with the values determined from X-ray intensity calculations for various Al³⁺ ions concentration. The Hyperfine magnetic field obtained from the Mössbauer absorption spectra decreases with increasing aluminum concentration suggesting the decrease in ferrimagnetic behavior and has been explained on the basis of super transferred hyperfine fields. The X-ray parameters, for example, lattice constant, X-ray density, bulk density, ionic radius, bond length, jump length of the A- and B-sites, oxygen positional parameter, A-site edge length, shared and unshared B-site edge lengths were determined and found affected by the Al³⁺ ions substitution.

Acknowledgments

The first author is grateful to Dr. Alok Banerjee and Dr. R. J. Chaudhary, scientists, UGC-DAE, Consortium for Scientific Research, University Campus, Khandwa Road, Indore (M.P.), India for providing experimental facilities.