Soft chemical synthesis and characterisation of some substituted ferrites

Journal of Alloys and Compounds 363 (2004) 257–262

Soft chemical synthesis and characterisation of some substituted ferrites Rodica Olar a,∗ , Mihaela Badea a , L. Diamandescu b , Elena Cristurean a , Dana Marinescu a , Doina Mihaila-Tarabasanu b , N. Stanica c , Maria Brezeanu a a

Department of Inorganic Chemistry, Bucharest University, 23 Dumbrava Rosie Strasse, Bucharest, Romania b IFA, National Institute of Material Physics, PO Box MG-7, Bucharest, Romania c Institute of Chemical Physics, 202 Spl. Independentei, Bucharest, Romania Received 15 November 2002; received in revised form 12 April 2003; accepted 12 April 2003

Abstract Substituted ferrites with the chemical composition Mx Fe1−x Fe2 O4 ·nH2 O (M: Mn(II), x: 0.35, n: 0.4; M: Ni(II), x: 0.38, n: 1; M: Zn(II), x: 0.45, n: 0.35) have been prepared by a soft chemical method using as starting materials magnetite and suitable acetates. These compounds have been characterised by elemental chemical analysis, thermal analysis, IR spectroscopy, X-ray diffraction on powder, transmission electron microscopy studies as well as Mössbauer spectroscopy and magnetic measurements at room temperature. The X-ray diffraction study indicates that the substituted ferrites adopt a cubic structure and the size of particles determined from the diffraction peak is fairly close to that indicated for agglomerates by transmission electron microscopy. The magnetic methods agree to the substitution of Fe(II) in the octahedral positions preserving the inversed spinel structure. © 2003 Elsevier B.V. All rights reserved. Keywords: Transition metal compounds; Chemical synthesis; Magnetically ordered materials; Magnetic measurements; Mössbauer spectroscopy

1. Introduction The interest for preparation of substituted ferrites is generated by the fact that such materials have particular physical properties leading to various applications in magnetic and electronic devices [1–3] as well as catalysts in processes like dehydrogenation [4], oxidation [5–8] and methylation [9]. Usually, the substituted ferrites are prepared by ceramic technique using stoichiometric amounts of the corresponding oxides or precursors (hydroxides, oxalates or carbonates) which could generate oxides by thermal decomposition. These solid state methods are expensive firstly because they employ high temperatures and the obtained samples are non-homogenous, leading to a troublesome control of stoichiometry. In the last years the non-conventional methods such as the sol–gel method [10,11], thermal decomposition of suitable complexes [12,13], coprecipitation [14–16] and the hydrothermal [17,18] method have been widely used for the synthesis of substituted ferrites. Previously it has been re-

∗

Corresponding author. E-mail address: [email protected] (R. Olar).

0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00452-3

ported that the slow reaction of Fe3 O4 and Cu(AcO)2 ·H2 O provides a new non-conventional method for copper ferrite formation [19]. It was observed that, small variation in the substitution degree of Fe(II) or Fe(III) leads to major changes of the magnetic and electric properties and also in the samples morphology. Some parameters like crystal field stabilisation energy (CFSE) of the metallic ions with different electronic configurations, the polarisation and repulsion effects could affect the total energy and the preferred symmetry of a certain compound. Thus, these parameters could lead to the crystallization of an inverse spinel non-distorted or distorted structure as well as a normal or mixed spinel structure. This paper reports the syntheses of manganese, nickel and zinc ferrite by a soft chemical method using as starting materials magnetite and suitable acetates in aqueous medium.

2. Experimental The syntheses were performed by adding 5 mmoles magnetite to solutions which contain 2.5, 7.5 or 25 mmoles M(CH3 COO)2 ·nH2 O (M: Mn, Ni, Zn) respectively in 200 ml water. Reaction mixtures were refluxed for 5 to 30 h.

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The resulting dark products were filtered off, washed several times with water and air dried. IR spectra were recorded in KBr pellets with a Bio-Rad FTIR 125 instrument in the 400–4000 cm−1 range. The thermal decomposition curves have been recorded by a q-1500D Paulik–Paulik–Erdey derivatograph in a static air atmosphere in the temperature range 20–1000 ◦ C at a heating rate of 10 K min−1 . The mass of sample was 50 mg. The Mössbauer spectra were recorded at room temperature using the standard transmission method with Promeda type equipment using a 10 mCi 57 Co source. X-ray powder diffraction patterns were measured on the Philips diffractometer using a Cu K␣ radiation source with wavelength λ = 1.54 Å. Glass plate photographs were obtained on a Philips Electronic Microscope CM120. Magnetic measurements were performed using an alternating current permeameter, at room temperature, using Ni-powder as standard. Metal salts were of analytical grade (Merck). The chemical analysis was performed by the usual micromethods.

3. Results and discussions In this paper, we report the obtaining of the substituted ferrites Mx Fe1−x Fe2 O4 ·nH2 O (M: Mn(II), x: 0.35, n: 0.4; M: Ni(II), x: 0.38, n: 1; M: Zn(II), x: 0.45, n: 0.35) by a soft chemical method. The samples of these substituted ferrites were obtained by Fe(II) substitution in a preformed spinelic lattice of Fe3 O4 with M(II) (M = Mn, Ni, Zn). The study reveals the following: • The substitution processes were studied for the magnetite:metallic acetate ratio 2:1, 2:3 (corresponding to the stoichiometric ratio for the total substitution of Fe(II)) and respectively 2:10; • for none of the reactant ratios a total substitution of Fe(II) was apparent; • the maximum value of the Fe(II) substitution by M(II) occurred when the reactants ratio was 2:3, or when excess acetate was used (Table 1); • the saturation concentration differs depending on the nature of the metallic ion that substitutes the Fe2+ ion, that is, it increases in the following order Mn (x = 0.35) < Ni (x = 0.38) < Zn ( x = 0.45);

• when the reactants ratio is 2:1, for equivalent conditions, the substitution degree is lower ( x = 0.15 for all the metallic ions); • for the same ratio of the reactants occurs a time graduation substitution of Fe(II) until it reaches the saturation concentration. The saturation concentration occurs after 25 h reaction time for all the systems that were studied at the constant reaction temperature. The substituted ferrites were characterised by chemical analysis, thermal analysis, IR spectroscopy, X-ray diffraction on powder, transmission electron microscopy as well as Mössbauer spectroscopy and magnetic measurements at room temperature. The composition of these mixed oxides deduced from the metal contents and the weight modification on heating is reported in Table 1. At the slight acidity resulting from the manganese acetate hydrolysis, the Mn(II) is not oxidised to Mn(III). This process occurs since the Fe(III) does not oxidise Mn(II) and, the Fe(II) could reduce at this pH the traces of Mn(III). The associated water content was estimated both by the thermogravimetric data and by the chemical analysis data. According to the literature data [20], the Mn(II) ion is not oxidised below 200 ◦ C in the case of the submicron manganese ferrites. IR spectra of magnetite and substituted ferrites are characteristically for the mixed oxides having a spinel structure and indicate the presence of associated water molecules [21]. The presence of water molecules are responsible for the appearance of a large strong band in the 3400–3500 cm−1 range, assigned to ν(OH) stretching vibrations and a medium band at 1640 cm−1 assigned to δ(OH) vibrational mode respectively. A third absorption band due to the ρ(OH) vibrational mode lies at about 1140 cm−1 . The bands assigned to metal–oxygen stretching vibrations, that are characteristic for spinel structure [22], appear in all spectra in the 570–590 and 450–500 cm−1 ranges respectively. The X-ray powder diffraction patterns for magnetite and nickel ferrite are shown in Fig. 1. The d-values and the mean diameter of particles (DXR ) determined with the Scherrer’s formula [23] and the mean size of agglomerates (DEM ) estimated from electronic microscopy are summarised in Table 2; the values of DXR and DEM show that the substituted ferrites are nanocrystals. As shown in Fig. 1, the magnetite has a cubic structure, substituted ferrites preserving this structure. The substitution

Table 1 Chemical analysis data Compound

Mn0.35 Fe0.65 Fe2 O4 ·0.4H2 O Ni0.38 Fe0.62 Fe2 O4 ·H2 O Zn0.45 Fe0.55 Fe2 O4 ·0.35H2 O

% Fe3+

% Fe

%M

% Residue

Calc.

Exp.

Calc.

Exp.

Calc.

Exp.

Calc.

Exp.

59.8 58.4 58.9

60.4 58.9 58.6

46.9 44.6 46.2

47.4 44.8 46.3

8.1 8.9 12.1

8.2 8.6 12.5

100.4 94.8 99.2

100.2 95.8 99.8

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of Fe(II) (r = 0.075 nm) with M(II) (M = Mn (r = 0.080 nm, Ni (r = 0.069 nm) and Zn (r = 0.074 nm)) leads to a modification of the distances between atomic planes, as well as of the lattice constant as shown in Table 2. Experimental values of the lattice constant were calculated from several high-angle lines (2θ > 45◦ ) for each sample. Fig. 2 shown the particle size histogram for Ni0.38 Fe0.62 Fe2 O4 ·H2 O (for 201 particles). The transmission electron microscopy studies confirmed that the mean size of agglomerates is fairly close to the size determined from the analysis of the diffraction peak broadening. 3.1. Mössbauer spectroscopy The room temperature Mössbauer (M) spectra are shown in Fig. 3. The characteristic parameters, hyperfine field (Hhf ), isomer shift (IS) and quadrupolar splitting (QS) are listed in Table 3 together with the relative abundance A (%). The solid lines in Fig. 3 are the results of the computer fit with lorentzian lines. Area fractions were estimated in the hypothesis of equal free fraction for different iron sites in the samples. The Mössbauer spectra of the investigated ferrites consist in a superposition of magnetic sextets corresponding to tetrahedral A and octahedral B coordinations of iron ions. As in the case of Fe3 O4 [24], the sharper six-line pattern is assigned to Fe(III) ions in tetrahedral coordination. The M parameters are close to those found for Fe3 O4 . The broader sextet can be attributed to iron ions with octahedral coordination B. A good fit was obtained by taking into account only one sextet to describe the iron ions in A site. For all the investigated ferrites the Hhf (≈ 490 KOe) and IS (≈ 0.27 mm/s)

Fig. 1. Diffractogram pattern of Ni0.38 Fe0.62 Fe2 O4 ·H2 O (a) and FeFe2 O4 (b).

Table 2 X-ray diffraction data and mean size of agglomerates from electron microscopy for magnetite and substituted ferrites Compound

FeFe2 O4

Mn0.35 Fe0.65 Fe2 O4 ·0.4H2 O

Ni0.38 Fe0.62 Fe2 O4 ·H2 O

Zn0.45 Fe0.55 Fe2 O4 ·0.35H2 O

(hkl)

d (Å)

I/I0 (%)

DXR (nm)

220 311 400 511 440 220 311 400 511 440 220 311 400 511 440 220 311 400 511 440

2.964 2.561 2.095 1.614 1.483 2.879 2.478 2.063 1.589 1.463 2.942 2.518 2.075 1.610 1.479 2.907 2.496 2.072 1.595 1.469

34 100 22 29 33 32 100 25 31 52 34 100 23 30 38 32 100 27 33 54

27.6 29.1 25.6 20.0 19.5 40.7 41.5 43.0 44.6 48.9 26.6 24.1 23.9 21.7 20.6 27.1 23.7 28.6 26.8 27.9

DmedXR (nm)

DmedEM (nm)

24.4

23.0

a0 [nm]

0.8385 0.8384

43.7

31.6 0.8255 0.8328

23.4

37.7 0.8360 0.8362

26.8

40.8 0.8287 0.8309

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Fig. 2. Particle size histogram for Ni0.38 Fe0.62 Fe2 O4 ·H2 O.

Fig. 3. Room temperature Mössbauer spectra for Mn0.35 Fe0.65 Fe2 O4 ·0.4H2 O (a), Ni0.38 Fe0.62 Fe2 O4 ·H2 O (b) and Zn0.45 Fe0.55 Fe2 O4 ·0.35H2 O (c) together with the computed curves.

are close to the characteristic values of Fe3 O4 within experimental error; these values indicate the exclusive presence of iron Fe(III) in tetragonal A site. The broader sextet can be attributed to iron ions with octahedral coordination B. The broadening of subspectrum B may be considered as being due to a distribution of hyperfine fields at the B site iron ions caused by variation of the number of M(II) (Mn, Ni, Zn) ions among the six nearest-neighbour B sites. Assuming a random occupation of the B sites by M(II) the relative intensity of the different B site subspectra can be approximated by the binomial distribution probability P(n, x) where n is the number of M(II) ions among the six nearest neighbouring B sites of the B site iron ion and x is the concentration of the M(II) at the B site (the Fe(II) substitution degree). In the case of Fe3 O4 it was accepted that, at room temperature, the shape and the characteristic hyperfine field at B site is a consequence of a rapid exchange occurring between Fe(II) and Fe(III) electrons [25]. The substitution of Fe(II) by the Mn(II), Ni(II) or Zn(II) will reduce the electronic exchange probability and the corresponding sextets will have a more pronounced Fe(III) behaviour; that means the hyperfine magnetic fields will increase with increasing of the number n of neighbouring substituting atoms. This effect is reflected by the fit results summarised in Table 3. For x≈0.35–0.45 only four sextets have relevant contribution to the spectrum (n = 0, 1, 2, 3) as it results from the calculation of binomial probabilities P(n, x). A good fit (Fig. 3) was obtained by taking into consideration four sextets that template the B pattern of M spectra. In the case of Mn0.35 Fe0.65 Fe2 O4 ·0.4H2 O our results are in good agreement with Lotgering and Van Diepen’s [26] calculations concerning the site occupancy of Mn(II). They found that Mn(II) occupy the octahedral sites exclusively in manganese bearing ferrites at x < 0.53. The calculated P(n, x) probabilities (n = 0, 1, 2, 3) are roughly in agreement with M relative areas of different B subspectra.

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Table 3 Room temperature Mössbauer parameters of substituted ferrites Sample

x

Hhf (KOe)

Mn0.35 Fe0.65 Fe2 O4 ·0.4H2 O

0.35

Ni0.38 Fe0.62 Fe2 O4 ·H2 O

IS (mm/s)

QS (mm/s)

499 443 464 488 506

0.26 0.93 0.63 0.67 0.74

0.14 −0.24 0.09 0.26 −0.38

64.1 7.6 15.6 9.0 3.7

A (Fe3+ ) B0 B1 B2 B3

0.38

493 447 459 483 509

0.27 0.73 0.62 0.65 0.39

0.11 0.04 0.08 0.15 0.21

55.0 10.9 20.1 10.3 3.7

A (Fe3+ ) B0 B1 B2 B3

Zn0.45 Fe0.55 Fe2 O4 ·0.35H2 O

0.45

495 446 461 482 508

0.27 0.74 0.62 0.69 0.40

0.10 0.08 0.1 0.09 0.32

58.4 9.0 19.2 9.8 3.6

A (Fe3+ ) B0 B1 B2 B3

Error

±0.002

±2

±0.01

±0.02

±0.7

The computer analysis of M spectra confirms the substitution of Fe(II) by Ni(II), Mn(II) and Zn(II) in the octahedral positions of the synthesised ferrites. 3.2. Magnetic measurements at room temperature The magnetisation curves for magnetite and substituted ferrites measured at room temperature are shown in Fig. 4, the values obtained for saturation magnetisation using the Langevin formula [27,28] being summarised in Table 4. From this data it can be seen that the values of saturation magnetisation for all substituted ferrites are smaller than the one for magnetite. The lower values of saturation magnetisation for all ferrites can be explained by their dimensions in the nanoparticle range. The magnetic moment of ferrites is a sum of magnetic moments of individual sublattices, the sublattice A consist-

Relative areas (%)

Site assignment

Table 4 The saturation magnetization values for nickel, magnetite and substituted ferrites Sample

M (␮em/g)

Ni FeFe2 O4 Mn0.35 Fe0.65 Fe2 O4 ·0.4H2 O Ni0.38 Fe0.62 Fe2 O4 ·H2 O Zn0.45 Fe0.55 Fe2 O4 ·0.35H2 O

54.0 78.6 70.1 59.4 63.2

ing of cations in tetrahedral positions and sublattice B with cations in octahedral positions. Usually, the AB-interaction is the strongest, AA-interaction is almost 10 times weaker and BB-interaction is the weakest. In the inversed ferrites, magnetic moments are mutually compensated and the resulting moment of the ferrite is due to the magnetic moment of divalent cations in the B-positions. Partial substitution of Fe(II) (3d6 , four unpaired electrons) in B positions with Zn(II) (3d10 , diamagnetic) or Ni(II) (3d8 , two unpaired electrons), preserving the inversed spinel structure, leads to the decrease of the ferromagnetic interaction BB that implies the reduction of the saturation magnetisation. The partial substitution of Fe(II) (3d6 , four unpaired electrons) with Mn(II) (3d5 , five unpaired electrons) in B sites should lead to an increase of the saturation magnetisation. The experimental value that is lower than expected could indicate that the spin arrangement is not collinear [29].

4. Conclusion

Fig. 4. Magnetization curves B: Ni; C: Ni0.38 Fe0,62 Fe2 O4 ; D: Zn0.45 Fe0,55 Fe2 O4 ; E: Mn0.35 Fe0,65 Fe2 O4 ; F: FeFe2 O4 .

A soft chemical protocol for preparation of Mn0.35 Fe0.65 Fe2 O4 ·0.4H2 O, Ni0.38 Fe0.62 Fe2 O4 ·H2 O and Zn0.45 Fe0.55 Fe2 O4 ·0.35H2 O was developed.

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The chemical analysis revealed that for both magnetite: metallic acetate 2:3 molar ratio or acetate excess the maximum substitution degree occurs. In all cases only a fraction of Fe(II) ions on the B sites of magnetite are substituted by M(II) ions. Moreover, a saturation of the substitution degree was observed for all studied systems after 25 h of thermal treatment at a constant temperature (in our case 100 ◦ C). The substitution level depends on the nature of the substituting cation. The substituted ferrites preserve the same cubic structure of magnetite as indicated by X-ray diffraction. The size of particles determined from analysis of the diffraction peak is fairly close to that indicated for agglomerates by transmission electron microscopy. Mössbauer spectroscopy indicates the preference of M(II) substitution for octahedral positions. The saturation magnetisation values are in good agreement with the partial substitution maintaining the inverse spinel structure.

Acknowledgements The present work was partially supported by a Grant for Scientific Research as well as by a CERES Project of the Romanian Ministry of Education and Research.

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