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Synthesis and characterization of Ni1−xZnxFe2O4 spinel ferrites from tailored layered double hydroxide precursors
Materials Research Bulletin 40 (2005) 1244–1255 www.elsevier.com/locate/matresbu
Synthesis and characterization of Ni1 xZnxFe2O4 spinel ferrites from tailored layered double hydroxide precursors Feng Li, Xiaofeng Liu, Qiaozhen Yang, Junjie Liu, David G. Evans, Xue Duan * Education Ministry Key Laboratory of Science and Technology of Controllable Chemical Reactions, University of Chemical Technology, P.O. Box 98, Beijing 100029, PR China Received 13 September 2004; received in revised form 19 January 2005; accepted 13 April 2005
Abstract In this paper, a series of pure Ni1 xZnxFe2O4 (0 x 1) spinel ferrites have been synthesized successfully using a novel route through calcination of tailored hydrotalcite-like layered double hydroxide molecular precursors of the type [(Ni + Zn)1 x yFey2+Fex3+(OH)2]x+(SO42 )x/2mH2O at 900 8C for 2 h, in which the molar ratio of (Ni2+ + Zn2+)/(Fe2+ + Fe3+) was adjusted to the same value as that in single spinel ferrite itself. The physico-chemical characteristics of the LDHs and their resulting calcined products were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Mo¨ssbauer spectroscopy. The results indicate that calcination of the as-synthesized LDH precursor affords a pure single Ni1 xZnxFe2O4 (0 x 1) spinel ferrite phase. Moreover, formation of pure ferrites starting from LDHs precursors requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring, because all metal cations on the brucite-like layers of LDHs can be uniformly distributed at an atomic level. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Layered compounds; A. Magnetic materials; B. Chemical synthesis; C. X-ray diffraction; C. Mo¨ssbauer spectroscopy
* Corresponding author. Tel.: +86 10 64451226; fax: +86 10 64425385. E-mail address: [email protected] (X. Duan). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.04.011
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1. Introduction Mixed metal oxides of the stoichiometry M2+Fe3+2O4 (M = Mn, Mg, Zn, Ni, Co, etc.) having the spinel structure, are amongst the most widely used magnetic materials [1], which have attracted a great deal of interest because of their diverse practical applications [2–4]. For example, nickel zinc ferrite materials are attractive for micro device applications in radio frequency coils and transformer cores owing to their high resistivity, low magnetic coercivity, low eddy current losses, high Curie temperature and chemical stability [5–8]. It is well known that the properties of ferrite materials are strongly influenced by the composition and microstructure of the materials, which are sensitive to their modes of preparation [6,9]. The usual method for preparing nickel zinc ferrites is the conventional ceramic method [10,11], which involves mechanical mixing of starting precursor materials (ZnO, NiO and Fe2O3) and following calcination process at high temperature with prolonged reaction time to achieve a single homogeneous phase by solid-state reaction between the reactants. However, if the temperature is too high or the reaction time is too long there are problems with Zn volatilization and phase separation. Wet chemical methods involving the coprecipitation of metal ions were also investigated comprehensively [12–14]. In these cases, it is difficult to prevent contamination of the product by cations arising from the precipitants or organic residues. Layered double hydroxides (LDHs), also known as hydrotalcite-like materials, are a class of synthetic two-dimensional nanostructured anionic clays whose structure can be described as containing brucitelike layers in which a fraction of the divalent cations have been replaced isomorphously by trivalent cations giving positively-charged sheets with charge-balancing anions between the layers [15]. LDHs have the general formula [M1 x2+Mx3+(OH)2]x+(An )x/nmH2O, where M2+ and M3+ are di- and trivalent cations, respectively, including Mg2+, Fe2+, Co2+, Cu2+, Ni2+ or Zn2+ and Al3+, Cr3+, Ga3+, Mn3+ or Fe3+, respectively; the value of x is equal to the molar ratio of M2+/(M2+ + M3+); and An is an anion, such as CO32 , SO42 , NO3 , F , Cl or PO43 [16]. Therefore, a large class of isostructural materials considered complementary to aluminosilicate clays with widely varied physico-chemical properties can be obtained by changing the nature of metal cation, the molar ratio of M2+/M3+ as well as the type of the interlayer anion. These materials are potential precursors for spinel ferrites since they are often formed with mixtures of the same cations and have been shown to have an absence of long-range cation ordering [17]. Calcination of LDHs at intermediate temperatures (450–600 8C) affords poorly crystalline mixed metal oxides [18]. Calcination above 750 8C is known to give spinels, but these are always mixed with the oxide of divalent metal [19]. This reflects the fact that in LDHs, the divalent cations are always present in greater amount than the trivalent cations (the x above is found typically in the range 0.2–0.33, corresponding to the molar ratio of M2+/M3+ of 2–4), whereas in a spinel the required molar ratio of M2+/M3+ is 0.5. In recent reports [20,21], we have shown that when the M2+/(Fe2+ + Fe3+) molar ratio in LDHs of the type [M1 x yFey2+Fex3+(OH)2]x+(A2 )x/2mH2O (M = Mg, Co, and Ni; A2 = CO32 or SO42 ) is 0.5, oxidation of all Fe2+ ions on calcination in air gives additional Fe3+ ions, thus overcoming the deficiency of trivalent ions, and leads to the formation of a pure MFe2O4 spinel ferrite with excellent magnetic property. Moreover, interest in LDHs is undoubtedly increasing because not only two but, more frequently, several cations may be accommodated in the structure, thus leading to a wide variety of multi-component compounds. Therefore, the aim of the present investigation is to synthesize a series of new tailored LDH precursors of the type [(Ni + Zn)1 x yFey2+Fex3+(OH)2]x+(SO42 )x/ xZnxFe2O4 (0 x 1) spinel 2mH2O with the composition appropriate for a range of pure Ni1
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ferrites, and to determine the physico-chemical characteristics of the calcined LDH products at 900 8C.
2. Experimental 2.1. Synthesis of samples Synthesis of layered double hydroxide sulfates containing Ni2+, Zn2+, Fe2+ and Fe3+ cations (Ni–Zn– Fe –Fe3+–LDHs). A mixture of Fe2(SO4)3, FeSO4, NiSO4 and ZnSO4 with the (Ni2+ + Zn2+)/Fe2+/Fe3+ molar ratio of 3/5/2 was dissolved in N2-saturated deionized water ([Ni2+ + Zn2+ + Fe2+ + Fe3+] = 0.8 mol/L). A base solution ([NaOH] = 1.5 M) in N2-saturated deionized water was added dropwise to the stirred mixture at 25 8C until the PH value reached 7.0. The resulting suspension was aged with stirring for 4 h in an N2 atmosphere. The mixture was cooled by adding N2-saturated deionized water at 0 8C, filtered and washed with N2-saturated deionized water at 0 8C and then N2-saturated ethanol at 0 8C. The final gelatinous precipitate was dried at room temperature and stored at 0 8C under N2. The synthesized LDHs precursors were calcined in air at 900 8C for 2 h at a heating rate of 10 8C/min and then the resulting products were slowly cooled to room temperature. 2+
2.2. Characterization Powder X-ray diffraction (XRD) patterns of the samples were recorded using a Shimadzu XRD-6000 diffractometer under the following conditions: 40 kV, 30 mA, graphite-filtered Cu Ka radiation (l = 0.15418 nm). The samples, as unoriented powders, were step-scanned in steps of 0.048 (2u) using a count time of 10 s/step. The observed interplanar spacings were corrected using elemental Si as an internal standard [d (1 1 1) = 0.31355 nm; JCPDS 27–1402]. Elemental analysis was performed using an inductively coupled plasma emission spectrometer (Ultima ICPS-7500) for metal ions and sulfur in samples. Samples were dried at 100 8C for 24 h prior to analysis, and solution was prepared by dissolving the sample in dilute hydrochloric acid (1:1). The H content was determined by Elementar Vario EL analyzer under the following conditions: carrier gas current 20 mL/min, oxygen current 20 ml/min, combustion temperature 950 8C and reduction temperature 550 8C. Room temperature Fourier transform infrared (FT-IR) spectra were recorded in the range 4000– 400 cm 1 with 2 cm 1 resolution on a Bruker Vector-22 spectrometer using the KBr pellet technique (1 mg of sample in 100 mg of KBr). X-ray photoelectron spectroscopy (XPS) measurements were carried out with a V.G. Scientific ESCALAB Mark II system and Mg Ka (hv = 1253.6 eV) as X-ray source. The hemispherical analyzer functioned with constant pass energy of 50 eV for high-resolution spectra. The binding energies (BE) were referenced to the C1s peak at 284.6 eV. The experimental curve fitted with a program that made use of a combination of Gaussian–Lorentzian lines. Mo¨ ssbauer spectra were recorded with an Oxford MS-500 instrument at 298 K. A radiation source of Co57 in an Rh matrix was used. The isomer shifts are reported relative to a-Fe. The solid lines are fitted with lorentzian lines. The accuracy in the fitted parameters is 0.03 mm/s for quadrupole splitting and 0.03 mm/s for isomer shift.
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Fig. 1. XRD patterns for Ni–Zn–Fe2+–Fe3+–LDHs with different Zn2+/Ni2+ molar ratios. Consecutively, from bottom to top, samples are: 0/3, 1/2, 1.5/1.5, 2/1 and 3/0.
3. Results and discussion 3.1. Structural properties of the Ni–Zn–Fe2+–Fe3+–LDHs precursors Fig. 1 illustrates the powder XRD patterns for as-synthesized Ni–Zn–Fe2+–Fe3+–LDHs with the (Ni2+ + Zn2+)/Fe2+/Fe3+ molar ratio of 3/5/2 in the synthesis mixture, whilst Table 1 summarizes the analytical properties and relevant structural parameters of LDHs obtained from their XRD patterns. Obviously, the XRD patterns exhibit the characteristic diffractions of hydrotalcite-like layered double hydroxide materials (JCPDS file 38–0487) [22] in each case and no other crystalline phases are present. Meanwhile, XRD patterns give a series of (0 0 l) peaks, such as 0 0 3, 0 0 6 and 0 0 9, appearing as narrow symmetric lines at low 2u angles, corresponding to the basal spacing and higher order diffractions with the relation among diffraction lines (d0 0 3 2d0 0 6 3d0 0 9), and give also several distinguishing Table 1 Structural and componential parameters of the synthesized Ni–Zn–Fe2+–Fe3+–LDHs Initial Zn2+/Ni2+ molar ratio
Constitute of products
3/0 [Zn0.34Fe0.262+Fe0.403+(OH)2](SO4)0.200.35H2O 2/1 [Ni0.11Zn0.22Fe0.272+Fe0.403+(OH)2](SO4)0.200.22H2O 1.5/1.5 [Ni0.16Zn0.17Fe0.252+Fe0.423+(OH)2] (SO4)0.210.37H2O 1/2 [Ni0.22Zn0.11Fe0.272+Fe0.403+(OH)2](SO4)0.200.30H2O 0/3 [Ni0.34Fe0.282+Fe0.383+(OH)2](SO4)0.19]0.39H2O a Lattice parameter a (= 2d1 1 0). b Crystallite size in c direction calculated from the Scherrer equation. c Crystallite size in a direction calculated from the Scherrer equation.
aa (nm)
Lcb (nm)
Lac (nm)
0.3160 0.3153 0.3145 0.3136 0.3128
20 19 16 14 13
28 86 75 56 34
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diffraction peaks between 30 and 508. It indicates that the synthesized samples consist of a single hydrotalcite-like phase without turbostratic layered structure. However, a clear difference in crystallinity, which was typically represented by the intensity and width of the (1 1 0) diffraction (the lower angle component of the characteristic doublet close to 2u = 598), occurs among the samples. From Table 1, it is seen that the experimental (Ni2+ + Zn2+)/(Fe2+ + Fe3+) molar ratio employed in the final product in the range of 0.49–0.52 is significantly different from that (=0.43) in corresponding synthesis mixture, but the values are reproducible within experimental error. It confirms that not all the metal cations can be coprecipitated completely. It is well known that LDHs is a hexagonal system, where the lattice parameter a is the average distance between two metal ions within the layers and reflects the density of metal ions stacking in 0 0 3 crystal plane. On the one hand, the incorporation of different M2+ ions into the brucite-like layers of LDHs can be ascertained from the change in the lattice parameter a. The value of the lattice parameter a (=2d1 1 0) can be calculated for LDH phases [23]. As shown in Table 1, the lattice parameter a increases with Zn content in LDHs, reflecting the fact that the Shannon ionic radii for Ni2+ and Zn2+ in octahedral coordination sites on the brucite-like layers are 0.083 and 0.088 nm, respectively. On the other hand, the LDH of the average crystallite size in the c direction (the stacking direction, perpendicular to the layers) may be estimated from the values of the fullwidth at half-maximum (FWHM) of the (0 0 3) diffraction peak by means of the Scherrer equation [L = 0.89l/b(u)cos u], where L is the crystallite size, l is the wavelength of the radiation (0.15418 nm) of the radiation used, u is the Bragg diffraction angle, and b(u) is the FWHM [24]. The average crystallite size in a direction may also be estimated from the FWHM of the (1 1 0) peak, although the errors are larger because of the weaker intensity of this peak. The crystallite sizes in a and c directions are shown in Table 1. Note that free-Ni LDH sample in the c direction is biggest, whereas that of free-Zn LDH
Fig. 2. FT-IR spectra for Ni–Zn–Fe2+–Fe3+–LDHs with different Zn2+/Ni2+ molar ratios. Consecutively, from bottom to top, samples are: 0/3, 1/2, 1.5/1.5, 2/1 and 3/0.
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sample is smallest. This is because incorporation of Zn2+ ion having a larger Shannon ionic radius than Ni2+ ion appears to increase the rate of growth and stacking of the brucite-like layers. Meanwhile, the crystallite size in a direction for as-synthesized LDHs as a function of Zn content lies in 28–86 nm range. Therefore, it can be concluded that the average crystallite size of as-synthesized LDH samples is in the nanometer range according to the values of crystallite size in both a and c directions. The FT-IR spectra for the LDHs in the region between 400 and 4000 cm 1 are shown in Fig. 2. The spectra do not show any significant variation with Zn content. Typical of this spectrum is the strong and broad absorption band between 3600 and 3200 cm 1 centered around 3450 cm 1 associated with a superposition of hydroxyl stretching bands arising from hydroxyl groups on the brucite-like layers and hydrogen-bonded interlayer water molecules. Another absorption band corresponding to a water deformation, d(H2O), is recorded around 1630 cm 1 [25]. The broad n3 absorption band around 1120 cm 1 and n4 absorption band around 610 cm 1 of interlayer sulfate ions in the LDHs can be observed. In the literature [26], the broadness of the n3 band for MgAl-LDHs sulfate has been interpreted as indicating a lowering of the symmetry of the sulfate from Td, which is probably also the case here. This may be associated with the lower charge density and polarizing power of Fe3+ compared with Al3+. The other bands observed in the low-frequency region of the spectrum correspond to lattice vibration modes and can be attributed to metal–oxygen and metal–hydroxyl vibrations. 3.2. Structural properties of the calcined LDHs Fig. 3 displays the powder XRD patterns for the materials obtained by calcination of LDHs at 900 8C for 2 h in air. It is obvious that calcination has destroyed the layered structure of the LDHs since no characteristic diffractions of LDHs are present. In each case the characteristic diffractions of a spinel ferrite phase with good crystallinity are observed, and no XRD patterns due to the existence of other
Fig. 3. XRD patterns for calcined Ni–Zn–Fe2+–Fe3+–LDHs with different Zn2+/Ni2+ molar ratios. Consecutively, from bottom to top, samples are: 0/3, 1/2, 1.5/1.5, 2/1 and 3/0.
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Fig. 4. The lattice parameter for NixZn1
xFe2O4
vs. Zn2+/(Ni2+ + Zn2+) molar ratio plot.
phases are observed. It confirms that the XRD patterns for calcined LDHs can be indexed to a pure spinel ferrite, in which the M2+/M3+ molar ratio of 0.5 is same as that of (Ni2+ + Zn2+)/(Fe2+ + Fe3+) in the LDH precursor. It suggests that all of the Fe2+ precursor ions have been oxidized to Fe3+ after calcination, and accordingly the obtained spinel ferrites have the general formula Ni1 xZnxFe2O4 (0 x 1). The lattice parameter for the cubic spinel phase as a function of the Zn2+/Ni2+ molar ratio in synthesis mixture is plotted in Fig. 4. Note that the introduction of different amount of Zn2+ ions gave a detectable change in the lattice parameter. With Zn content the value of the lattice parameter increases from 0.834 to 0.844 nm, as probably attributed to the progressive dissolution of Zn2+ into the spinel structure. Of course, the lattice parameter can also be affected by the distribution of metal cations in the spinel structure [27], which can directly influence the magnetic performance of materials. On the other hand, it can be seen from Fig. 3 that the calcined LDHs exhibit wide diffraction lines, probably indicative of a smaller scattering domain size. The average crystallite size may be estimated from the (3 1 1), (5 1 1) and (4 4 0) diffraction peaks by means of the Scherrer equation. The average crystallite size for spinel ferrites, lying in the 28–55 nm range, is presented in Table 3. To assess chemical states of iron cations on the surface of the calcined materials, all samples were submitted to Fe 2p spectra investigation of XPS shown in Fig. 5. Calculating the relative intensities of spectral components have been performed by the fitting procedure of the experimental XPS peaks from the analysis of the Fe 2p3/2 regions. All the spectra generally show a main peak (peak I) at BE of around 710.5 eV, accompanied by a satellite line visible at BE of around 718.5 eV, only indicative of the presence of Fe3+ cations [28,29]. Meanwhile, the presence of the peak around 713.0 eV between peak I and its satellite peak indicates that the Fe3+ species exist in more than one chemical state. More possibly, the two chemical states may be related to different coordination environment of the Fe3+, its tetrahedral (A) or octahedral (B) environment of Fe3+ cations in spinel structure: FeA3+ at higher binding energy and FeB3+ at lower binding energy. According to the proportions of the two chemical states of the Fe3+ listed in Table 2, taking into account the main peak as well as its satellite line, it is concluded that the increase in Zn content favors thus FeB3+ rather than FeA3+ cations. This is because of the high affinity of Zn2+ ions in tetrahedral sites of spinel structure. It is interesting to note that a small amount of Fe3+ ions in the A-sites presents on the surface of
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Fig. 5. XPS of the Fe 2p regions in the NixZn1 xFe2O4 with different Zn2+/Ni2+ molar ratio in systhesis LDH precursors Consecutively, from bottom to top, samples are: 0/3, 1/2, 1.5/1.5, 2/1 and 3/0.
Table 2 XPS characteristics of Fe 2p3/2 region of the calcined Ni–Zn–Fe2+–Fe3+–LDHs Initial Zn2+/Ni2+ molar ratio 3/0 2/1 1.5/1.5 1/2 0/3 a
Ia (%)
Binding energy (eV) FeB3+
FeA3+
FeB Sat3+
711.01 710.33 710.07 710.47 710.39
714.08 714.01 713.92 713.87 712.92
719.08 – – – 718.10
Intensity of the FeA3+ peak in percentage of the total Fe 2p3/2 area.
12.5 25.5 27.5 34.0 43.0
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obtained nanocrystalline ZnFe2O4, consistent with previous reports [30–32]. It is well known that in bulk form, ZnFe2O4 is a normal spinel with Zn2+ ions in the A-sites and Fe3+ ions in the B-sites. Therefore, the presence of Fe3+ ions in the A-sites observed in the present study shows that the cation distribution of nanocrystalline ZnFe2O4 has changed from normal to mixed spinel type. Generally, the chemical states of the iron species characterized by means of an ordinary XPS route only should be considered representative of the surface or external layers composition of the synthesized ferrite samples. As a result, use of a single precursor with cations uniformly distributed with no long order facilitates the synthesis of a homogeneous spinel phase [17]. The close structural relationship between the LDH precursor and its calcinations products is also a key factor. Bellotto et al. [33] have shown that collapse of the layered structure on heating an LDH at around 400 8C gives a poorly crystalline mixed metal oxide which can best be described as spinel-like phase. This phase preserves the particle morphology of the LDH [34], suggesting a topotactic transformation. In this case the (1 1 0) diffraction of the LDH
Fig. 6. Mo¨ ssbauer spectra for calcined Ni–Zn–Fe2+–Fe3+–LDHs with different Zn2+/Ni2+ molar ratios. Consecutively, from bottom to top, samples are: 0/3, 1/2, 1.5/1.5, 2/1 and 3/0.
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Table 3 The crystallite sizes and Mo¨ ssbauer parameters of nickel zinc ferrites produced by calcination of Ni–Zn–Fe2+–Fe3+–LDH Initial Zn2+/Ni2+ molar ratio
La (nm)
Hhfb (kOe)
ISc (mm/s)
3/0
55
...
0.34
2/1
36
... 202.2 (B) f 326.3 (B)
1.5/1.5
28
1/2
39
0/3
37
a b c d e f
QSd (mm/s)
We (mm/s)
Relatively areas (%)
0.46
0.20
100.0
0.35 0.34 0.24
1.13 0.09 0.08
0.48 0.98 0.79
23.6 43.4 33.0
... 394.5 (B) 452.4 (B) 515.6 (A)
0.27 0.33 0.27 0.38
0.45 0.05 0 0.23
0.30 0.77 0.39 0.14
2.5 51.3 43.1 3.1
485.0 (B) 514.1 (A)
0.26 0.36
0 0.02
0.30 0.19
83.3 16.7
0.23 0.20
56.9 43.1
490.2 (B) 0.24 0.01 524.5 (A) 0.35 0.02 Crystallite size using Scherrer equation. Hyperfine field. Isomer shift. Quadrupole splitting. The linewidth of subspectrum. A and B in parentheses refer to the A-site Fe3+ and B-site Fe3+ ions, respectively.
transforms to the (4 4 0) spinel diffraction. Furthermore, the fact that the spinel is produced from a single solid precursor rather than a mixture means that the calcinations process requires a much shorter time and lower temperature, leading to a lower chance of side-reactions occurring. 3.3. Magnetic properties for the synthesized ferrites Mo¨ ssbauer spectra of ferrites shown in Fig. 6 were measured in zero applied magnetic fields at room temperature. The Mo¨ ssbauer solid lines for the samples were fitted using sextets, which can be ascribed to Fe3+ ions in A and B sites according to the values of magnetic hyperfine field, and doublet representing paramagnetic behavior. Correspondingly, the characteristic parameters of Mo¨ ssbauer spectra including hyperfine field H values (Hhf), isomer shift (IS), quadrupole splitting (QS) and the relative area of subspectra are listed in Table 3. It can be found that according to the Mo¨ ssbauer fitting parameters, the obtained ZnFe2O4 phase exhibits a paramagnetic state at room temperature, and the substituted Ni1 xZnxFe2O4 (x = 0.5 and 0.66) phases exhibit a canted ferromagnetic structure characterized by a significant reduction of magnetic hyperfine field or by the collapse of the six lines of ferrimagnetic spectra, and the substituted Ni1 xZnxFe2O4 (x = 0.33) and NiFe2O4 phase exhibit collinear ferromagnetic ones. The result is closely consistent with those reported by Leung [35] and Macedo [36]. It also reveals that the nickel richer ferrite samples are in the magnetically ordered state arising from the superexchange interaction between Fe3+ ions in tetrahedral and octahedral sites, basically due to the affinity of Ni2+ in octahedral sites. Moreover, the observation of a doublet contribution in the Mo¨ ssbauer spectra of the zinc richer samples could be attributed to a superparamagnetic behavior of nanosized ferrite samples [35].
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4. Conclusion A series of pure Ni2+-, Zn2+-, Fe2+- and Fe3+-containing layered double hydroxide sulfates with an (Zn2+ + Ni2+)/(Fe2+ + Fe3+) molar ratio of 0.5 in the synthesis product, [(Ni + Zn)1 x yFey2+Fex3+ (OH)2]x+(SO42 )x/2mH2O, have been prepared by coprecipitation method. Powder X-ray diffraction shows the presence of only one good crystalline phase. The detailed study on the structure of LDHs materials has shown that change in the composition with Zn content results in small but significant changes in the parameter lattice a and the average crystalline size in the c direction. Powder X-ray diffraction, X-ray photoelectron spectroscopy and Mo¨ ssbauer spectroscopy show that calcination of the type tailored LDH precursors affords a pure Ni1 xZnxFe2O4 (0 x 1) spinel ferrite. The major advantage of the new method is that it affords uniform distribution of all metal cations at an atomic level in the LDH precursors; hence the formation of spinel ferrites starting from the LDHs requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring. The studies on magnetic properties of the synthesized ferrites clearly demonstrate that the zinc richer ferrite samples exhibit strong superparamagnetic relaxation while the nickel richer ferrite samples show resolved ferrimagnetic spectra with reduced magnetic hyperfine fields.
Acknowledgements We gratefully acknowledge the financial support from the NSFC (20371006, 20306003), Beijing Nova Program (2003B10) and program for new century excellent talents in university (NCET-04-0120).
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Synthesis and characterization of Ni1 xZnxFe2O4 spinel ferrites from tailored layered double hydroxide precursors Feng Li, Xiaofeng Liu, Qiaozhen Yang, Junjie Liu, David G. Evans, Xue Duan * Education Ministry Key Laboratory of Science and Technology of Controllable Chemical Reactions, University of Chemical Technology, P.O. Box 98, Beijing 100029, PR China Received 13 September 2004; received in revised form 19 January 2005; accepted 13 April 2005
Abstract In this paper, a series of pure Ni1 xZnxFe2O4 (0 x 1) spinel ferrites have been synthesized successfully using a novel route through calcination of tailored hydrotalcite-like layered double hydroxide molecular precursors of the type [(Ni + Zn)1 x yFey2+Fex3+(OH)2]x+(SO42 )x/2mH2O at 900 8C for 2 h, in which the molar ratio of (Ni2+ + Zn2+)/(Fe2+ + Fe3+) was adjusted to the same value as that in single spinel ferrite itself. The physico-chemical characteristics of the LDHs and their resulting calcined products were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Mo¨ssbauer spectroscopy. The results indicate that calcination of the as-synthesized LDH precursor affords a pure single Ni1 xZnxFe2O4 (0 x 1) spinel ferrite phase. Moreover, formation of pure ferrites starting from LDHs precursors requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring, because all metal cations on the brucite-like layers of LDHs can be uniformly distributed at an atomic level. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Layered compounds; A. Magnetic materials; B. Chemical synthesis; C. X-ray diffraction; C. Mo¨ssbauer spectroscopy
* Corresponding author. Tel.: +86 10 64451226; fax: +86 10 64425385. E-mail address: [email protected] (X. Duan). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.04.011
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1. Introduction Mixed metal oxides of the stoichiometry M2+Fe3+2O4 (M = Mn, Mg, Zn, Ni, Co, etc.) having the spinel structure, are amongst the most widely used magnetic materials [1], which have attracted a great deal of interest because of their diverse practical applications [2–4]. For example, nickel zinc ferrite materials are attractive for micro device applications in radio frequency coils and transformer cores owing to their high resistivity, low magnetic coercivity, low eddy current losses, high Curie temperature and chemical stability [5–8]. It is well known that the properties of ferrite materials are strongly influenced by the composition and microstructure of the materials, which are sensitive to their modes of preparation [6,9]. The usual method for preparing nickel zinc ferrites is the conventional ceramic method [10,11], which involves mechanical mixing of starting precursor materials (ZnO, NiO and Fe2O3) and following calcination process at high temperature with prolonged reaction time to achieve a single homogeneous phase by solid-state reaction between the reactants. However, if the temperature is too high or the reaction time is too long there are problems with Zn volatilization and phase separation. Wet chemical methods involving the coprecipitation of metal ions were also investigated comprehensively [12–14]. In these cases, it is difficult to prevent contamination of the product by cations arising from the precipitants or organic residues. Layered double hydroxides (LDHs), also known as hydrotalcite-like materials, are a class of synthetic two-dimensional nanostructured anionic clays whose structure can be described as containing brucitelike layers in which a fraction of the divalent cations have been replaced isomorphously by trivalent cations giving positively-charged sheets with charge-balancing anions between the layers [15]. LDHs have the general formula [M1 x2+Mx3+(OH)2]x+(An )x/nmH2O, where M2+ and M3+ are di- and trivalent cations, respectively, including Mg2+, Fe2+, Co2+, Cu2+, Ni2+ or Zn2+ and Al3+, Cr3+, Ga3+, Mn3+ or Fe3+, respectively; the value of x is equal to the molar ratio of M2+/(M2+ + M3+); and An is an anion, such as CO32 , SO42 , NO3 , F , Cl or PO43 [16]. Therefore, a large class of isostructural materials considered complementary to aluminosilicate clays with widely varied physico-chemical properties can be obtained by changing the nature of metal cation, the molar ratio of M2+/M3+ as well as the type of the interlayer anion. These materials are potential precursors for spinel ferrites since they are often formed with mixtures of the same cations and have been shown to have an absence of long-range cation ordering [17]. Calcination of LDHs at intermediate temperatures (450–600 8C) affords poorly crystalline mixed metal oxides [18]. Calcination above 750 8C is known to give spinels, but these are always mixed with the oxide of divalent metal [19]. This reflects the fact that in LDHs, the divalent cations are always present in greater amount than the trivalent cations (the x above is found typically in the range 0.2–0.33, corresponding to the molar ratio of M2+/M3+ of 2–4), whereas in a spinel the required molar ratio of M2+/M3+ is 0.5. In recent reports [20,21], we have shown that when the M2+/(Fe2+ + Fe3+) molar ratio in LDHs of the type [M1 x yFey2+Fex3+(OH)2]x+(A2 )x/2mH2O (M = Mg, Co, and Ni; A2 = CO32 or SO42 ) is 0.5, oxidation of all Fe2+ ions on calcination in air gives additional Fe3+ ions, thus overcoming the deficiency of trivalent ions, and leads to the formation of a pure MFe2O4 spinel ferrite with excellent magnetic property. Moreover, interest in LDHs is undoubtedly increasing because not only two but, more frequently, several cations may be accommodated in the structure, thus leading to a wide variety of multi-component compounds. Therefore, the aim of the present investigation is to synthesize a series of new tailored LDH precursors of the type [(Ni + Zn)1 x yFey2+Fex3+(OH)2]x+(SO42 )x/ xZnxFe2O4 (0 x 1) spinel 2mH2O with the composition appropriate for a range of pure Ni1
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ferrites, and to determine the physico-chemical characteristics of the calcined LDH products at 900 8C.
2. Experimental 2.1. Synthesis of samples Synthesis of layered double hydroxide sulfates containing Ni2+, Zn2+, Fe2+ and Fe3+ cations (Ni–Zn– Fe –Fe3+–LDHs). A mixture of Fe2(SO4)3, FeSO4, NiSO4 and ZnSO4 with the (Ni2+ + Zn2+)/Fe2+/Fe3+ molar ratio of 3/5/2 was dissolved in N2-saturated deionized water ([Ni2+ + Zn2+ + Fe2+ + Fe3+] = 0.8 mol/L). A base solution ([NaOH] = 1.5 M) in N2-saturated deionized water was added dropwise to the stirred mixture at 25 8C until the PH value reached 7.0. The resulting suspension was aged with stirring for 4 h in an N2 atmosphere. The mixture was cooled by adding N2-saturated deionized water at 0 8C, filtered and washed with N2-saturated deionized water at 0 8C and then N2-saturated ethanol at 0 8C. The final gelatinous precipitate was dried at room temperature and stored at 0 8C under N2. The synthesized LDHs precursors were calcined in air at 900 8C for 2 h at a heating rate of 10 8C/min and then the resulting products were slowly cooled to room temperature. 2+
2.2. Characterization Powder X-ray diffraction (XRD) patterns of the samples were recorded using a Shimadzu XRD-6000 diffractometer under the following conditions: 40 kV, 30 mA, graphite-filtered Cu Ka radiation (l = 0.15418 nm). The samples, as unoriented powders, were step-scanned in steps of 0.048 (2u) using a count time of 10 s/step. The observed interplanar spacings were corrected using elemental Si as an internal standard [d (1 1 1) = 0.31355 nm; JCPDS 27–1402]. Elemental analysis was performed using an inductively coupled plasma emission spectrometer (Ultima ICPS-7500) for metal ions and sulfur in samples. Samples were dried at 100 8C for 24 h prior to analysis, and solution was prepared by dissolving the sample in dilute hydrochloric acid (1:1). The H content was determined by Elementar Vario EL analyzer under the following conditions: carrier gas current 20 mL/min, oxygen current 20 ml/min, combustion temperature 950 8C and reduction temperature 550 8C. Room temperature Fourier transform infrared (FT-IR) spectra were recorded in the range 4000– 400 cm 1 with 2 cm 1 resolution on a Bruker Vector-22 spectrometer using the KBr pellet technique (1 mg of sample in 100 mg of KBr). X-ray photoelectron spectroscopy (XPS) measurements were carried out with a V.G. Scientific ESCALAB Mark II system and Mg Ka (hv = 1253.6 eV) as X-ray source. The hemispherical analyzer functioned with constant pass energy of 50 eV for high-resolution spectra. The binding energies (BE) were referenced to the C1s peak at 284.6 eV. The experimental curve fitted with a program that made use of a combination of Gaussian–Lorentzian lines. Mo¨ ssbauer spectra were recorded with an Oxford MS-500 instrument at 298 K. A radiation source of Co57 in an Rh matrix was used. The isomer shifts are reported relative to a-Fe. The solid lines are fitted with lorentzian lines. The accuracy in the fitted parameters is 0.03 mm/s for quadrupole splitting and 0.03 mm/s for isomer shift.
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Fig. 1. XRD patterns for Ni–Zn–Fe2+–Fe3+–LDHs with different Zn2+/Ni2+ molar ratios. Consecutively, from bottom to top, samples are: 0/3, 1/2, 1.5/1.5, 2/1 and 3/0.
3. Results and discussion 3.1. Structural properties of the Ni–Zn–Fe2+–Fe3+–LDHs precursors Fig. 1 illustrates the powder XRD patterns for as-synthesized Ni–Zn–Fe2+–Fe3+–LDHs with the (Ni2+ + Zn2+)/Fe2+/Fe3+ molar ratio of 3/5/2 in the synthesis mixture, whilst Table 1 summarizes the analytical properties and relevant structural parameters of LDHs obtained from their XRD patterns. Obviously, the XRD patterns exhibit the characteristic diffractions of hydrotalcite-like layered double hydroxide materials (JCPDS file 38–0487) [22] in each case and no other crystalline phases are present. Meanwhile, XRD patterns give a series of (0 0 l) peaks, such as 0 0 3, 0 0 6 and 0 0 9, appearing as narrow symmetric lines at low 2u angles, corresponding to the basal spacing and higher order diffractions with the relation among diffraction lines (d0 0 3 2d0 0 6 3d0 0 9), and give also several distinguishing Table 1 Structural and componential parameters of the synthesized Ni–Zn–Fe2+–Fe3+–LDHs Initial Zn2+/Ni2+ molar ratio
Constitute of products
3/0 [Zn0.34Fe0.262+Fe0.403+(OH)2](SO4)0.200.35H2O 2/1 [Ni0.11Zn0.22Fe0.272+Fe0.403+(OH)2](SO4)0.200.22H2O 1.5/1.5 [Ni0.16Zn0.17Fe0.252+Fe0.423+(OH)2] (SO4)0.210.37H2O 1/2 [Ni0.22Zn0.11Fe0.272+Fe0.403+(OH)2](SO4)0.200.30H2O 0/3 [Ni0.34Fe0.282+Fe0.383+(OH)2](SO4)0.19]0.39H2O a Lattice parameter a (= 2d1 1 0). b Crystallite size in c direction calculated from the Scherrer equation. c Crystallite size in a direction calculated from the Scherrer equation.
aa (nm)
Lcb (nm)
Lac (nm)
0.3160 0.3153 0.3145 0.3136 0.3128
20 19 16 14 13
28 86 75 56 34
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diffraction peaks between 30 and 508. It indicates that the synthesized samples consist of a single hydrotalcite-like phase without turbostratic layered structure. However, a clear difference in crystallinity, which was typically represented by the intensity and width of the (1 1 0) diffraction (the lower angle component of the characteristic doublet close to 2u = 598), occurs among the samples. From Table 1, it is seen that the experimental (Ni2+ + Zn2+)/(Fe2+ + Fe3+) molar ratio employed in the final product in the range of 0.49–0.52 is significantly different from that (=0.43) in corresponding synthesis mixture, but the values are reproducible within experimental error. It confirms that not all the metal cations can be coprecipitated completely. It is well known that LDHs is a hexagonal system, where the lattice parameter a is the average distance between two metal ions within the layers and reflects the density of metal ions stacking in 0 0 3 crystal plane. On the one hand, the incorporation of different M2+ ions into the brucite-like layers of LDHs can be ascertained from the change in the lattice parameter a. The value of the lattice parameter a (=2d1 1 0) can be calculated for LDH phases [23]. As shown in Table 1, the lattice parameter a increases with Zn content in LDHs, reflecting the fact that the Shannon ionic radii for Ni2+ and Zn2+ in octahedral coordination sites on the brucite-like layers are 0.083 and 0.088 nm, respectively. On the other hand, the LDH of the average crystallite size in the c direction (the stacking direction, perpendicular to the layers) may be estimated from the values of the fullwidth at half-maximum (FWHM) of the (0 0 3) diffraction peak by means of the Scherrer equation [L = 0.89l/b(u)cos u], where L is the crystallite size, l is the wavelength of the radiation (0.15418 nm) of the radiation used, u is the Bragg diffraction angle, and b(u) is the FWHM [24]. The average crystallite size in a direction may also be estimated from the FWHM of the (1 1 0) peak, although the errors are larger because of the weaker intensity of this peak. The crystallite sizes in a and c directions are shown in Table 1. Note that free-Ni LDH sample in the c direction is biggest, whereas that of free-Zn LDH
Fig. 2. FT-IR spectra for Ni–Zn–Fe2+–Fe3+–LDHs with different Zn2+/Ni2+ molar ratios. Consecutively, from bottom to top, samples are: 0/3, 1/2, 1.5/1.5, 2/1 and 3/0.
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sample is smallest. This is because incorporation of Zn2+ ion having a larger Shannon ionic radius than Ni2+ ion appears to increase the rate of growth and stacking of the brucite-like layers. Meanwhile, the crystallite size in a direction for as-synthesized LDHs as a function of Zn content lies in 28–86 nm range. Therefore, it can be concluded that the average crystallite size of as-synthesized LDH samples is in the nanometer range according to the values of crystallite size in both a and c directions. The FT-IR spectra for the LDHs in the region between 400 and 4000 cm 1 are shown in Fig. 2. The spectra do not show any significant variation with Zn content. Typical of this spectrum is the strong and broad absorption band between 3600 and 3200 cm 1 centered around 3450 cm 1 associated with a superposition of hydroxyl stretching bands arising from hydroxyl groups on the brucite-like layers and hydrogen-bonded interlayer water molecules. Another absorption band corresponding to a water deformation, d(H2O), is recorded around 1630 cm 1 [25]. The broad n3 absorption band around 1120 cm 1 and n4 absorption band around 610 cm 1 of interlayer sulfate ions in the LDHs can be observed. In the literature [26], the broadness of the n3 band for MgAl-LDHs sulfate has been interpreted as indicating a lowering of the symmetry of the sulfate from Td, which is probably also the case here. This may be associated with the lower charge density and polarizing power of Fe3+ compared with Al3+. The other bands observed in the low-frequency region of the spectrum correspond to lattice vibration modes and can be attributed to metal–oxygen and metal–hydroxyl vibrations. 3.2. Structural properties of the calcined LDHs Fig. 3 displays the powder XRD patterns for the materials obtained by calcination of LDHs at 900 8C for 2 h in air. It is obvious that calcination has destroyed the layered structure of the LDHs since no characteristic diffractions of LDHs are present. In each case the characteristic diffractions of a spinel ferrite phase with good crystallinity are observed, and no XRD patterns due to the existence of other
Fig. 3. XRD patterns for calcined Ni–Zn–Fe2+–Fe3+–LDHs with different Zn2+/Ni2+ molar ratios. Consecutively, from bottom to top, samples are: 0/3, 1/2, 1.5/1.5, 2/1 and 3/0.
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Fig. 4. The lattice parameter for NixZn1
xFe2O4
vs. Zn2+/(Ni2+ + Zn2+) molar ratio plot.
phases are observed. It confirms that the XRD patterns for calcined LDHs can be indexed to a pure spinel ferrite, in which the M2+/M3+ molar ratio of 0.5 is same as that of (Ni2+ + Zn2+)/(Fe2+ + Fe3+) in the LDH precursor. It suggests that all of the Fe2+ precursor ions have been oxidized to Fe3+ after calcination, and accordingly the obtained spinel ferrites have the general formula Ni1 xZnxFe2O4 (0 x 1). The lattice parameter for the cubic spinel phase as a function of the Zn2+/Ni2+ molar ratio in synthesis mixture is plotted in Fig. 4. Note that the introduction of different amount of Zn2+ ions gave a detectable change in the lattice parameter. With Zn content the value of the lattice parameter increases from 0.834 to 0.844 nm, as probably attributed to the progressive dissolution of Zn2+ into the spinel structure. Of course, the lattice parameter can also be affected by the distribution of metal cations in the spinel structure [27], which can directly influence the magnetic performance of materials. On the other hand, it can be seen from Fig. 3 that the calcined LDHs exhibit wide diffraction lines, probably indicative of a smaller scattering domain size. The average crystallite size may be estimated from the (3 1 1), (5 1 1) and (4 4 0) diffraction peaks by means of the Scherrer equation. The average crystallite size for spinel ferrites, lying in the 28–55 nm range, is presented in Table 3. To assess chemical states of iron cations on the surface of the calcined materials, all samples were submitted to Fe 2p spectra investigation of XPS shown in Fig. 5. Calculating the relative intensities of spectral components have been performed by the fitting procedure of the experimental XPS peaks from the analysis of the Fe 2p3/2 regions. All the spectra generally show a main peak (peak I) at BE of around 710.5 eV, accompanied by a satellite line visible at BE of around 718.5 eV, only indicative of the presence of Fe3+ cations [28,29]. Meanwhile, the presence of the peak around 713.0 eV between peak I and its satellite peak indicates that the Fe3+ species exist in more than one chemical state. More possibly, the two chemical states may be related to different coordination environment of the Fe3+, its tetrahedral (A) or octahedral (B) environment of Fe3+ cations in spinel structure: FeA3+ at higher binding energy and FeB3+ at lower binding energy. According to the proportions of the two chemical states of the Fe3+ listed in Table 2, taking into account the main peak as well as its satellite line, it is concluded that the increase in Zn content favors thus FeB3+ rather than FeA3+ cations. This is because of the high affinity of Zn2+ ions in tetrahedral sites of spinel structure. It is interesting to note that a small amount of Fe3+ ions in the A-sites presents on the surface of
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Fig. 5. XPS of the Fe 2p regions in the NixZn1 xFe2O4 with different Zn2+/Ni2+ molar ratio in systhesis LDH precursors Consecutively, from bottom to top, samples are: 0/3, 1/2, 1.5/1.5, 2/1 and 3/0.
Table 2 XPS characteristics of Fe 2p3/2 region of the calcined Ni–Zn–Fe2+–Fe3+–LDHs Initial Zn2+/Ni2+ molar ratio 3/0 2/1 1.5/1.5 1/2 0/3 a
Ia (%)
Binding energy (eV) FeB3+
FeA3+
FeB Sat3+
711.01 710.33 710.07 710.47 710.39
714.08 714.01 713.92 713.87 712.92
719.08 – – – 718.10
Intensity of the FeA3+ peak in percentage of the total Fe 2p3/2 area.
12.5 25.5 27.5 34.0 43.0
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obtained nanocrystalline ZnFe2O4, consistent with previous reports [30–32]. It is well known that in bulk form, ZnFe2O4 is a normal spinel with Zn2+ ions in the A-sites and Fe3+ ions in the B-sites. Therefore, the presence of Fe3+ ions in the A-sites observed in the present study shows that the cation distribution of nanocrystalline ZnFe2O4 has changed from normal to mixed spinel type. Generally, the chemical states of the iron species characterized by means of an ordinary XPS route only should be considered representative of the surface or external layers composition of the synthesized ferrite samples. As a result, use of a single precursor with cations uniformly distributed with no long order facilitates the synthesis of a homogeneous spinel phase [17]. The close structural relationship between the LDH precursor and its calcinations products is also a key factor. Bellotto et al. [33] have shown that collapse of the layered structure on heating an LDH at around 400 8C gives a poorly crystalline mixed metal oxide which can best be described as spinel-like phase. This phase preserves the particle morphology of the LDH [34], suggesting a topotactic transformation. In this case the (1 1 0) diffraction of the LDH
Fig. 6. Mo¨ ssbauer spectra for calcined Ni–Zn–Fe2+–Fe3+–LDHs with different Zn2+/Ni2+ molar ratios. Consecutively, from bottom to top, samples are: 0/3, 1/2, 1.5/1.5, 2/1 and 3/0.
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Table 3 The crystallite sizes and Mo¨ ssbauer parameters of nickel zinc ferrites produced by calcination of Ni–Zn–Fe2+–Fe3+–LDH Initial Zn2+/Ni2+ molar ratio
La (nm)
Hhfb (kOe)
ISc (mm/s)
3/0
55
...
0.34
2/1
36
... 202.2 (B) f 326.3 (B)
1.5/1.5
28
1/2
39
0/3
37
a b c d e f
QSd (mm/s)
We (mm/s)
Relatively areas (%)
0.46
0.20
100.0
0.35 0.34 0.24
1.13 0.09 0.08
0.48 0.98 0.79
23.6 43.4 33.0
... 394.5 (B) 452.4 (B) 515.6 (A)
0.27 0.33 0.27 0.38
0.45 0.05 0 0.23
0.30 0.77 0.39 0.14
2.5 51.3 43.1 3.1
485.0 (B) 514.1 (A)
0.26 0.36
0 0.02
0.30 0.19
83.3 16.7
0.23 0.20
56.9 43.1
490.2 (B) 0.24 0.01 524.5 (A) 0.35 0.02 Crystallite size using Scherrer equation. Hyperfine field. Isomer shift. Quadrupole splitting. The linewidth of subspectrum. A and B in parentheses refer to the A-site Fe3+ and B-site Fe3+ ions, respectively.
transforms to the (4 4 0) spinel diffraction. Furthermore, the fact that the spinel is produced from a single solid precursor rather than a mixture means that the calcinations process requires a much shorter time and lower temperature, leading to a lower chance of side-reactions occurring. 3.3. Magnetic properties for the synthesized ferrites Mo¨ ssbauer spectra of ferrites shown in Fig. 6 were measured in zero applied magnetic fields at room temperature. The Mo¨ ssbauer solid lines for the samples were fitted using sextets, which can be ascribed to Fe3+ ions in A and B sites according to the values of magnetic hyperfine field, and doublet representing paramagnetic behavior. Correspondingly, the characteristic parameters of Mo¨ ssbauer spectra including hyperfine field H values (Hhf), isomer shift (IS), quadrupole splitting (QS) and the relative area of subspectra are listed in Table 3. It can be found that according to the Mo¨ ssbauer fitting parameters, the obtained ZnFe2O4 phase exhibits a paramagnetic state at room temperature, and the substituted Ni1 xZnxFe2O4 (x = 0.5 and 0.66) phases exhibit a canted ferromagnetic structure characterized by a significant reduction of magnetic hyperfine field or by the collapse of the six lines of ferrimagnetic spectra, and the substituted Ni1 xZnxFe2O4 (x = 0.33) and NiFe2O4 phase exhibit collinear ferromagnetic ones. The result is closely consistent with those reported by Leung [35] and Macedo [36]. It also reveals that the nickel richer ferrite samples are in the magnetically ordered state arising from the superexchange interaction between Fe3+ ions in tetrahedral and octahedral sites, basically due to the affinity of Ni2+ in octahedral sites. Moreover, the observation of a doublet contribution in the Mo¨ ssbauer spectra of the zinc richer samples could be attributed to a superparamagnetic behavior of nanosized ferrite samples [35].
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4. Conclusion A series of pure Ni2+-, Zn2+-, Fe2+- and Fe3+-containing layered double hydroxide sulfates with an (Zn2+ + Ni2+)/(Fe2+ + Fe3+) molar ratio of 0.5 in the synthesis product, [(Ni + Zn)1 x yFey2+Fex3+ (OH)2]x+(SO42 )x/2mH2O, have been prepared by coprecipitation method. Powder X-ray diffraction shows the presence of only one good crystalline phase. The detailed study on the structure of LDHs materials has shown that change in the composition with Zn content results in small but significant changes in the parameter lattice a and the average crystalline size in the c direction. Powder X-ray diffraction, X-ray photoelectron spectroscopy and Mo¨ ssbauer spectroscopy show that calcination of the type tailored LDH precursors affords a pure Ni1 xZnxFe2O4 (0 x 1) spinel ferrite. The major advantage of the new method is that it affords uniform distribution of all metal cations at an atomic level in the LDH precursors; hence the formation of spinel ferrites starting from the LDHs requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring. The studies on magnetic properties of the synthesized ferrites clearly demonstrate that the zinc richer ferrite samples exhibit strong superparamagnetic relaxation while the nickel richer ferrite samples show resolved ferrimagnetic spectra with reduced magnetic hyperfine fields.
Acknowledgements We gratefully acknowledge the financial support from the NSFC (20371006, 20306003), Beijing Nova Program (2003B10) and program for new century excellent talents in university (NCET-04-0120).
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