XAS (XANES and EXAFS) Investigations of Nanoparticulate Ferrites

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J. Phys. Chem. C 2007, 111, 6252-6262

XAS (XANES and EXAFS) Investigations of Nanoparticulate Ferrites Synthesized Continuously in Near Critical and Supercritical Water Merete Hellner Nilsen,*,† Camilla Nordhei, Astrid Lund Ramstad, and David G. Nicholson Department of Chemistry, Norwegian UniVersity of Science and Technology, Høgskoleringen 5, N-7491 Trondheim, Norway

Martyn Poliakoff School of Chemistry, The UniVersity of Nottingham, UniVersity Park, Nottingham, United Kingdom NG72RD

Albertina Caban˜ as Department of Physical Chemistry, UniVersidad Complutense de Madrid, 28040 Madrid, Spain ReceiVed: May 2, 2006; In Final Form: February 15, 2007

Nanoparticulate ferrites (39-105 nm), including magnetite (Fe3O4) and materials containing additional metals (cobalt, zinc, and nickel), have been synthesized continuously in near-critical and supercritical water. For comparison, a cobalt ferrite (particle size, 16 nm) was synthesized by a conventional hydrothermal procedure. The local metal environments of the iron atoms and the additional metal in the ferrites have been studied by X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) in order to determine the type of spinel structure. Previously reported results for the bulk-phase materials show that Fe3O4 (magnetite), CoFe2O4, and NiFe2O4 exhibit inverse spinel structure, while ZnFe2O4 is a normal spinel. Our results show that the inverse spinel structures extend to the nanoregime. For the ZnFe2O4 material, XANES and EXAFS show that the sample adopts the normal structure in the same size regime. All of the materials are very polydisperse. In the case of CoFe2O4 synthesized under supercritical conditions the EXAFS clearly indicates that the particle sizes are weighted heavily toward 10 nm. To interpret the EXAFS, it was necessary to calculate actual or apparent multiplicities of the different interactions. These are based on 1/ of the metal atoms occupying the tetrahedral positions and 2/ the octahedral positions of the spinel structure. 3 3

Introduction The combined electronic and magnetic properties exhibited by ferrite spinels have found applications in catalysis, magnetic media, memory cores, and high-frequency devices.1,2 Below a certain dimension, we enter a regime where these properties are affected by crystal or particle size. Over a very wide size range the properties of solid-state materials are constant and seemingly independent of particle size. These bulk, or macroscopic, properties are effectively dominated by the crystal structure from which all of the quantum forces stemming from the constituent atoms are averaged. However, as crystal sizes progressively decrease, a stage is reached below which this averaging is no longer valid. The size-dependent onset of quantum effects is due to the increasing fraction, and hence influences, of surface atoms relative to those within the interior. For most materials, 30-50% of the constituent atoms in 3 nm diameter spherical particles actually lie at the surface. It is around this size limit that surface energy dominates physiochemical properties. Hence, manipulating the dimensions of particles is a central aspect in designing useful materials. The possibility of designing nanophase ferrites through an environmentally friendly synthetic procedure and, at the same * To whom correspondence should be addressed. Phone: +47 22857014, Fax: + 47 22855441. E-mail: [email protected]. † Present address: Department of Chemistry, University of Oslo, P. O. Box 1033, Blindern, N-0314 Oslo, Norway.

time, fast and continuous was the impetus behind a study on syntheses in near-critical and supercritical water.3 Heating a fluid toward its critical point causes major changes in its physical properties. In the case of water (critical point: Tc ) 374 °C; Pc ) 218 atm) the transformation is especially significant because it changes from a polar liquid to one that is virtually nonpolar and miscible with gases and organic compounds. These changes are spread over a fairly wide temperature range so that around 200 °C (near critical) the change in properties is significant enough to make the system synthetically interesting.4 A major reason for interest in the ferrite system stems from varying degrees of flexibility in distributing various cations within the overall spinel AB2O4 structure (see below). This distribution is mainly a function of the electronic configuration and valence state of the metals within the constraints of the structure, but also the method of preparation and of particle size (>10 nm) can be influential.5 Thus, depending on the metals, the two types of cation, A and B, frequently exhibit some disordering over two octahedral and one tetrahedral site per formula unit.6 The temperature dependency of this disorder for a number of spinels with different cation combinations has been extensively discussed in the literature.7 The cation distribution for a normal spinel can be formally represented by (A2+)tet(2B3+)oct(O2-)4 and for a generalized disordered system by (A1-xBx)tet(AxB2-x)octO4, where x is the inversion parameter.8-10 For the special case in which x ) 1 the spinel is termed “inverse”. Included in this study is the ferrite ZnFe2O4, which

10.1021/jp0626723 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/10/2007

Nanoparticulate Ferrites Synthesized in Water

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6253

TABLE 1: Summary over the Samples of Synthetic Conditions and Characterizationa

sample

reacn temp/°C

Fe3O4 CoFe2O4d CoFe2O4 NiFe2O4 NiFe2O4 ZnFe2O4

200 178 200 200 400 400

additional products XRD analysisb Fe (vw) Ni (w) Ni (vw) Fe (w); Fe3O4 (vw)

particle sizec/nm

code

61 16 39 38 43 105

Fe(61) CoFe(16) CoFe(39) NiFe(38) NiFe(43) ZnFe(105)

a For additional information, see ref 4. b w, weak; vw, very weak (see ref 4). c These sizes are based on the largest particles in these materials (except CoFe(16)). In addition, there are small particles of ca. 10 nm. See text and ref 4 for further discussion. d This material is synthesized by a conventional hydrothermal procedure (see Experimental Section).

exert a significant influence. This is because the oxygen atoms at or near the surface need to satisfy different bonding requirements than those located in the interior and this has an effect on the electronic properties of the metals. For particles larger than 10 nm these designations are still reasonable.13 Structural characterization of these samples is crucial to rationalizing their properties because the method of preparation can affect the cation distribution over the spinel structure. This paper focuses mainly on a series of ferrite spinels that have been synthesized in near-critical or supercritical water.3 These materials have previously been partly characterized by X-ray powder diffraction and transmission electron microscopy. Whereas X-ray diffraction yields information on long-range structural aspects, X-ray spectroscopy (XAS) provides complementary details on the electronic environments of the metals and on the short-range structure. Indeed XAS comes into its own for small nanoparticles. Since the early 1990s, a large number of studies have established that XAS is important for characterizing these type of systems, particulary with regard to spinels with particle sizes within the nanoregion.14-20 Previous XAS work on ferrites has been directed toward comparative studies rather than explicit determinations of local environments about the metal atoms.6,11,15,21,22 We report here an XAS study on some normal and inverse spinel nanoferrites (38-105 nm) synthesized in near-critical and supercritical water. For comparison, a cobalt ferrite (16 nm) was synthesized by a conventional hydrothermal method. More detailed information from the XAS was extracted by using calculated apparent multiplicities (see below) in the EXAFS study. Experimental Section

Figure 1. Powder X-ray diffraction pattern (Cu KR radiation) of the hydrothermally synthesized CoFe(16).

is reported to have a normal spinel structure in the bulk phase (no degree of inversion; x ) 0),10,11 and Fe3O4, CoFe2O4, and NiFe2O4, which in the bulk adopt the completely inverse spinel structure [(B3+)tet(A2+B3+)oct(O2-)4].12 The validity of the designation “normal” and “inverse” weakens for very small particles because then surface effects

Syntheses. All the compounds except CoFe2O4 (16 nm) were synthesized hydrothermally by reacting mixtures of Fe(II) and different metal(II) acetates (Co(II), Zn(II), and Ni(II)) in nearcritical and supercritical water (size range, 38-105 nm) using a flow reactor as previously described.3,23 In every case nanophase spinels with “bimodal” particle size distributions (small particles of ca. 10 nm and larger aggregates up to 100 nm) were obtained. Sample CoFe2O4 (16 nm) was synthesized

Figure 2. Normalized and first derivative Fe K-edge XANES of the model compounds: (a) octahedral iron in Fe2O3, (b) a mixture in the inverse spinel, magnetite, Fe3O4, and (c) tetrahedral iron in FePO4.

6254 J. Phys. Chem. C, Vol. 111, No. 17, 2007

Nilsen et al.

Figure 3. Normalized and first derivative Fe K-edge XANES of (a) ZnFe(105), (b) NiFe(38), (c) NiFe(43), (d) CoFe(39), (e) CoFe(16), and (f) Fe(61).

in the following manner: the pH of the cobalt and iron nitrate solutions were adjusted to 8.5 by adding aqueous ammonia (25%), and the resulting co-precipated mixture was heated in Teflon-lined Parr vessels at 178 °C at autogenous pressure for 4 h. After the hydrothermal reaction, the solid material was separated from the solution by centrifugation. The solid materials were washed with water and dried at 110 °C.24,25 Characterization. All the materials were characterized by X-ray powder diffraction (XRD). In selected cases, transmission electron microscopy (TEM) was used to verify the particle sizes obtained from XRD.3 The relative amounts of the different metals in the ferrites were determined by atomic absorption (AA) spectroscopy. For some samples, the local relative amounts of the metals were determined by energy-dispersive X-ray analysis (EDX).3 Powder X-ray diffraction data were collected using a Phillips EXPERT and a Siemens D 5005 diffractometer. In both cases, KR radiation from a Cu target was used. The peak broadening was obtained by measuring the width of the strongest diffraction peak at high 2θ in order to give a more accurate result (the 440-reflection). This reflection does not overlap with others. The Scherrer equation using shape factor 0.9 was used in calculating particle sizes (microstrain was not considered).26 X-ray Absorption Data Collection. The data were collected at the European Synchrotron Radiation Facility (ESRF), Grenoble, France, using the facilities of the Swiss-Norwegian beam-

TABLE 2: Apparent Multiplicities in EXAFS, Based on the General Expression (A1-xBx)tet(AxB2-x)octO4, Where A is the Divalent, B the Trivalent Cation, and x the Inversion Parameter apparent multiplicity in EXAFS spinel structure AB2O4

(A0.5B0.5)((A0.5B1.5)O 4

(B)(AB)O4

calculation

A K-edge

Normal (x ) 0) A-edge: A ) 4 B-edge: B ) 6

4

6

5

5.5

6

5

Partly Inverse (x ) 0.5) A-edge: 1 /2 × 4 ) 2 1 /2 × 6 ) 3 B-edge: 1 /4 × 4 ) 1 3 /4 × 6 ) 4.5 Totally Inverse (x ) 1) A-edge: A)6 B-edge: 1 /2 × 4 ) 2 1 /2 × 6 ) 5

B K-edge

lines (SNBL). The electron energy was 6 GeV with a maximum current of 200 mA. The spectra were recorded at room temperature at the iron, cobalt, zinc, and nickel K-edges. The



Figure 4. Experimental (s) and calculated (--) k3-weighted Fe EXAFS and its Fourier transform for (a) Fe3O4 (bulk) and (b) Fe(61) (nanophase) (k-range, 2-16 Å-1).

monochromator and gas ion chambers are described elsewhere.27 The samples were prepared as described previously.27 The energy calibration was checked by measuring the spectra of the 0.005 µm thick metal foils. Several scans of each material were collected and summed. EXAFS Data Analyses. After formatting and correcting for dark currents, the spectra were summed, deglitched, backgroundsubtracted, and calibrated to yield the EXAFS function χobs i (k) using the Daresbury programs, EXCALIB and EXBACK.28 The edge positions were determined from the first inflection points of the derivative spectra. The EXCURVE9829 program (curvedwave theory with ab initio phase shifts being calculated from within the program) was used to fit the EXAFS (k3-weighting scheme) which was Fourier filtered over a wide range (1.025.0 Å). This filter removes the low-frequency contributions to the EXAFS below 1 Å, but does not smooth the spectrum or remove any noise. The spectra of the model compounds, magnetite (Fe3O4),30 cobalt oxide (CoO),31 nickel oxide (NiO),32 and zinc oxide (ZnO)33 were used to check the validity of the ab initio phase shifts and to establish the AFAC (amplitude reduction due to many-electron processes).34 The AFAC values were used in the analyses of the unknowns because this reduces residual systematic errors in the multiplicities.27 During least-squares fitting it is important to avoid correlation effects between parameters that strongly affect the EXAFS amplitude and between those that influence the frequency of the EXAFS oscillations. Therefore, the EXAFS spectra were least-squares-fitted using k1- and k3-weighted data because it has been shown that optimizing the k1- and k3-weighted fits reduces the degree of coupling between the highly correlated parameters (N, 2σ2 and r, EF), giving a solution common to

both weighting schemes.35,36 The k3-weighting scheme used in the refinement compensates for the diminishing photoelectron wave at higher k. All of the spectra were treated in exactly the same manner, and the validity of the data reduction and fitting procedures was checked against the spectra of the reference compounds. The refinements were carried out to minimize the fit index (FI):

FI )

2 - χcalc ∑i [k3(χexp i i )]

where χexp and χcalc are the experimental and theoretical i i EXAFS, respectively.28,29 If meaningful results are to be obtained, it is essential to identify the maximum number of independent parameters, Nind, that may be varied in the EXAFS analysis. This is given by Nind ) (2∆k∆R/π) + 2, where ∆k is the extent of the data in k-space and ∆R the range of distance being modeled.37 Another constraint is the smallest separation of shells that can be resolved. This is given by π/∆k.38 Results and Discussion Characterization. X-ray powder diffraction shows that the materials synthesized in near-critical or supercritical water correspond to a similar ferrite phase. In some cases other biproducts and unreacted metal salts were detected, as previously reported.3 Minor quantities of iron and nickel appear in some of the samples synthesized in supercritical water. Metallic iron originates from the disproportionation reaction of iron(II) as extensively discussed in ref 3. Nickel seems to come from the reduction of Ni(II), the reducing agent being a form of


Nilsen et al.

TABLE 3: Results of the EXAFS Fe K-Edge Curve-Fitting for Ferritesa Interaction bulk-phase Fe3O4 (model compd)c Fetetr-O Feoct-O Feoct‚‚‚Feoct Fetetr‚‚‚Feoct Fe(61)c Fetetr-O Feoct-O Fe-Fe Feoct‚‚‚Feoct Fetetr‚‚‚Feoct CoFe(16) Fetetr+oct-O Feoct‚‚‚Feocte Fetetr‚‚‚Feoct CoFe(39) Fetetr+oct-O Feoct‚‚‚ Feocte Fetetr‚‚‚Feoct NiFe(43)c Fetetr+oct-O Feoct‚‚‚Feocte Fetetr‚‚‚Feoct NiFe(38) Fetetr+oct-O Feoct‚‚‚Feocte Fetetr‚‚‚Feoct ZnFe(105) Feoct-O Fe-Fe Feoct‚‚‚Feoct Feoct‚‚‚Zntetr

N

r/Å

2σ2/Å2

EF/eV

Rb/%

TABLE 4: Results of EXAFS Co, Ni, and Zn K-Edge Curve-Fittinga interaction

N

2σ2/Å2

r/Å

EF/eV

R/%

Co K-Edge CoFe(16) Cooct-O Cooct‚‚‚Feoctb Cooct‚‚‚Fetetr CoFe(39) Cooct-O Cooct‚‚‚Feoctb Cooct‚‚‚Fetetr

1.3 4.0 4.0 8.0

1.87(1) 2.03(1) 2.982(8) 3.474(6)

0.004(2) 0.014(3) 0.016(2) 0.016(1)

-1.1(7)

33.9

1.8(3) 3.1(4) 0.5d 4.4(4) 10(1)

1.87d 2.03d 2.47(2) 2.963(7) 3.472(4)

0.08(3) 0.08(2) 0.007(3) 0.018(2) 0.018(2)

-0.3(3)

37.2

5.1(4) 5(1) 7(1)

1.953(7) 2.975(8) 3.477(7)

0.016(2) 0.016(4) 0.014(3)

5.5(3) 4(1) 4(1)

1.98(1) 2.982(1) 3.482(8)

0.019(2) 0.025(5) 0.018(4)

0.6(6)

30.6

4.9(6) 4(1) 10(2)

1.95(1) 2.97(1) 3.459(8)

0.021(3) 0.020 (5) 0.018(3)

1.3(9)

33.3

4.7(4) 5(1) 8(1)

1.95(1) 2.97(1) 3.466(6)

0.021(3) 0.023(5) 0.016(2)

1.3(8)

30.6

See footnotes in Table 3 for details. b The peak is a composite of both iron and A cation (A ) Co, Ni). The shell is refined as iron.

4.1(5) 0.5d 4.1(8) 5(2)

2.01(1) 2.482(2) 2.985(6) 3.483(3)

0.014(3) 0.021(5) 0.011(2) 0.014(4)

-3.1(7)

38.8

ing the intensities of target atom with different valences. These peaks are diagnostically useful because their positions and intensities are also sensitive to the immediate symmetry about the metal. For example, the preedge feature is several times more intense for noncentrosymmetric tetrahedral iron(III) environments than for centrosymmetric octahedral. An example of the former is the model compound FePO4. The other model compounds, Fe2O3 and Fe3O4, contain the metal atoms in octahedral and both tetrahedral and octahedral sites within the oxide sublattices, thereby giving less intense preedge peaks. The latter compound is an inverse spinel in which iron(III) occupies the tetrahedral sites, and both iron(II) and iron(III) occupy the octahedral sites. Examples of the symmetry-dependent intensities of iron K-edge preedge peaks for the three model compounds are shown in Figure 2. The preedge peak intensity for the inverse spinel is approximately a weighted average of tetrahedral and octahedral intensities. Figure 3 shows the normalized XANES at the Fe K-edge for the ferrites and their first derivative spectra. The preedge peaks for the cobalt and nickel ferrites are more intense than the preedge peak in Fe3O4. This accords with these materials (including the 40 nm particles) being inverse spinels. Of the two cobalt samples, the lower intensity preedge peak in CoFe(39) is consistent either with a partially inverse structure or a mixture of phases. In the case of the zinc ferrite, ZnFe(105), the preedge peak is considerably weaker, but still stronger than the preedge peak (octahedrally coordinated Fe(III)) in Fe2O3. Taking into account that ZnFe(105) contains small amounts of Fe3O43 (which will strengthen the preedge peak), the results support zinc ferrite being a normal spinel with iron in octahedral sites, as reported in the literature.10 The first derivative of the XANES spectra is useful because it highlights the features around the absorption edge and facilitates the determination of transition energies. The characteristic triplet in the derivative iron edge spectrum of the Fe3O4 model is common to most of the nanophase as also observed in Co3O4 and CoAl2O4 systems.36 The XANES spectrum for

6.2(7) 6(1) 7(2)

2.02(1) 2.95(1) 3.48(1)

0.022(4) 0.020(4) 0.019(5)

1.4(9)

39.5

7.2(5) 1.4(6) 3(4)

2.05(7) 2.97(1) 3.49(2)

0.024(3) 0.010(5) 0.017(1)

1.9(6)

33.5

i K-Edge

0(1)

30.5

NiFe(43) Nioct-O Ni-Ni Nioct‚‚‚Feoctb Nioct‚‚‚Fetetr NiFe(39) Nioct-O Ni-Ni Nioct‚‚‚Feoctb Nioct‚‚‚Fetetr

Fe2(OH)2.3 The reaction conditions, composition, and particle size of the samples are summarized in Table 1. Also the XRD of the CoFe2O4 material synthesized under hydrothermal conditions shows the spinel structure as exemplified in Figure 1. All of the diffraction peaks that are indexed match the literature values for this compound.39 The XRD pattern exhibits considerable line broadening consistent with the fine-particle nature of the cobalt ferrite. The calculated size of CoFe2O4 particles using the Scherrer equation was 16 nm. XANES. The preedge and XANES regions of the absorption spectrum are important because they contain electronic information on the immediate environment of the absorbing atom which in principle can be translated into spatial or geometrical information. This information complements that provided by the EXAFS region which is restricted to interatomic distances. For K-edge absorption spectra of first row transition metals there are preedge features stemming from 1s f 3d transitions that are forbidden by dipole selection rules and hence of weak intensity. The preedge intensity is also valence-dependent since the electronic configuration has to be factored in when compar-

2.04(1) 2.46(3) 2.942(8) 3.46(1)

0.014(3) 0.01(1) 0.015(4) 0.015(7)

2(1)

43.0

4.5(6) 3.0(8) 5(1) 4(1)

2.03(1) 2.48(1) 2.92(1) 3.47(1)

0.014(3) 0.016(3) 0.020(8) 0.017(7)

3(1)

40.3

-1(1)

42.9

Zn K-Edge ZnFe(105) Zntetr-O Zntetr‚‚‚Feoct

a The EXAFS refinements give information about multiplicity (N), bonding distance (R), and thermal vibration (Debye-Waller factor, 2σ2). EF is the refined correction of Fermi energy in vacuum, compared to E0 in EXBACK. The standard deviation in the last significant digit as calculated by EXCURV98 is given in parentheses. These estimates, however, will in cases of high correlation between parameters lead to an overestimation of accuracy as the standard deviations for bonding distances are (0.01 Å for small r-values and (0.04 Å for r-values exceeding 3 Å. The deviation for 2σ2 is (20%. b The statistical R-factor is defined in the text and gives indication of the quality of fit in k-space. c The fixed multiplicities of the model are taken from references and fixed in the refinements (see text). d Fixed values. e The peak is a composite of both iron and A cation (A ) Co, Ni). The shell is refined as iron.

5.9(7) 0.5(7) 6(1) 4(2)

4.5(4) 22(4)

1.948(9) 3.505(7)

0.012(3) 0.028(2)

a



Figure 5. Experimental (s) and calculated (--) k3-weighted Fe EXAFS and its Fourier transform for (a) CoFe(39) and (b) CoFe(16) (k-range, 2-12 Å-1).

Fe(61) (Fe3O4) is very similar to that of the bulk Fe3O4 model (Figures 2 and 3), which indicates that the surface and nearsurface interactions in the 61 nm system are dominated by those from the interior. The traces of iron metal3 contamination are not visible in the XANES other than a slightly elevated preedge peak consistent with the metal absorption edge. For all the other ferrite samples, the iron absorption edge shifts to higher energies relative to Fe3O4, as iron changes a higher valence state. However, in NiFe2O4 and ZnFe2O4 the edge energies do not match those of the iron(III) references (Fe2O3 and FePO4) as might be expected. Whereas the cobalt ferrites (CoFe(16) and CoFe(39)) both show iron(III), the edges for the nickel and zinc ferrites (NiFe(38), NiFe(43), and ZnFe(105)) indicate that a fraction of the iron content is divalent. Both NiFe(38) and NiFe(43) contain minor amounts of nickel metal which may reflect nickel deficiency in the ferrite and the concomitant reduction of some of the iron(III) to iron(II) to maintain electroneutrality. Indeed, the XRD pattern for the nickel samples match that of NiFe2O4 and/or nickel-deficient (Ni,Fe)Fe2O43. ZnFe(105) contains Fe3O4 and iron metal as additional phases, which explains the observed shift in absorption edge energy. The unresolved peaks in the derivative spectrum (Figure 3) show that a mixture of phases is present, mainly including contributions from ZnFe2O4 and Fe3O4. Both the lack of features in the firstderivative spectra of CoFe(39) and the EXAFS (see below) show unexpected deviations from the spinel structure given that the particle size is as large as 39 nm. (This is also observed at the cobalt edge. The XANES were also measured at the second metal (Co, Ni, or Zn) K-edges in the mixed ferrites (figures not shown). In the case of the bulk inverse ferrites, CoFe2O4 and NiFe2O4,

it is known that the second metal occupies the octahedral sites.12 The cobalt and nickel edge XANES show that this is also the case for the nanophase materials. Consistent with octahedral coordination, the nickel ferrites have edge energies and features similar to that of the octahedral nickel(II) reference NiO. The peaks in the first-derivative spectra of the samples are less prominent than in the reference, which may be due to the samples being nanoparticulate. Nickel metal in NiFe(43) (Table 1) is manifested as a shoulder in the preedge region which increases in NiFe(38) as the metal content increases. As expected, this is accompanied by a weakened white line. The cobalt K-edge XANES spectra for the cobalt ferrites are similar to that of the CoO reference, indicating octahedral coordination.17,40 Since zinc in ZnFe2O4 is a divalent 3d10 atom, it does not exhibit a preedge feature in the zinc XANES.41 Hence, the diagnostic test for tetrahedrally coordinated transition metal environments cannot be used. However, there are large differences between XANES of the ZnO reference (octahedrally coordinated) and ZnFe(105). Since the valence state is restricted to divalent, these differences suggest that zinc in the ferrite occupies the tetrahedral sites rather than the octahedral. EXAFS. In analyzing the EXAFS of mixed environments of the same edge, it is necessary to take into account the relative weighting of the occupied sites. In Fe3O4, for example, although the actual coordination number of iron on the 1/3 tetrahedral sites is 4 and iron on the 2/3 octahedral sites is 6 from the point of view of the XAS the composite multiplicities are actually 4 × 1/3 and 6 × 2/3, respectively. In this work we use the term apparent multiplicities to distinguish from multiplicities of compounds containing identical sites at the given edge. In


Nilsen et al.

Figure 6. Experimental (s) and calculated (--) k3-weighted Co EXAFS and its Fourier transform for (a) CoFe(39) and (b) CoFe(16) (k-range, 2-13 Å-1).

principle, the value of the multiplicities enable distinctions to be made between normal, inverse, and partly inverse (Table 2). Since in a normal spinel (AB2O4) the divalent (A) and trivalent (B) cations occupy exclusively the tetrahedral and octahedral sites, respectively, the first shell multiplicities for both A and B K-edges correspond to the coordination numbers. For an inverse spinel, B(AB)O4, the A site is now octahedral with the B cations being distributed over the remaining tetrahedral and octahedral sites. It follows then taking into account the relative weighting of the occupied sites that the multiplicity of the first B-O shell at the B edge is now apparently 5. Some spinels, especially in small particles (

XAS (XANES and EXAFS) Investigations of Nanoparticulate Ferrites

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