CROATICA CHEMICA ACTA CCACAA 70 (2) 719-734 (1997)

ISSN-00l1-1643CCA-2448 Original Scientific Paper

Formation and Characterization of NiFe204

Svetozar Musić, a Davor Balzar;" Stanko Popović, a,b Marijan Gotić,aIlana Czak6-Nagy, c and Samet Dalipi"

aRuder Bošković Institute, P. O. Box 1016, 10001 Zagreb, Croatia

bDepartment of Physics, Faculty of Science, University of Zagreb,p. o. Box 162, 10001 Zagreb, Croatia

"Department of Nuclear Chemistry, Eotoos Lorand University,p. O. Box 32, 1518 Budapest, Hungary

Received January 25, 1996; revised June 4, 1996; accepted June 7, 1996

Synthesis of nickel ferrite, NiFe204, was performed applying thethermal treatment of the corresponding mixed metal hydroxides orthe solid state reaction between NiO and a-Fe203. The sampleswere studied by X-ray diffraction, Fourier transform IR spectro-scopy and 57Fe Mčssbauer spectroscopy. Ball-milling of NiFe204caused a decrease of hyperfine magnetic fields corresponding toFe3+ ions in tetrahedral and octahedral sites, an increase of theMossbauer spectral line widths, as well as a slight increase of iso-mer shifts. It was supposed that the ball-milling of NiFe204 hadmore influence on the degree of inversion than on other structuralproperties of the spinel. It was found that the heating temperature,and not the heating time, had the ultimate effect on NiFe204 mi-crostructure. Samples heated up to 500 °C showed a pronouncedsize-correlated diffraction line broadening, corresponding to the co-herent domain size of about 13 nm, and rather small crystallinedisorder. Samples heated at temperature s above "" 1000 °C hadmuch larger crystallites, exhibiting very small disorder.

720 S. MUSIĆ ET AL.

INTRODUCTION

Nickel ferrite is an inverse spinel in which the tetrahedral or A-sitesare occupied by Fe3+ ion, and the octahedral or B-sites by Fe3+ and Ni2+ions. Fully inverse nickel ferrite is described by the structural formula(Fe3+)A[Ni2+Fe3+]BO~-.Nickel ferrite and substituted nickel ferrites are thematerials of interest for the application in electronic technology. These ma-terials, besides specific magnetic properties, are characterized by a strongabsorption of electromagnetic radiation in the microwave frequency rangeand, due to these properties, they have a potential use in devices for tele-communication measurements, or as surface coatings on aircraft or misslestructures.' Nickel ferrites of different stoichiometry are also found in therust formed by corrosion of nickel containing alloys.

Scientists and engineers have investigated nickel ferrite, as well as sub-stituted nickel ferrites from different standpoints. For instance, Morrish andHaneda'' prepared NiFe204 particles of average~sizes 250 ± 50, 800 ± 200and 1300 ± 200 A. The authors suggested a non-collinear magnetic structurenear the surface of NiFe204 particles. In accordance with this suggestion,small NiFe204 particles consisted of a core with the usual spin arrangementand asurface layer with magnetic moments inclined to the direction of thenet magnetization. A non-collinear magnetic structures near the surface ofy-Fe203, 57Fe surface-enriched y-Fe203, 57Fe-dopedCr02 and c-Fe particleswere also observed.i'

Mixed hydroxides Ni(OH)2IFe(OH)3,containing up to x = 0.10 NiO, werethermally treated in order to prepare NiFe204.4NiO was found to be solublein a-F~203 at 550 "C (XNiO = 2%), and the formation of NiFe204 was alsoobserved for a bigger content of NiO. When the stoichiometric ratio forNiFe204 formation was used, the precursor of NiFe204 was observed at""185 "C, which was transformed to amorphous ferrite at ""275 "C and tocrystalline NiFe204 above 400 "C. Morozumi et al.5 hydrothermally treatedthe mixed hydroxides, Ni(OH)2IFe(OHh, up to 250 "C, Formation of nickelferrite precursor was observed at 75 "C, and of nickel ferrite at temperaturesabove 100 -c.

Mixed hydroxides, Ni(OHhlFe(OHh, containing different molar fractionsof NiO and Fe203, were thermally treated up to 800 or 1100 cC.6Oxide phasesa-Fe203, NiO and NiFe204 were detected in the samples prepared at 800 CC.The samples prepared at 1100 "C contained one more phase, with the struc-ture similar to stoichiometric NiFe204. Formation of oxide phases, generatedduring the synthesis of NiFe204 by thermal decomposition of the mixed salts,Ni(N03)~e(N03)3· nH20, NiO: Fe203= 1 : 1, was also investigated." Peev etal.8 examined the thermal decomposition of the salt Nio.33Feo.67(NH4MS04h.. 6H20 using thermal analysis and Mossbauer spectroscopy. The optimumtemperature for NiFe204 formation was found to be 900 "C.

FORMATION AND CHARACTERIZATION OF NiFe,O, 721

Mossbauer spectra of several samples, prepared in the system ~i,;Fe3-x04'O::; x::; 1, were recorded at room temperature.f The rapid electron exchangebetween Fe2+ and Fe3+ ions in the same lattice site was shown. MacKenzieand Cardile-? investigated the formation of nickel ferrite from NiO andspinel iron-sand found on the west coast of New Zealand's North Island.

Spherical and hollow NiFe204 particles were produced using the methodof aerosol pyrolysis.U Aerosol droplets, containing a dilute aqueous solutionof Ni(N03)2 + Fe(N03hwith stoichiometric ratio for NiFe204 formation, werethermally treated between 450 and 810 ac.

The princip al corrosion product, formed in the stainless steel primarycooling circuit of a PWR (Pressurized Water Reactor), is generallyacceptedto be a nonstoichiometric nickel ferrite, NixFe3_x04'0.45 < x < 0.75.12Raw13investigated variously prepared samples of magnetite and nickel ferrite insimulation of corrosion products. The Ni-substituted magnetite (x "" 0.5) andNiFe204 were detected in the rust formed by the heating of Fe-Ni alloys inair at 635 aC.14 Lenglet et al.15 characterized the oxidation products formedon stainless steel (18Cr-10Ni-2Mn) at 900-1100 ac using the X-ray diffrac-tion, conversion electron Mossbauer spectroscopy and Fourier transformspectroscopy. Cr203, MnCr204 and a-Fe203 were detected as oxidation prod-ucts on the steel surface, but not NiFe204, NiO and NiCr204'

The aim of the present work was to obtain more information about thechemical and structural properties of NiFe204 formed under different ex-perimental conditions. Special attention was paid to the influence of me-chanical treatment on the structural properties of NiFe204'

EXPERIMENTAL

All chemicals were ofp.a. purity. Twomethods of the NiFe204 preparation wereused. Samples 81 to 86 were prepared using mixed hydroxides obtained from theNi(N03)2 + Fe(N03)3 solution by addition of the NaOH solution. The mixed hydrox-ides Ni(OHhlFe(OH)s, NiO: Fe203 = 1 : 1, were washed from »neutral- electrolyteand then heated at different temperatures, as given in Table I. A LKO II furnacewith Kanthal heaters was used at temperatures above 1000 "C. Samples 87 to 811were prepared using the solid state reaction between NiO and a-Fe203 in stoichio-metric ratio for the NiFe204 formation. The mixture of NiO and a-Fe203 , used forthe preparation of samples 87 and 88, was mechanically activated for 15 to 60 min-utes, respectively, while the time of mechanical activation in the preparation of sam-ples 89, 810 or 811was 3 hours. A planetary mill by Fritsch (pulverisette 5, agatebowl with balls of 99.9% 8i02) was used. Mechanically treated oxide powders werepressed into tablets using a Carver press, and then heated at high temperatures,as given in Table I.

All samples were studied using the X-ray powder diffraction (XRD), Fouriertransform infrared (FT-IR) spectroscopy and 57Fe Mossbauer spectroscopy. XRDmeasurements were performed at room temperature (RT) using a Philips counter

722 s. MUSIĆ ET AL.

TABLE I

Thermal treatment of the samples and their phase composition, as determined byX-ray diffraction

8ample Heating Heating Phase Remarkstempo/ -c time / h composition200 1300 1400 1

81 800 1 NiFe204 Sharp diffraction linesnoo 11200 11350 3

82 500 2 NiFe204 + a-Fe203 (few %)

83 500 4 NiFe204 + a-Fe203 (few %) Broad diffraction lines

NiFe204 + a-Fe203 (few %)All samples show

84 500 8 practically equal85 500 24 NiFe204 + a-Fe203 (few %) diffraction patterns

86 500 48 NiFe204 + a-Fe203 (few %)200 1300 1400 1

87 800 NiFe204noo 11200 11350 3 Very little broadened

diffraction lines in200 1 comparison to 81300 1400 1

88 800 1 NiFe204noo 11200 11350 3

89 200 1 a-Fe203 + NiO300 1 NiO-broadeneddiffraction lines200 1 8imilar diffraction

810 300 1 a-Fe203 + NiO patterns400 1200 1

811300 1 NiFe204 + a-Fe203 (-5%) Very little broadened400 1 diffraction lines800 5

diffractometer, model MPD 1880, with monochromatized eu-Ka radiation (graphitemonochromator). FT-IR spectra (RT) were recorded using a Perkin-Elmer spectrome-ter, model 2000. The FT-IR spectrometer was coupled to a personal computer loadedwith the IR Data Manager (IRDM) program. Specimens for FT-IR measurementswere prepared by pressing the samples with spectroscopically pure KBr or polyeth-

FORMATION AND CHARACTERIZATION OF NiFe,O, 723

ylene in small pellets. Mossbauer spectra (RT) were recorded on a spectrometer pro-duced by WISSEL. The spectra were fitted using the SIRIUS program.J''

RE8ULT8 AND DI8CU88ION

Phase composition of the studied samples, as determined by XRD, isgiven in Table I.

NiFe204 as asingle phase was obtained after heating the mixed hydrox-ides, Ni(OH):!Fe(OH)3, NiO: Fe203 = 1 : 1, up to 1100 or 1350 °C, and thecorresponding X-ray diffraction patterns contained sharp diffraction lines. 'At 500 °C, for the heating time from 2 to 48 hours, NiFe204 was formed asthe dominant phase, exhibiting a pronounced diffraction broadening. Thesesamples also contained a small amount of U-Fe203.It was reported that nickelferrite could be also obtained from mixed hydroxides, Ni(OH)2IFe(OH)3,attemperatures below 500 °C;4,5however, in that case, its stoichiometry andcrystallinity varied significantly, and generally other oxide phases were pre-sent in the samples.

Lattice constant of NiFe204 (space group Fd3m) was determined in thepresent work using u-Al203 (corundum by Merck, 99.999%) as internalstandard (space group R3c, lattice constants, in terms of hexagonal axes,a = 0.4758, c = 1.2991 nm at 26 °C, ICDD Powder Diffraction File (card no.10-173). For sample 81, utilizing diffraction lines at the highest possibleBragg angles, well resolved in spectral doublet components KU1U2(Figure1), a value of a = 0.83377(3) nm was obtained at 26 °C. For samples 82 to86 , having broad diffraction lines, not resolved in spectral components (Fig-ure 2), practically the same value was obtained, a = 0.8338(1) nm.

8amples 87 to 811,prepared by the solid state reaction between NiO +U-Fe203 had also practically the same lattice constant as the ones obtainedby precipitation from hydroxides. 8amples 87, 88 and 811, heated to hightemperature, showed very little broadened diffraction lines of NiFe204 incomparison with sample 81. However, samples 89 and 810,heated up to 300and 400 °C, respectively, contained U-Fe203and NiO, which showed broaddiffraction lines.

8amples 82 to 86 showed a pronounced diffraction-line broadening, ascompared to all the other samples heated at a much higher temperature. Ac-cording to the accepted theories,17,18both »size- and »strain- effects causediffraction-line broadening. The size term does not necessarily correspond tothe size of particles or grains, but to the size of domains distinctly definedby incoherent diffraction, which can be caused by stacking (deformation) ortwin (growth) faults, small-angle boundaries due to dislocation ordering, orsimilar extended lattice defects. That is why the domain size, determinedby diffraction methods, is usually underestimated in relation to a size ob-

724

3000

2500

2000

1500

1000...-.. 500en--en.•...~ O~Oo~.o'H 1600en~cl) 1400.•...~ 1200

1000800600400200

O

S. MUSIĆ ET AL.

+ Observed data 311 + (a)--Refined pattem.......... Componcnt line profiles

220

222

~ s i.. A

30 32 34 36 38

+ Observed data 440 (b)+

--Refmed pattem.......... Component line profiles

511

---!.l '.V' Vr

56 58 602e;o

62 64

Figure 1. X-ray diffraction pattern of sample Sl. Refined pattern was obtained throughthe profile-fitting least-squares minimization procedure described in the text, Differencebetween the observed and refined patterns is plotted around the zero-intensity line.The Mi11erindices of dim-action lines of NiFe204 are denoted, Note sharp diffractionlines, which make 311 and 222 completely resolved.

tained by other, most1y optical methods (microscope, laser size analyzer,etc.). The strain term includes contribution from any disruption of a regularlattice, such as dislocations and point defects. Therefore, there may be atleast two reasons for substan tial line broadening of samples 82 to 86 , smalldomain size and/or disorder at crystallographic sites, which is possible in thespinel structure. However, the fact that diffraction patterns of samples 82to 86 and particularly the full width at half maximum (FWHM) of their dif-fraction lines did not change noticeably, regardless of heating time (see Ta-

FORMATION AND CHARACTERIZATION OF NiFe,O, 725

1000 (a)900800700600500400300 220

200100

O

30

311

35 45

+ Observed data--Refined pattem----------Component line profil es

400

40

500 (b)

400

300

200

100

o ..

+ Observed data--Refined pattem---..----.Component line profiles

511

440

...--.....•_ ...••......531

6028/0

6550 55

Figure 2. X-ray diffraction patterns of sample S3.Refined pattern was obtained throughthe profile-fitting least-squares minimization procedure described in the text. Differencebetween the observed and refined patterns is plotted around the zero-intensity line.The Miller indices of diffraction lines of NiFe204 are denoted. Stars indicate posi-tions of diffraction lines of a small amount of a-Fe203 impurity. Note broad diffrac-tion lines, where 222 is partially hidden under the much stronger 311 line.

ble I), favours the former assumption. A rough estimate from FWHM usingthe Scherrer equatiorr'f

(1)

(A, is the X-ray wavelength, e is a diffraction and eo the Bragg angle, andthe integral breadth [3(2e) was approximated with FWHM yields an average

726 S. MUSIĆ ET AL.

coherent volume-weighted domain size of 13 nm. Nevertheless, we under-took the more detailed line-broadening analysis to further clarify this point.

It is widely accepted that the Stokes Fourier deconvolution method.š" fol-lowed by the Warren-Averbach analysis.l'' gives the lea st biased approach.However, in cases of substantialline overlapping, the Stokes method generallyfails due to unresolved tails of diffraction lines. Figure 2 shows charac-teristic parts of the diffraction pattern of sample S3' where large overlappingof almost all lines is visible. We used a different profile-fitting approach.P!which uses pre-set analytical functions to model different contributions toline broadening and minimizes the residual between the computed and ob-served diffraction patterns. The subsequent line-broadening analysis isbased on the »double-Voigt- method.F Before any line-broadening analysis,the instrumental contribution to the broadening must be accounted for. Thisis accomplished by careful measurement of a suitable standard specimenshowing a minimal physical (structural, specimen) broadening. Sometimes,the adequate standard can be the same material that was properly an-nealed. Figure 1 shows quite sharp diffraction lines of sample Sl. However,a comparison with the diffraction pattern of NIST standard reference ma-terial 660 LaB6 showed, though not large, some residual broadening ofNiFe204 in sample Sl. Moreover, we chose to use LaB6 as a standard, to beable to compare the parameters of samples Sl and S3.

In the fitting procedure, the instrumentalline profile is synthesized frompre-determined parameters which define instrumental broadening. This lineprofile is convoluted with a pre-set Voigt function which models the physi-cally broadened line profile. After the background is added, the thus calcu-lated pattern is compared to the observed data. The refinable parametersof the Voigt function are adjusted until the least-squares residual reaches aminimum. All the refined parameters of the physically broadened line pro-files for seven strongest NiFe204 reflections for both Sl and S3 samples arepresented in Table II. Standard uncertainties and particularly the pattern-discrepancy factors indicate somewhat larger relative errors for sample Sl>as a consequence of a similar broadening of Sl and LaB6 diffraction lines.Figure 3 presents the so-called Williams on-Hall plot23for sample S3. The in-tegral breadths, {3(s) = {3(2e)cos8jA, are plotted as a function of the variablein reciprocal space, s = 2sin8lA. It is evident that the integral breadth of 222diffraction line is underestimated due to its overlap with the much stronger311 line (Figure 2). We excluded that particular line from all the subsequentcalculations for sample S3 , whereas all the lines for sample Sl were used.All other integral breadths approximately fall on the straight line, whichmeans that broadening is isotropic. This dismisses a possible existence ofgrowth fauIts in grains because it would cause an [hkl]-dependent broaden-ing and line shifts, which were not observed either, in FCC spinel structure.l''Approximately horizontal straight line indicates small strain. More detailed

FORMATION AND CHARACTERIZATION OF NiFe20,

TABLE II

727

Parameters of the Voigt function, which models the physically broadened lineprofile, as refined in the profile fitting for samples Sl and S3: Cauchy (Lorentz),

{Je, and Gauss, {JG, integral breadths. The corresponding full width at halfmaximum (FWHM) is calculated from the Voigt integral breadth, {J. Weighted

pattern-discrepancy (residual) factor, Rwp = 100[2:wi(Ii-Iei)2/2:w/rl1l2, where to,are weights of the observed intensities, h and lei are calculated intensities

Sample hkl Rwpl%FWHM/O

220311222400422511440

0.019(4)*0.009(1)< 10-5

0.0140.036(3)0.044(2)0.033(1)

0.036(9)0.019(4)

0.048(27)0.019(5)< 10-5

< 10-5

< 10-5

0.0490.0250.0480.0290.0360.0440.033

0.0410.0210.0450.0230.0230.0280.021

5.195.195.194.305.737.817.81

220311222400422511440

0.663(26)0.836(11)0.001(2)

0.738(22)0.888(36)0.988(14)0.912(10)

0.485(24)0.335(14)0.725(18)0.552(20)0.430(37)0.346(19)0.463(12)

0.9820.9970.7261.1041.1221.1381.171

0.7260.6810.6820.8190.7830.7670.822

3.623.623.623.622.662.662.66

* Numberts) in the parentheses denote the standard uncertainty (s.u.) of the least significantdigit(s). Only s.u.'s of refinable parameters are given.

TABLE III

Parameters obtained by diffraction line broadening analysis for samples Sl and S3:volume-weighted, (D)v, and surface-weighted, (D)s domain sizes, root-mean-square

strain averaged over a distance L in domains (e2(L»1I2

Sample (D)v/nm (D)sl nm286(7)*9.5(3)

265(7)5.9(2)

0.23(1)2.1(5)

* Number in the parenthesis denotes the standard uncertainty of the least significant digit.

analysis of line broadening is presented in Table III. Although the strain is10 times larger for sample S3 than for Sv it still has a negligible influenceon the broadening in comparison with the size effect. For instance, an esti-mate of strain by using the Stokes and Wi1son24 relation

{3(2()) coteoe= 4 (2)

728 S. MUSIC ET AL.

0.12220 311o 0

400o 422 511 440

ODD

0.14

'§ 0.10---r-.~~0.08

222o

0.06

3 4 6 7

Figure 3. Williamson-Hall plot for sample 83.

would require a strain of 0.012 to cause a line broadening comparable to thedata from Figure 2 (we took {3(2e) = 1° at 40° 2e). This is underlined by thefact that the value of volume-weighted domain size is reasonably close tothe estimation from the Scherrer equation. It must be emphasized that thethus obtained domain size is an either volume-weighted or surface-weightedaverage thickness measured in the direction of the diffraction vector (or-thogonal to the diffracting planes). Therefore, the real domain dimensionwill depend on its shape. In this particular case, it is safe to assume someisotropic shape. For instance, for a distribution of uniform spheres of diame-ter D it follows that D = 4 (D)v / 3, which gives the approximate average truedomain dimension of 12.7(4) nm. This method goes beyond determination ofa mere average value of domain size. It allows for the calculation of the do-main-size distribution function. Figure 4 shows volume-weighted domain-size distribution functions for both 81 (Figure 4a) and 83 (Figure 4b) sam-ples. The largest domain size for sample 81 estimates to about 600 nm,whereas only 30 nm for 83,

In conclusion, it is evident that the heating temperature, but not theheating time, has the ultimate effect on the particle size. Sample 81> heatedup to 1350 °C, shows a minimum diffraction-line broadening. It indicateswell-defined and large crystallites without many crystal defects or site dis-orders.

Figure 5 shows characteristics parts of the FT-IR spectra recorded forsamples 81, 82, 84 and 86, FT-IR spectrum of NiFe204 (sample 81) is char-acterized by two bands with transmittance minima at 592 and 400 cm-1 andshoulders at 534, 468 and 342 cm". The FT-IR spectra of samples 82 to 86are characterized by the bands at 603 and 406 crrr", The shape of FT-IR

FORMATION AND CHARACTERIZATION OF NiFe20. 729

<aj

100 200 300 400 500 600

(b)

Figure 4. Volume-weighted domain-size distrihution function normalized to the unitarea. The average volume-weighted domain size is defined as a mean of this distri-hution: (a) sample Sl; (h) sample S3.

o 10 20L/nm

30

speetra of samples S7 and S8 was similar to that of sample Sl eonfirmingthe XRD results obtained for samples Sl' S7 and S8' Samples S9 and SlO'prepared by heating the mixture of NiO: a-Fe203 = 1 up to 300 and 400 "C,respeetively, did not show formation of niekel ferrite. When the temperatureof heating was inereased up to 800°C, NiFe204 and", 5% of a-Fe203 werefound as the produets of the solid state reaetion (sample S11)'

The appearanee of two very strong IR bands, at '" 600 and '" 400 cm-i,is typieal of the inverse spinel NiFe204.6,25In previous work," it was ob-served that the IR band s of niekel ferrite shifted from 600 to 604 cm=! andfrom 417 cm"! to '" 400 cm+ when the maximum temperature of heatingehanged from 700 to 1350°C. The observed IR shifts were diseussed interms of erystallattiee ordering and/or ehange in stoiehiometry of niekel fer-rite. The shoulder at 529-536 cm:' was pronouneed best for niekel ferrite

730 s. MUSrĆ ET AL.

600 400Wave number/cm'

Figure 5. Characteristicparts of the FT-IRspectra of samplesSl, S2,S4and S6.Thespectra were recordedat room temperature.

samples prepared at maximum temperature of 1350 "C, and this effect wasnot dependent on the preparation procedure such as chemical coprecipita-tion, solid state reaction between corresponding oxides or thermal decompo-sition of the mixed salts Ni(N03VFe(N03)3· nH20. Incorporation of Cu2+ions in NiFe204 caused a shift of IR bands at 608 and 417 cnr ' to smallerwave numbers and this was explained by the effect of Cu2+ (the Jahn-Tellerion) on the crystal field state ofthe Ni2+ ions.26The related Raman spectrumof NiFe204 was discussed by Graves et al.27

The present work indicates that NiFe204 as asingle phase could be pre-pared by a simple thermal treatment of Ni(OH)2 and Fe(OH)3 without ce-ramic sintering. This means that distribution of Ni2+ and Fe3+ ions in themixed hydroxide particles favours the formation of NiFe204' i.e., that thereare no preconcentration regions of Ni2+ or Fe3+ ions. Very probably, thiscould be related to the formation of NiFe204 precursor already during theprecipitation of mixed hydroxide Ni(OHh/Fe(OH)3 or in the early stage ofits ageing.

The effect of mechanical treatment of NiFe204 on its structural proper-ties was investigated by Pavlyukhin et al.28 They observed a very strong ef-fect on the Mossbauer spectrum. After 10 minute s of mechanical treatmentof NiFe204, an intensive central doublet was ohserved at room temperature,while a symmetric central doublet was only present in the room temperature

FORMATION AND CHARACTERIZATION OF NiFe,O, 731

spectrum after 30 minutes of mechanical treatment. The authors concludedthat the observed effect is a consequence of the transfer of cations from thetetrahedral to vacant octahedral sites in the spinel structure. They also sug-gested that the disordering of anion and cation sites may lead to an X-rayamorphous structure.F

In contrast to the work by Pavlyukhin et al.,28 such a dramatic influenceof mechanical treatment on the properties of NiFe204 was not observed inthe present work. Figure 6 shows Mossbauer spectra of NiFe204 before andafter ball-milling for 27 hours at room temperature. No central quadrupledoublet was observed. However, the calculated Mossbauer parameters (Ta-ble IV) indicated a decrease of hyperfine magnetic fields Ml from 495 to 485and M2 from 526 to 516 kOe, during a ball-milling period up to 27 hours.The parameters of sextet Ml correspond to Fe3+ cations in tetrahedral sites,while the parameters of sextet M2correspond to Fe3+ cations in octahedralsite s in NiFe204. The mechanical treatment of NiFe204 also caused an in-crease of the spectral line width and a slight increase of isomer shifts, asshown in Table IV. The FT-IR spectra (Figure 7) of the samples, obtainedafter ball-milling of NiFe204 did not showa significant change with the ball-milling time. A small change in the intensities of the shoulders at 534 and332 cm"! was visible. On the basis of Mossbauer and FT-IR measurements,

TABLE IV

57Fe Mossbauer parameters of NiFe204 before and after ball-millingof different duration

Ball-milling Spectral Isomer shift* Quadrupole Hyperfine Line widthsplitting magnetic fieldtime Ih component 8lmms-1 ~q/mms-l HlkOeTlmm s-I

O Ml 0.237 -0.002 495 0.480M2 0.352 -0.001 526 0.440

3 Ml 0.240 -0.002 495 0.525M2 0.353 -0.002 527 0.461

6 Ml 0.238 -0.001 495 0.491M2 0.350 -0.001 527 0.437

9 Ml 0.257 0.021 487 0.531M2 0.356 0.010 517 0.449

15 Ml 0.269 0.010 486 ~.506M2 0.367 0.002 516 .463

21 Ml 0.265 0.008 484 0.J47M2 0.366 -0.002 515 0.471

27 Ml 0.270 0.011 486 0.529M2 0.368 -0.001 516 0.468

* Relative to a-Fe.

732 S. MUSIĆ ET AL.

a)

-10 -5 O SVELO CITY j(mm/s)

10

Figure 6. Mossbauer speetra (RT) of niekel ferrite (a) before and (b) after its ball-milling for 27 hours.

no conclusion can be drawn on the formation of an amorphous-like phaseas a consequence of the ball-milling of NiFe204 under given experimental

3 h01)o 6 hc<Il

.§ 9 hVJ

1a!:l

15 h01)

.~~ 21 h

~27 h

800 600 400Wave number/cm"

200

Figure 7. FT-IR speetra of nickel ferrite before and after ball-milling of different du-ration.

FORMATION AND CHARACTERIZATION OF NiFe,O, 733

conditions. It can be supposed that the ball-milling of NiFe204 has a greaterinfluence on the degree of inversion than on other structural properties ofthis spinel. However, we must keep in mind that Ni2+ has high preferencefor octahedral sites, as compared to Fe3+. For this reason applied-field Moss-bauer spectroscopic measurements must be performed, in the future to allowamore appropriate conclusion about the effect of'ball-milling on the NiFe204structure. The effect of mechanical grinding on the structural and magneticproperties of BaFe12019 was also monitored by Mossbauer spectroscopy.29-33However, the mechanism of mechanochemical changes in M-type hexagonalferrite is different than in the spinel ferrite NiFe204, due to the specificstructure of BaFe12019.34

Acknowledgements. - The author s gratefully acknowledge the support of (a) In-ternational Atomic Energy Agency (contract No 7681IRB), (b) National ResearchFoundation of Hungary (contract No TOO 7266) and (c) The USA (National Instituteof Standards and Technology) - Croatia (Ministry of Science and Technology) JointFund (grant No JF106).

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SAZETAK

Nastanak i svojstva NiFe204

Svetozar Music, Davor Balzar, Stanko Popovic, Marijan Gotic,Ilona Czaho-Nagy i Samet Dalipi

NiFe204 je pripravljen grijanjem mjesovitoga metalnog hidroksida odgovara-juceg sastava ili reakcijom NiO i a-Fe203 u cvrstom stanju. Pripravljeni uzorci istra-zivani su rentgenskom difrakcijom, FT-IR spektroskopijom i 57Fe Mossbauerovomspektroskopijom. Intenzivno mljevenje NiFe204 uvjetovalo je smanjenje hiperfinogamagnetskog polja iona Fe3+ u tetraedarskim i oktaedarskim polozajima, povecanjesirine Mossbauerovih spektralnih linija i malo povecanje izomernih pomaka. Zaklju-ceno je da mljevenje NiFe204 ima vise utjecaja na stupanj inverzije NiFe204 negona ostala svojstva tog spinela. Utvrdeno je da temperatura grijanja, a ne vrijemegrijanja, ima odlucujuce djelovanje na mikrostrukturu NiFe204 . Uzorci grijani do500°C pokazali su izrazito sirenje rentgenskih difrakcijskih linija, uzrokovano ve-licinom kristalita od priblizno 13 nm i relativno malom kristalnom neuredenoscu,Uzorci grijani iznad ~ 1000 °C posjedovali su puno vece kristale koji su pokazivalivrlo malu kristalnu neuredenost.