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Physica B 354 (2004) 27–34

www.elsevier.com/locate/physb

Ni- and Zn-doped hematite obtained by combustion of mixedmetal oxinates

C.A. Barreroa, J. Arpeb, E. Sileoc, L.C. Sancheza, R. Zyslerd, C. Saragovib,�

aInstituto de Fısica, Universidad de Antioquia, A.A. 1226 Medellın, ColombiabDepartamento de Fısica, Comision Nacional de Energıa Atomica (CNEA), Centro Atomico Constituyentes, Av General Paz 1499, 1650,

San Martın, Buenos Aires, ArgentinacINQUIMAE y Departamento de Quımica Inorganica, Analıtica y Quımica Fısica, Facultad de Ciencias Exactas y Naturales,

Universidad de Buenos Aires, Pabellon II, Piso 3, Ciudad Universitaria, C1428EHA, Buenos Aires, ArgentinadCentro Atomico Bariloche (CNEA), 8400 S.C. de Bariloche, RN, Argentina

Abstract

We have studied the changes in the structural and magnetic properties of hematite obtained by a new method based

on the combustion of mixed Fe(III)0.95, Ni(II)0.05- and Fe(III)0.95, Zn(II)0.05-oxinates. Thermogravimetric analysis

shows that the decomposition of the mixed oxinates proceeds in an overall weight loss formed at least by two stages and

the weight loss is related to the formation of probably CO2, N2 and H2O, and a solid residue. X-ray diffraction and

Mossbauer spectroscopy reveal that the residue of the thermally treated Fe(III)0.95, Zn(II)0.05-oxinate is a–Fe2O3 and

ZnFe2O4, whereas in the case of Fe(III)0.95, Ni(II)0.05-oxinate is Ni-doped a–Fe2O3 and NiFe2O4. Results suggest low

Ni incorporation into the hematite structure. However, it was difficult to establish if Zn-for-Fe substitution in hematite

had taken place. The comparison between the two hematites obtained in this work with a non-doped one shows

reduction in the cell parameters, increment in the saturation hyperfine fields, and reduction in the Neel temperatures.

The presence of the dopant, vacancy sites and lattice distortion are responsible for these effects.

r 2004 Elsevier B.V. All rights reserved.

PACS: 76.80.+y; 75.30.Hx; 91.60.Pn; 75.50.Ee

Keywords: Mossbauer spectrometry; Doped hematite; Neel temperature; Morin transition

e front matter r 2004 Elsevier B.V. All rights reserve

ysb.2004.09.013

ng author. Tel.: +54 11 6772 7160; fax:

1.

ss: saragovi@cnea.gov.ar (C. Saragovi).

1. Introduction

Hematite, a–Fe2O3, is a fascinating iron oxidewhich has been the subject of intense research for along time. Pure hematite is paramagnetic abovethe Neel temperature, TN, of about 965 K. At

d.

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C.A. Barrero et al. / Physica B 354 (2004) 27–3428

room temperature it is weakly ferromagnetic (WF)and at 265 K (the Morin temperature, TM), itundergoes a phase transition to an antiferromag-netic (AF) state [1,2]. Recently, literature reportsincreasing efforts to the search for new methods ofpreparation, with emphasis on doping. It is welldocumented that doping can affect the magneticbehavior, among other physical properties. Cationsubstitution always lowers the Neel temperatureTN. However, it has been found that althoughmost dopants decrease TM, Rh3+, Ru3+, Ir4+

increase it [2]. The most studied dopant isundoubtedly Al3+ [1–4]. Comparatively very fewworks have been reported on Zn2+- and Ni2+-doped hematites, partly because the usual wetchemical methods produce hematites with little orno dopant content. Singh et al. [5] prepared Ni-hematites by coprecipitation of the mixed Fe andNi oxides from nitrate solutions at appropriatepH. They could substitute Ni2+-for-Fe3+ up to anextent of 6 mol%. This metal replacement was alsoaccompanied by OH� for O2� substitution. Theydo not use Mossbauer spectroscopy to character-ize their samples. On the other hand, Sidhu et al.[6,7] determined the distributions of Ni and Zn inhematites obtained by thermal treatment ofnatural and synthetic Ni- and Zn-doped magne-tites. They found that during the thermal trans-formation much of the Ni and Zn were ejected toform a surface layer rich in these elements. Thisejection is more pronounced for Zn than Ni andalso more for synthetic than for natural samples.Interestingly, they do not report the formation ofZn-doped hematites. In contrast, Ayub et al. [8]were able to substitute very small amounts ofZn2+-for-Fe3+ in the hematite lattice, which wereprepared by hydrothermal techniques. They foundthat the presence of Zn completely suppressed theMorin transition down to 18 K using Mossbauerspectroscopy.

An alternative method of hematite preparationbased on combustion of metal oxinates, whichwere obtained by homogeneous precipitation, hasrecently appeared in the literature. Da Costa et al.[9] have successfully obtained Al-doped hematitesfrom metal oxinates. They found that this methodhas the advantage that the particle size was notsubstantially reduced by the Al-content, an effect

that is very common in hematite produced by wetchemical methods. In that paper, the possibility ofusing the combustion of metal oxinates to prepareother metal-substituted hematites was put for-ward. Saragovi et al. [10] have employed thismethod to obtain Ni-hematites. The resultsindicated that the substitution of Ni2+-for-Fe3+

in hematite was less than 5.3 mol% and that it wasaccompanied by oxygen and structural vacancysites. In the present investigation, the changesproduced in the structural and magnetic propertiesof hematite upon Ni and Zn substitution, obtainedfrom the combustion process of mixed metaloxinates, are reported. The formation of Ni- andZn-hematite involves the incorporation of divalentions (Ni2+, Zn2+) for trivalent Fe3+, in contrastwith the work by da Costa et al. [9], where Fe3+ ispartially substituted by Al3+.

2. Experimental

The metal-substituted hematites were preparedfrom homogeneous precipitation of Fe(III) andnickel(II) or zinc(II) containing oxinates obtainedaccording to the procedure described in Ref. [9].Samples were named RDM5, where M is the typeof metal (nickel or zinc) and 5 represents theatomic percentage (at%) defined as 5 M/(M+Fe)in the original mixed oxinates. The oxinates weretreated thermally at 700 1C in a tubular furnaceunder a constant air flow of 3 h. The solidsobtained were named RDM5C.

Thermogravimetric experiments were performedusing a Universal V2.5H TA Instruments Modulewith samples of about 15 mg under a constant flowof synthetic dry air in the temperature range20–800 1C. Heating rate was 10 1C/min.

X-ray diffraction (XRD) patterns were recordedin a Siemens D5000 diffractometer using a CuKaradiation. Generator settings were 40 kV, 37 mA.Divergence, scattered and receiving slits were 11, 11and 0.2 mm, respectively. A curved graphitemonochromator was used. Data were collected inthe 2y range 27.500–130.001, with scanning step of0.0201 and a counting time of 18 s/point. The stepwidth assured a minimum of about 12 intensitypoints for the narrower peaks. The data were

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Table 1

Thermal behavior of the mixed oxinates. Weight-loss percen-

tage values obtained theoretically and through the TGA curves:

first and second stage temperatures were obtained from the first

derivative of the TG curves. The experimental weight loss was

obtained by weighting the samples before and after combustion

in the tubular furnace

Sample RdNi5 RdZn5

First stage temperature (1C) 329 343

Second stage temperature (1C) 461 460

Experimental wt loss (%) 83.1 83.9

TGA wt loss (%) 81.3 82.0

C.A. Barrero et al. / Physica B 354 (2004) 27–34 29

analyzed using the GSAS system [11]. TheThompson–Cox–Hasting pseudo-Voigt function[12] was used for fitting peak profiles.

Mossbauer spectra (MS) were recorded fromroom temperature (RT) to 15 K in the standardtransmission geometry, using a 25 mCi Co57/Rhsource. Absorbers were carefully prepared tooptimize both a good compactness and anadequate heat transmission. Analysis of thespectra was performed using the Normos [13]and the Recoil [14] least-squares fitting programs.Doublets and sextets of Lorentzian lines wereused, doublets in the paramagnetic region andsextets in the magnetic region. The spectra werecalibrated referring to a–Fe.

3. Results and discussion

The TGA curves are similar for both samples(see Fig. 1), and similar to the ones reported by daCosta and co-workers [9] for iron- and aluminum-containing oxinates. The decomposition process

Fig. 1. Thermogravimetric curves for iron and zinc oxinate

RDZn5 (solid line), and for iron and nickel oxinate RDNi5

(dashed line).

involves one weight loss formed, at least, by twostages. The temperature of the first stage starts atabout 329 1C for RdNi5 (see Table 1) and at�343 1C for RdZn5, whereas the second stagestarts at about 460 1C for both the samples. Abovethe second stage, weight loss remains constant.This temperature was reported to be of 452 1C fornon-doped iron-containing oxinate [9]. The pro-cess starts with the decomposition of the organicmoiety to give non-characterized gases, probablyCO2, N2 and H2O, and a solid residue identified asa hematite-like phase impurified with variablequantities of a ferrite-like phase.

The overall weight loss is in fair agreement withthe stoichiometry

2Fe1�xMxðC9H6NOÞ3�xðsÞ þ AO2ðgÞ

¼ ðFe1�xMxÞ2O3�2xðOHÞ2xðsÞ

þ BCO2ðgÞ þ CH2OðgÞ þ DN2ðgÞ; ð1Þ

where A, B, C, and D are appropriate stoichio-metric quantities. Small differences in the totalweight loss between the experimental and the TGvalues (see Table 1) may be ascribed to thepresence of a ferrite-like phase that forms togetherwith the hematite, as found in the XRD analyses,and also to the presence of oxygen vacancies in thehematite structure caused by the thermal treat-ment, and according to

ðFe1�xMxÞ2O3�2xðOHÞ2x

! xH2OðgÞ þ ðFe1�xMxÞ2O3�xðsÞ: ð2Þ

Interatomic potential calculations support thisassumption [8]. It is interesting to mention that

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Fig. 2. 57Fe MS recorded from nickel-doped (upper part) and

zinc-doped (lower part) hematites at 190 K.

C.A. Barrero et al. / Physica B 354 (2004) 27–3430

da Costa et al. [9] also found impurities in their Al-hematite obtained from oxinates with xX6 at%.

The XRD patterns for both samples show twophases, hematite (PDF: 33-0664) and trevorite,NiFe2O4 (PDF: 10-0325) for RDNiC and hematite(PDF: 33-0664) and franklinite, ZnFe2O4 (PDF:22-1012) for RDZn5C. XRD diagrams for samplesRdNi5C, and RdZn5C have been simulated usingthe Rietveld method [11]. Hematite sizes foundusing a standard reference material (LaB6) are 95and 132 nm, respectively. The calculated latticeparameters and the unit-cell volume for bothphases are shown in Table 2. Data for purehematite from Ref. [9] is also included forcomparison. Simulated Rietveld data indicatesthat the hematite-like phase in samples RDNi5Cand RDZn5C present cell parameters that aresmaller than those presented for pure hematiteobtained by the same synthesis procedure. In thecase of RDNi5C, this fact perhaps indicates theinclusion of Ni(II) in the structural framework,and although the ionic radius of Ni(II) (0.70 A) islarger than that of Fe(III) (0.64 A), Eq. (2) showsthat substitution of Ni(II)-for-Fe(III) in samplestreated at 700 1C must be accompanied by oxygenvacancies. In the case of RDZn5C it is difficult toestablish if the lattice contraction could be due toany Zn-for-Fe substitution. The normally occu-pied trivalent iron sites in hematite are octahedral.It has been reported that Zn2+ (0.74 A) does nothave any octahedral crystal-field stabilizationenergy and may not be incorporated into thehematite lattice because of its large size and lowvalency [6,7]. In this way, it is possible that all Znhas migrated toward the surface of the particles.Thus a possible explanation of the smaller latticeparameters found for RDZn5C could be the

Table 2

Lattice constants, unit-cell volume and relative abundance of hematite

for the least significant figures of the data shown. The esd values are

Hematite parameters

a ¼ b (A) c (A)

Rd0C 5.038 13.759

RdNi5C 5.0368(2) 13.754(1)

RdZn5C 5.0361(1) 13.7519(2)

presence of more oxygen, and perhaps iron,vacancies than for the other samples. Finally, itshould be mentioned that by increasing theamount of Ni or Zn in our samples, in the caseof the first ones only hematite-like and ferritephases are obtained, whereas besides these twophases, ZnO is also observed in the Zn-dopedsamples.

Fig. 2 shows characteristic Mossbauer spectra(MS). In the MS for RDNi5C, two sextets were

and ferrite phases in the samples. Values in parentheses are esd

taken from the final cycle of the Rietveld refinement

Ferrite parameters

Volume (A3) a ¼ b ¼ c Volume (A3)

302.18(3) 8.340(1) 580.1(3)

302.052(5) 8.4424(1) 601.7(3)

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used for the NiFe2O4 phase according to previousdata [15] and their relative area were found foreach nominal doping. One or two more sextetswere used for the hematite phase according to theirmagnetic state, i.e., AF, WF or coexistence of both[2,3,16] and some of the parameters derived fromthe Mossbauer analysis are listed in Table 3. Inorder to calculate x in the chemical formula of Ni-hematite, (Fe1�xNix)2O3�x, the relative spectralareas determined from the spectrum at 15 K havebeen used and the assumption of a stoichiometricNi-ferrite. For the hematite to Ni-ferrite areas theratio obtained is �89/11. In this way a value ofx � 0:001 is obtained, suggesting a very small Ni-for-Fe substitution in hematite.

In the MS for RDZn5C, one doublet was usedfor the ZnFe2O4 phase according to previous data[17]. The bulk ZnFe2O4 is a normal spinel in whichthe divalent cations, Zn2+, occupy the A sites onlywhile Fe3+ cations occupy B sites. A sitesrepresent the tetrahedral sites and B the octahedralone. Other occupancy can occur depending on the

Table 3

Parameters derived from the Mossbauer analysis of the

hematite phase. TM: Morin temperature, DTM: range of

temperatures over which TM occurs, TN: Neel temperature,

BAF(0): saturation AF hyperfine field, BWF(0): saturation WF

hyperfine field, 2eAF(0): saturation value for the AF quadrupole

shift, 2eAF(0): saturation value for the WF quadrupole shift, dI:

intrinsic isomer shift, yM: characteristic Mossbauer tempera-

ture, x: nickel (or zinc) content in hematite, and z: number of

neighboring iron ions

Cation 0 Ni Zn

Data

from

Ref. [8]

TM (K) 263 26975 26475

DTM (K) 25 5075 3275

TN (K) 88372 87473

BAF(0) (T) 54.1

(80 K)

54.470.1 54.770.1

BWF(0) (T) 53.670.1 54.370.1

DB=BAF(0)–BWF(0) 0.8 0.4

2eAF(0) (mm/s) 0.4170.01 0.4070.01

2eWF(0) (mm/s) �0.2070.01 �0.2170.01

dI (mm/s) 0.6270.02 0.6270.03

yM (K) 513780 5427132

x 0.001

z 8.28 8.19

fabrication procedure. The magnetic order of bulkZnFe2O4/franklinite is antiferromagnetic with alow ordering temperature, TN ¼ 10:5 K: Themagnetic properties of the ZnFe2O4 spinel, alreadystudied by Schiessl et al. [18], are out of the scopeof this paper and will not be mentioned for now.As with the MS for RDNi5C, one or two moresextets were used for the hematite phase inRDZn5C according to their magnetic state. Thehematite parameters derived from the Mossbaueranalysis are shown in Fig. 3. For the hematite toZn-ferrite areas the ratio determined from thespectrum at 15 K is �84/16. Under the sameassumption as for RDNi5C, an even lower valueof x is obtained in Zn-for-Fe substitution,suggesting that Zn may not have entered into thehematite structure.

The Morin temperature, TM, is the temperatureat which 50% of the hematite spectral areafraction is in the WF state. It is found that withinthe error bars, TM is about the same for both thesamples and for the non-doped hematite. The factthat the TM values remain invariable in oursamples could be related to metals, oxygenvacancies and lattice distortions that produceddifferent changes in the temperature and magni-tudes of both the magnetic dipolar and the singleion anisotropies, which determine the Morintransition, such a way that at the end there is noappreciable effect on TM [19]. It is interesting tonote that the a and c cell parameters for bothsamples lie well below and well to the right of theline that separates hematites that exhibit Morintransition from the ones that do not, in the c–a

plot proposed by Dang et al. [19]. On the otherhand, sample RDNi5C exhibits a broader range oftemperatures DTM over which the transitionoccurs, in comparison to RDZn5C and the non-doped hematite (see Table 3).

The quadrupole shifts of the AF phases exhibitweak temperature dependence probably accountedby the thermal expansion of the lattice. For boththe samples, the saturation values for AF quadru-pole shifts were about 0.41 mm/s, independent ofthe type of metal. These values suggest parallelalignment of the spins with the [1 1 1] direction. Onthe other hand, saturation values for WF quadru-pole shifts of about �0.20 mm/s were found for

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both the samples, suggesting orientation of thespins in the basal (1 1 1) plane.

The characteristic Mossbauer temperature yM

and the intrinsic isomer shift dI were derived bytaking the average of several pair of data pointsusing the RECOIL program [14], which is basedon the Debye model for the lattice vibrations.These values are similar to the ones reported by DeGrave and Vandenberghe [16] for natural hema-tite. There is no clear effect of the Ni and Zn on dI;and within the error bars, on yM (Table 3).

The saturation value BAF(0) for the hyperfinefield of the AF spin state in both the samples wascalculated by fitting Brillouin curve for S ¼ 5

2to

the experimental data in the temperature rangewhere only the AF phase is present. The results ofthe fits are seen in the solid lines of Fig. 4. In thisfigure, the dotted lines do not represent fits to theexperimental data. Instead, these are Brillouinfunctions calculated for S ¼ 5

2; assuming the same

Neel temperatures for the given sample, asobtained for the AF curve, but changing thesaturation hyperfine field for the WF phase(BWF(0)). The experimental data lies on or abovethe dotted lines in all the cases. The parametersderived from these fits are also listed in Table 3.The BAF(0) value for RDZn5C is slightly largerthan for RDNi5C, but both are slightly larger thanfor the one reported for non-doped hematite(54.1 T). Next, BWF(0) is lower for RDNi5C thanfor RDZn5C. This difference is also reflected inDB defined as BAF(0)–BWF(0). It is observed thatTN is lower for RDZn5C than for RDNi5C, andthat both are lower with respect to the onepublished for single crystal pure hematite of960 K. In another work [10], we have found thatthe increment of Ni into the hematite structure,prepared by a method similar to the one used here,reduces TN. Unfortunately, for comparison

Fig. 3. Temperature dependence of relevant Mossbauer para-

meters for the hematite phase in sample RDZn5C. Solid spheres

and open-up triangles represent the experimental data for the

AF and WF phases, respectively. Due to the fact that the center

shift is the same for both phases at all temperatures, we have

used solid squares to represent the data. The solid lines in the

upper part of the figure do not represent fits, but they serve to

guide the naked eye.

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Fig. 4. Temperature dependence of the magnetic hyperfine field

for hematite phases in all samples. Solid spheres and open-up

triangles represent the experimental data for the AF and WF

phases respectively. The solid lines represent fits to the data in

the AF region using the Brillouin function for S ¼ 52: As

explained in the text, the dotted lines represent Brillouin curves

for which the experimental data lies on or above.

C.A. Barrero et al. / Physica B 354 (2004) 27–34 33

purposes, there is no reported value of TN for anon-doped hematite obtained by the combustionprocess of Fe-oxinates [9]. Taking into considera-tion the observed and reported data, the reductionfor TN in our samples can be partly explained bythe fact that in order to balance charge andconsidering the high temperature of combustion,there can exist vacancy sites. These sites reduce thesuperexchange magnetic paths. Additionally, if Znhas entered into the crystalline structure, it alsohelps to understand this reduction due to itsdiamagnetic character. In a mean-field approxima-tion TN�zJ where z is the number of neighbors Feions and J the superexchange constant, using thevalues of TN ¼ 960 K and z ¼ 9 (3 nn+6 nn) forthe non-doped hematite, z values of 8.28, and 8.19are obtained for RdNi5C and RDZn5C, respec-tively (see Table 3). Additionally, the contraction

of the lattice due to the vacancies makes theFe–O–Fe angles relevant for superexchange andthe Fe–O lengths to change [20]. In the case of Ni-hematite, assuming that there is a Ni-for-Fesubstitution, the experimental effective magnetonnumbers of Ni2+ and Fe3+ are of 3.2 and 5.9,respectively [21], which reduces the strength for thesuperexchange interactions [17]. It is important tomention that Saragovi et al. [10] have suggestedthat structural vacancies, besides the oxygen andiron vacancies, are also present in Ni-hematites,but the magnitude of those is difficult to quantify.This result is based on the fact that the hematite/(stoichiometric) ferrite relative spectral area, cal-culated by assuming that the breaking of themagnetic interactions is only due to Ni and oxygenvacancy, is higher than those obtained directlyfrom the Mossbauer spectra.

4. Conclusions

The crystallographic and magnetic properties ofhematite produced via the combustion of mixedFe- and Zn-(or Ni-)oxinates at 5 at% nominalcomposition were investigated. The decompositionof (Ni, Fe)-oxinate produces Ni-hematite andNiFe2O4 phases, whereas that of (Zn, Fe)-oxinateproduces hematite and ZnFe2O4 phases. Ourresults suggest low Ni incorporation into thehematite structure, but it is difficult to know ifany Zn-for-Fe substitution has happened. TM isfound to be the same for both hematites andsimilar to the ones reported for the non-dopedhematite. TN is lower for the hematite from (Zn,Fe)-oxinate than for Ni-hematite, and both arelower in comparison to TN for single crystalhematite. BWF(0) and DB were found differentfor both samples. These differences could berelated to the slight incorporation of Ni into thehematite lattice and to the lower presence ofvacancy sites for RDNi5C in comparison toRDZn5C. The present result suggests that themicrostructures of the samples contain differentlocal environments partially produced by thepresence of oxygen and perhaps iron vacancies,and in the case of Ni-doped hematite, by thepresence of the Ni and also by structural

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C.A. Barrero et al. / Physica B 354 (2004) 27–3434

vacancies. In fact, a totally stoichiometric hematitecould not be assumed with the preparationprocedure used, even with no-doping.

Acknowledgements

To Geraldo Magela da Costa for kindlyproviding the iron and nickel oxinates. Thanksalso goes to COLCIENCIAS (Colombia) andCONICET (Argentina) through the CIAM pro-gram for financial support. C.A.B. acknowledgesCODI (Universidad de Antioquia).

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