Cobalt Substitution in CuFe$_{2}$O$_{4}$ spinel and its influence on the crystal structure and phonons

0.97 MB·English

by  M. D. P. Silva

Cobalt Substitution in CuFe O spinel and its
2 4
3 influence on the crystal structure and phonons
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M. D. P. Silva, F. C. Silva, F. S. M. Sinfrônio, Alexandre Rocha Paschoal, E. N.
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1 Silva,† and Carlos William de Araujo Paschoal∗,¶,k
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i UniversidadeFederaldoMaranhão,UniversidadeFederaldoCeará, DepartamentodeFísica,
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CCET, UniversidadeFederaldoMaranhão,65085-580,SãoLuís -MA, Brazil,and Department
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ofMaterialsScienceandEngineering,UniversityofCaliforniaBerkeley, 94720-1760,Berkeley -
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a CA, UnitedStates
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E-mail: [email protected]
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1 Abstract
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Inthisworknanometric spinelCo Cu Fe O powderswereobtained bypolymeric pre-
1 1−x x 2 4
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7 cursorsmethodatseveralannealingtemperaturesbetween700and1200◦C.Thesampleswere
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characterized bymeansofX-raypowderdiffraction, confirmingtheidealinversespinelstruc-
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3
1 ture for CoFe2O4 sample and the tetragonal distorted inverse spinel structure for CuFe2O4
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sample. BasedonFWHMevaluation,weestimatedthatcrystallitesizesvariesbetween27and
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r 37nmforthenon-substituted samples. Theoptical-active modesweredetermined byinfrared
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andRamanspectroscopies. Thephononspectrashowedalocaltetragonaldistortionformixed
samples.
∗Towhomcorrespondenceshouldbeaddressed
†DepartamentodeQuímica,CCET,UniversidadeFederaldoMaranhão,65085-580,SãoLuís-MA,Brazil
‡DepartamentodeFísica,UniversidadeFederaldoCeará,CampusdoPici,60455-760,Fortaleza-CE,Brazil
¶DepartamentodeFísica,CCET,UniversidadeFederaldoMaranhão,65085-580,SãoLuís-MA,Brazil
§Department of Materials Science and Engineering, University of California Berkeley, 94720-1760, Berkeley -
CA,UnitedStates
kAuthorwhosecorrespondenceshouldbeaddressed:Phone[phone];Fax:[phone]
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Keywords: Spinel,CoFe O ,CuFe O ,Raman,Infrared, X-raypowderdiffraction
2 4 2 4
Introduction
Multiferroicsaresingularmaterialsthatexhibitsimultaneouslytwoferroicorders,usuallyelectric
andmagnetic,withamagnetoelectriccouplingbetweentheminsomecases. Thesematerialshave
attracted much attention due to their potential technological applications. As such, multiferroic
compounds bring new functionalities to spintronics and new device possibilities, such as memo-
ries, sensors, actuators and tunable filters.1–5 The most investigated multiferroic compound is the
perovskite BiFeO .6–14 However, the long search for the control of electrical properties by mag-
3
neticfieldshasbeenrecentlyledtoinagroupofmaterialsknownas"frustratedmagnets",suchas:
the perovskites REMnO 15–17 and REMn O 18–21 (with RE being rare earth ions), in the spinels
3 2 5
CuFeO 22,23 and CoCr O ,24 and the Y-type hexagonal ferrites (Ba, Sr) Zn Fe O ,25 among
2 2 4 2 2 12 22
others.
Therestrictednumberofpuremultiferroiccompoundsisaconsequenceoftheatomicarrange-
ment specificity, since, theoretically, only 13 point groups can lead to the multiferroic behavior.
In addition, by definition, ferroelectrics are insulators (3d transition metal oxides typically have
ionswhichhaveanelectronicdistributiond0),whileferromagnetsneedconductionelectrons. The
difficulties associated with the combination of electric and magnetic responses in a single phase
compoundcanbesolvedbymakingtwo-phasemultiferroiccompositesconsistingofaferroelectric
component (e.g. PbZr Ti O - PZT) and a ferromagnetic component (e.g. CoFe O ).26–38 In
1−x x 3 2 4
suchcomposites,themagnetoelectriceffectisduetotheinteractionbetweenelasticcomponentsof
ferroelectricandferromagneticconstituents. Insuchcase, anelectricfieldinducesavoltageinthe
ferroelectricwhichistransferredtotheferromagnet,thatcausesthemagnetizationofthematerial.
Sincethemagnetoelectriceffectishigherifthecouplingattheinterfaceishigher,compoundswith
large surface areas (as in multilayer thin films and nanometric powders) and strongly ferroelastic
areparticularlyinteresting. Thisapproachopensnewroutestogetamagnetoelectricresponsewith
2
thespecificchoice,relationshipandmicrostructureofthe constituents. Infact,themagnetoelectric
coefficientsatroomtemperaturewhichwasachievedexceedthosefoundinlow-temperaturesingle
phasesamplesinthreetofiveordersofmagnitude. Thus,magnetoelectricmultiferroiccomposites
are on the boundary of the technological applications.3,4,39,40 So, as the spinels have been contin-
uouslyemployedtoobtaingoodmultiferroiccomposites,acareful attentionhasbeen giveninlast
years to thesynthesisofspinelsinlast years fordifferent methods.41–61
Besides their extensively investigated ferromagnetic properties, Li-based spinels has been ex-
tensivelyinvestigatedtobeappliedaselectrodeinbatteries.????? Also,binaryspinelferriteslike
CdFe O , NiFe O , ZnFe O and CuFe O also show significant gas-sensing activity.62 More-
2 4 2 4 2 4 2 4
over,CuFe O showshighelectronicconductivity,highthermalstabilityandhighactivityasgreen
2 4
anodeforaluminumelectrolysis.63Anotherimportantbinaryspinel-typeferriteisthecobalt ferrite
CoFe O due to its high cubic magnetocrystalline anisotropy, high coercivity, and moderate sat-
2 4
uration magnetization, being such properties severely affected by to the concentration of divalent
metalcation.59????
UsuallyCoFe O crystallizesinaspinelstructure. Thecationicdistributioninthespinelstruc-
2 4
turecanbedescribedbythechemicalformula(Fe Co )[Fe Co ]O ,where()and[]denote
1−x x 1+x 1−x 4
the tetrahedral A and octahedral B sites, respectively. The inversion parameter x is equal to 0 for
inverse spinels and to 1, when the spinel is normal. CoFe O spinel structure is predominantly
2 4
inverse (x=0), with Co2+ ions occupying mainly B sites while Fe3+ ions are distributed almost
equally on A and B sites. The inversion index (1−x) depends on the thermal history of the sam-
ple.64 By the other side, CuFe O has a tetragonally deformed spinel structure, that is stretched
2 4
along the < 011 > direction.65 Yokoyama et al.66 observed changes in the crystal structure of
nanosized CuFe O powders obtained by coprecipitation followed by annealing. They observed
2 4
that the copper spinel is cubic at temperatures below 300 oC and tetrahedral over 400 oC. The
formation of considerable quantities of Cu+ in the lattice is the mechanism responsible for the
transition from tetragonal to cubic structure.66 As showed by Kester et al.,67 through reduction
reaction of quenched samples of CuFe O , the formation of Cu+takes place, but its fraction in
2 4
3
the B-sublattice strongly depends on the synthesis procedures and on the subsequent temperature
treatment. Nanosized particles ofCuFe O , obtained by a classical ceramic technology,have also
2 4
been studied.68 High temperature treatments lead to structural and magnetic surface disorders,
which can be induced by the dispersion of different copper ions in the sublattices, by the arising
of cations and oxygen vacancies, by the structure amorphisation, among others. Also, when the
spinel is synthesized using classical solid state route with accurate stoichiometry (x=1), it has a
tetragonalstructure.69
However, independently of the cation present in the lattice, the physicochemical properties of
such spinel-type ferrites depend on their microstructural properties, which are related, in turn, to
the preparation methods of these compounds. Thus, several routes were used to produce these
binary oxides such as hydrolysis,70 ball-milling, solid state reaction, co-precipitation and sol gel
methods, combustion processing etc.71 Since the synthesis route is crucial overthe spectroscopic
and structural properties of the spinel-typeferrites, the aim of this work was to evaluate the effect
ofcobaltisovalentsubstitutioninCuFe O spinelsnanoparticlesobtainedbypolymericprecursors
2 4
method.
Materials and Methods
All samples were synthesized using the Polymeric Precursor Method, as follow: ferric chloride
hexahydrate(FeCl .6H O,Isofar),coppersulphatepentahydrate(CuSO .5H O,Isofar)andcobalt
3 2 4 2
chloride hexahydrate (CoCl .6H O, Vetec) were used as purchased without further purification
2 2
(papurity). TheprecursorsolutionsofFe, Cu andCo wereprepared by addingtherawsolidsinto
an aqueous solution of citric acid (C H O , Nuclear), using stoichiometric quantities. Therefore,
6 8 7
ethylene glycol (C H O , Nuclear) was added to the metallic solution, according to a molarratio
2 6 2
1:3. This solutionwas heated at 110 ◦C for5 h in an ovento promotepolymerization. Soon after,
thepolymerizedgelwasheatedat300◦Cfor1h,underairatmosphere,toburntheorganicmatter
and form a black solid mass (primary calcination). Such carbonaceous mass was grounded until
4
its particles were 100 Mesh sized and heat-treated at 300 ◦C for12 hours (secondary calcination),
under a high oxygen atmosphere, in order to produce oxygen vacancies in the solids. Finally,
suchprecursorpowderswereannealedbetween700–1200 ◦C(ternarycalcination)inairfor4hin
Al O crucibles, and thedesired spinelcompoundsresulted.
2 3
CrystalstructuresoftheannealedpowderswereexaminedusinganX-raydiffractometerXpert
MPD (Panalytical), with Co Ka radiation (40 kV and 40 mA), speed of 0.02◦ q s−1 and value
ranging from 10 to 100 ◦. X-ray powderdiffraction(XRD) patterns were compared with theJoint
CommitteePowderDiffractionStandards(JCPDS) dataforthephaseevaluation.
Crystallite sizes (D) of the samples were determined from X-ray line broadening analyzes,
employingtheScherrer’s equation:
Kl
D= (1)
b cosq
where l is the X-ray radiation wavelength (l =1.78896 Å), K is the Scherrer constant, b is the
FWHM ofthepeak (in radians)andq isthepeak angularposition.
The infrared spectra were obtained using an IR prestige-21 infrared spectrometer(Shimadzu),
applyingKBras dispersantagent (1:100wt./wt.) in themidrange: 400 upto 1000cm−1.
The confocal Raman spectra were acquired using an alpha 300 system microscope (Witec,
Ulm, Germany), equipped with a highly linear (0.02%) stage, piezo-driven, and an objective lens
from Nikon (20x, NA = 0.40). A Nd:YAG polarized laser (l = 532 nm) was focused with a
diffraction-limited spot size (0.61l /NA) and the Raman light was detected by a high sensitivity,
back illuminated spectroscopic CCD behind a 600 g/mm grating. The final power at the end of
the objective lens used to focus on the sample was 3 mW. The spectrometer used was an ultra-
high throughput Witec UHTS 300 with up to 70% throughput, designed specifically for Raman
microscopy. The integration time and number of accumulations were in average 60 s and 3, re-
spectively.
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Crystalline Structure and Group Theory
Both CoFe O and CuFe O crystallizes according to inversespinel-based structure. Particularly,
2 4 2 4
CuFe O assumes a body centered tetragonal distorted inverse-spinel structure belonging to the
2 4
space group I4 /amd(D19), in which there are four molecules per unit cell (Z =4). In this struc-
1 4h
ture Fe/Cu, Fe and O atoms occupy 8d (C ), 4a (D ) and 16h (C ) Wyckoff sites, respectively.
2h 2d s
On the other hand, CoFe O shows a face centered cubic inverse-spinel structure, belonging to
2 4
the space group Fd3m(O7), with eight molecules perunit cell (Z =8), in which Fe/Co, Fe and O
h
atomsoccupy 16d(D ), 8a(T )and 32e(C )Wyckoffsites,respectively.
3d d 3v
Since each structure contains 14 atoms in the primitiveunit cell, there are 42 degrees of free-
dom and, consequently, 42 phonons are permitted for both structures. Using the Factor Group
Analysis,72 the zone-center vibrational modes distribution was decomposed in terms of the irre-
duciblerepresentationsforbothO andD factorgroups(Table1). Duetothesesiteoccupations,
h 4h
for both structures, FeB/M (M = Co or Cu) atoms do not contribute to the Raman-active phonon
spectra. Thus, any Raman assignments can be attributed to (Fe)A and O ions, while the infrared
spectramay beinfluenced by allconstituentions.
CuFe O andCoFe O spinelRamanandinfraredspectrapatternswerealsoanalyzedaccord-
2 4 2 4
ingtothequasi-moleculardescription,usingtheinternal modesfortheFeO tetrahedron.73 So,in
4
thiscase the structureswas described as formed by two differentsub-latticegroups: FeB/M atoms
and (FeO )5− tetrahedron. Tables 2 and 3 summarize the FeO group vibrations obtained by the
4 4
correlationdiagrams forcubicand tetragonalstructures, respectively.
According to theFeO symmetry,n (A ) and n (F )are assignedto theFe–O symmetricand
4 1 1 3 2
asymmetric stretchings, respectively; while n (E) and n (F ) vibrational modes are attributed to
2 1 2
the symmetric and asymmetric Fe-O bendings. The T(F ) vibrational mode is assigned to the
2
FeO tetrahedron translation motion. Finally, the Raman and infrared modes for both cubic and
4
tetragonalinversespinels,disregardingtheacousticand silentmodes,are showninTable4.
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Table1: FactorgroupanalysisfromCoFe O (cubic)and CuFe O (tetragonal)
2 4 2 4
Structure Atoms Sites Sitesymmetry Irreducible representations
Fe/Co 16d D A ⊕E ⊕2F ⊕F
3d 2u u 1u 2u
Fe 8a T F ⊕F
d 1u 2g
O 32e C A ⊕A ⊕E ⊕E ⊕F ⊕2F ⊕2F ⊕F
3v 1g 2u g u 1g 1u 2g 2u
Total A ⊕2A ⊕E ⊕2E ⊕F ⊕5F ⊕3F ⊕2F
Cubic (cid:0)O7(cid:1) 1g 2u g u 1g 1u 2g 2u
h Acousticmodes F
1u
IR modes 4F
1u
Raman modes A ⊕E ⊕3F
1g g 2g
Silentmodes 2A ⊕2E ⊕F ⊕2F
2u u 1g 2u
Fe/Cu 8d C A ⊕2A ⊕B ⊕2B ⊕3E
2h 1u 2u 1u 2u u
Fe 4a D A ⊕B ⊕E ⊕E
2d 2u 1g g u
O 16h Cv 2A ⊕A ⊕A ⊕2A ⊕2B ⊕B ⊕B
s 1g 1u 2g 2u 1g 1u 2g
⊕2B ⊕3E ⊕3E
2u g u
Total 2A ⊕2A ⊕A ⊕5A ⊕3B ⊕2B ⊕B
Tetragonal (cid:0)D19(cid:1) 1g 1u 2g 2u 1g 1u 2g
4h ⊕4B ⊕4E ⊕7E
2u g u
Acousticmodes A ⊕E
2u u
IR modes 4A ⊕6E
2u u
Raman modes 2A ⊕3B ⊕B ⊕4E
1g 1g 2g g
Silentmodes 2A ⊕A ⊕2B ⊕4B
1u 2g 1u 2u
Table2: Correlationcharts ofthephononsymmetryforAFe2O4 inthe(cid:0)O7h(cid:1)cubicstructure.
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Table3: Correlationcharts ofthephononsymmetryforAFe2O4 in the(cid:0)D14h9(cid:1)tetragonal phase.
Table4: Raman andInfrared modescorrespondingtothecubic andtetragonal inversespinels
Spinel structure Activity Assignments
Raman 2T(B ⊕E )+L(E )+n (A )+2n (A ⊕B )+2n (B ⊕E )+
1g g g 1 1g 2 1g 2g 3 1g g
D19(CuFe O ) 2n (B ⊕E )
4h 2 4 4 1g g
IR 5T(2A ⊕3E )+L(E )+2n (A ⊕E )+2n (A ⊕E )
2u u u 3 2u u 4 2u u
Raman T(F )⊕n (A )⊕n (E )⊕n (F )⊕n (F )
O7 (CoFe O ) 2g 1 1g 2 g 3 2g 4 2g
h 2 4 IR 2T(2F )⊕n (F )⊕n (F )
1u 3 1u 4 1u
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Results and Discussions
Structural properties
Figures1and2presenttheXRDpatternsfortheseveralCo Cu Fe O spinelpowdersasafunc-
1−x x 2 4
tion of the annealing temperature. The results confirm the formation of the single phase spinel,
as indicated by the JCPDS 00-034-0425 (tetragonal CuFe O ) and JCPDS 00-022-1086 (cubic
2 4
CoFe O )patterns,exceptedforthoseannealedat700and800◦CCu-basedpurespinelsthatcon-
2 4
tain a -Fe O rhombohedral (JCPDS 00-013-0534). It is important to point out that a -Fe O is
2 4 2 4
often found as a secondary phase in spinel synthesis, as previously reported by Sun et al.62 and
Mathew et al.74 In addition, for the cobalt substituted ferrites, it can be noticed than even minor
Co2+ inclusions induce a cubic lattice. The full-width at half maximum (FWHM) (monitoring
thepeak (400))inCoFe O decreases withtheincreaseoftheannealingtemperature, as shownin
2 4
Figure 3, showingthat theaverage crystallitesize is becominglarger, whilean inversebehavioris
observed in CuFe O (monitoring the peak (211)). Besides, for Co Cu Fe O samples, the
2 4 0.25 0.75 2 4
FWHM hardly changes, except when synthesized at 900 ◦C and above. This behavior could be
associated to minor lattice distortion imposed by the spatial competition between octahedral and
tetrahedral sites. Such fact is confirmed by Figure 4, since Co-substituted ferrites present bigger
average crystallite sizes, as compared to the CuFe O structures. Furthermore, the average crys-
2 4
tallitesizetendstoenlargewiththeannealing(26upto54nm). Specifically, CuFe O crystallites
2 4
show mean size between 32 and 35 nm, much lowerthan thevalueproposed by Sun ıet al.,62 that
estimated the range in between 75 - 110 nm for the ferrites. In opposition, CoFe O crystallite
2 4
sizeswereintherange28-37nm,beingalsosmallerthanthosereportedbyGaikwadetal.,75 i.e.,
60 -80nm,but biggerthantheoneobtainedbyValdés-Solís et al.,76 thatwas 20nm.
Vibrational properties
Figure 5 shows the Raman spectra obtained for CoFe O and CuFe O pure annealed spinels.
2 4 2 4
CuFe O spinel,synthesizedat700and800◦C(Figure5b),infactcorrespondstothe a -Fe O as
2 4 2 4
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(cid:7)(cid:19)(cid:9)
(cid:6)(cid:1)(cid:2)(cid:2)(cid:10)(cid:20)(cid:17)
(cid:6)(cid:6)(cid:2)(cid:2)(cid:10)(cid:20)(cid:17)
(cid:6)(cid:2)(cid:2)(cid:2)(cid:10)(cid:20)(cid:17)
(cid:18)(cid:2)(cid:2)(cid:10)(cid:20)(cid:17)
(cid:5)(cid:2)(cid:2)(cid:10)(cid:20)(cid:17)
(cid:15)(cid:2)(cid:2)(cid:10)(cid:20)(cid:17)
(cid:1)(cid:2) (cid:3)(cid:2) (cid:4)(cid:2) (cid:5)(cid:2) (cid:6)(cid:2)(cid:2)
(cid:1)θ(cid:10)(cid:7)(cid:11)(cid:12)(cid:13)(cid:14)(cid:12)(cid:12)(cid:9)
(cid:7)(cid:8)(cid:9)
(cid:6)(cid:1)(cid:2)(cid:2)(cid:16)(cid:17)
(cid:6)(cid:6)(cid:2)(cid:2)(cid:16)(cid:17)
(cid:6)(cid:2)(cid:2)(cid:2)(cid:16)(cid:17)
(cid:18)(cid:2)(cid:2)(cid:16)(cid:17)
(cid:5)(cid:2)(cid:2)(cid:16)(cid:17)
(cid:15)(cid:2)(cid:2)(cid:16)(cid:17)
(cid:1)(cid:2) (cid:3)(cid:2) (cid:4)(cid:2) (cid:5)(cid:2) (cid:6)(cid:2)(cid:2)
(cid:1)θ(cid:10)(cid:7)(cid:11)(cid:12)(cid:13)(cid:14)(cid:12)(cid:12)(cid:9)
Figure1: XRDpatternsasfunctionoftheannealingtemperaturefor(a)CoFe O and(b)CuFe O
2 4 2 4
spinels.
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