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Hindawi Publishing CorporationInternational Journal of SpectroscopyVolume 2013 Article ID 272846 11 pageshttpdxdoiorg1011552013272846

Research ArticleCrystal Defects and Cation Redistribution Study onNanocrystalline Cobalt-Ferri-Chromites by PositronAnnihilation Spectroscopy

Kunal B Modi1 Nimish H Vasoya2 Vinay K Lakhani3

Tushar K Pathak4 and P M G Nambissan5

1 Department of Physics Saurashtra University Rajkot Gujarat 360005 India2Om Shanti Engineering College Morbi Road Rajkot Gujarat 360030 India3 Department of Physics Bahauddin Science College Junagadh Cujarat 362001 India4Government Engineering College Kalawad Road Rajkot Gujarat 360005 India5 Applied Nuclear Physics Division Saha Institute of Nuclear Physics Kolkata West Bengal 700064 India

Correspondence should be addressed to Kunal B Modi kunalbmodi2003yahoocom

Received 5 March 2013 Accepted 7 May 2013

Academic Editor Hicham Fenniri

Copyright copy 2013 Kunal B Modi et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Positron lifetime and Doppler broadening measurements were carried out on nanocrystalline (grain size sim60ndash65 nm) samplesof the Cr3+-substituted cobalt ferrite system with general chemical formula CoCrxFe2minusxO4 (119909 = 00minus20) synthesized by thecoprecipitation technique The results indicated selective trapping of positrons in large vacancy clusters initially at the tetrahedral(A-) sites and then with Cr3+-substitution up to concentration (119909) = 07 at the octahedral (B-) sites The results are consistentwith the cation distribution determined from X-ray diffraction line intensity calculations which indicated partial inversion of theinverse spinel ferrite subsequent stabilization over a range of substitution (119909 = 07 to 17) and finally the full inversion to the normalspinel chromite (CoCr

2O4 119909 = 20) In the intermediate range of substitution lattice contraction prevented a fraction of Co2+ ions

released from the (B-) sites from entering the tetrahedral sites and these vacancies at the (A-) sites trapped positrons Althoughthe samples were composed of nanocrystalline grains only an insignificant fraction of positrons were diffused and annihilated atthe grain surfaces since the grain sizes and the thermal diffusion length of positrons nearly overlapped

1 Introduction

Crystalline materials with the characteristic spinel structurecomprising of well-designated tetrahedral (A-) and octahe-dral (B-) sites constitute a very interesting class of condensedmatter systems evoking interest even from the very funda-mental science viewpoint [1 2] Cobalt ferrite (CoFe

2O4)

Cobalt chromite (CoCr2O4) and their solid solutions with

a typical crystalline structure XY2O4are candidate materials

in this class where X is normally a divalent and Y is atrivalent ionThesematerials have attracted a large number ofchemists physicists andmetallurgists to study their differentaspects including the structure and properties using boththeoretical modeling and wide variety of experimental tools[3ndash5] Ferrites composed of nanometer-sized particles elevate

this interest to new dimensions as the very large networkof interfaces will then play a decisive role in controllingthe atomic transport and spatial rearrangement of atomswithin the structure So far as the tools for investigating suchdetails are concerned conventional experimental methodssuch as X-ray diffraction (XRD) and transmission electronmicroscopy (TEM) have helped to obtain substantial infor-mation on a macroscopic scale But defect-specific probessuch as positron annihilation spectroscopy are needed topinpoint their role in the postsynthesis and characterizationtreatments suchmaterials have to undergoWe present in thispaper the results of our investigation carried out onCoFe

2O4

both on the nascent mother sample and samples in which theFe3+ ions are replaced byCr+3 ions that is CoCrxFe2minusxO4 x=00ndash20 The purpose of the work is double sided Initially we

2 International Journal of Spectroscopy

will present the important observations which were obtainedfrom more conventional experimental tools such as XRDTEM and energy dispersive analysis of X-rays (EDAX) andhighlight the changes occurring at the different stages ofCr3+-substitutionThe emphasis will be thereafter diverted todemonstrate the ability of positron annihilation techniquesto sense such changes The latter is of importance sincesuch works are scarcely available in the literature so far andsecondly it offers a viable investigative probe for such studieswhich are highly essential in the current scenario of novelmaterials and arising challenges in their understanding

According to the existing literature cobalt ferrite(CoFe

2O4) is an inverse spinel and taken to be collinear

ferrimagnet [6] while cobalt chromite CoCr2O4 is a

normal spinel with a canted ferromagnetic structure and itsCurie temperature is 97K [7] Previous studies of magneticproperties and Mossbauer spectroscopy on mixed cobalt-ferri-chromites CoCrxFe2minusxO4 of coarse-grain compositionindicated that canting of magnetic structure is observedwhen Co2+ is present at the tetrahedral (A-) site [8 9] Whilemagnetization measurements on the same system couldbe explained by Neelrsquos model as far as the series remainedinverse spinel they deviated significantly when they beganto have normal spinel structure [10] Recently structural andmagnetic properties of nanocrystalline CoCrxFe2minusxO4 (0 le xle 10) system prepared by the sol-gel autocombustion routehave been studied by Tolesha et al [11]

The redistribution of cations when one species is replacedby ions of neighboring elements in the periodic table has beenof tremendous significance in modifying the properties aswell as in giving rise to new phenomena and processes [68 9] The transformation of a nearly complete inverse spinelferrite CoFe

2O4 to a normal spinel chromite CoCr

2O4

through successive replacement of the Fe3+ ions by Cr3+ ionshas been found to generate drastic changes in the cationicdistribution in the structure owing to lattice contractionas well as the likely presence of vacancy-type structuraldefects Although substitution effects generally prompt suchredistributions concomitant lattice contraction or expansioncan also influence it and quite often it will result in thegeneration of structural defects in the form of unoccupiedlattice sites The latter are potential sites for investigation bypositron annihilation experiments Such a defect-sensitivespectroscopic probe will be of immense benefit as it canpinpoint the origin and evolution of such defects and theirdominating role over the redistribution of ions in the latticeAs is popularly known the positron lifetimes and Dopplerbroadening of the electron-positron annihilation gammaray spectrum are directly related to the electron densityand momentum distribution in a material and hence theinformation carried by the signals of annihilation can unravelthe material properties in the atomic scale [12]

2 Experimental Procedure

The spinel ferrite system CoCrxFe2minusxO4 with variable com-positions with the range of x = 00 01 03 07 11 1315 17 18 19 and 20 were prepared by air oxidation of

an aqueous suspension containing Co2+ Fe2+ and Cr3+cations in stoichiometric proportions The starting solutionswas prepared by mixing 50mL of aqueous solutions ofCoSO

4sdot7H2O FeSO

4sdot7H2O and Cr

2(SO4)3sdot6H2O in proper

proportions A 2M solution of NaOH was prepared as aprecipitant The starting solution (pH sim 35) was added intothe precipitant so that the solubility product constants for thehydroxides of the cation are exceeded and their sequentialprecipitation can be avoidedThe suspension (pH= 105) thusformed containing dark green intermediate precipitates wasthen heated and kept at 333 K for 1 hour During the heatingoxygen gas was bubbled uniformly into the suspension to stirit and to promote oxidation reaction until all the intermediateprecipitates changed into dark brownish precipitates of thespinel ferrite The samples were filtered washed by acetoneseveral times and dried at 473K under vacuum

The formation of the ferrite powders by the oxidationmethod consists of oxidation by air (O

2) bubbling through

an aqueous solution containing ferrous ions (Fe2+) and otherdivalent ions (M2+) after an alkaline solution (ROH) has beenadded This is according to the chemical reaction Fe2+ +M2+ + ROH + O

2rarr M2+(Fe3+)

2O4where ROH is NaOH

KOH NH4OH and so forth Ferrite powder of high homo-

geneity and purity andwith sufficient control over the particlesize can be achieved by this method

The stoichiometry of the powdered samples was con-firmed from EDAXmeasurementsThe compositional valueswere determined within an accuracy of 1 X-ray diffractiondata were collected using a Philips (PW 1710) automatedX-ray powder diffractometer with Cu K

120572radiation graphite

monochromator and Xe-filled proportional counter Datawere collected in the angle range 5∘ndash80∘ TEM images of thesamples were taken using a TECNAIK 20 (Philips) micro-scope operated at 200 kV For the TEM observations thepowder was first dispersed in amyl acetate by ultrasonicationand then the suspensions were dropped on a copper grid witha carbon film The grain sizes and shapes were determinedfrom the TEM pictures

Positron annihilation studies of the powdered sampleswere performed in the conventional way The radioactiveisotope 22Na in the form of 22NaCl dissolved in dilute HClwas deposited and dried on a thin (sim2120583m) Ni foil andfolded to form the source of positrons It was kept immersedin the volume of the sample taken in a glass tube Carewas taken to ensure that the powdered sample surroundedthe source from all sides sufficiently enough to ensureannihilation of positrons within it Further the sample withthe source embedded in it wasmaintained under dry vacuumconditions during the experiments This is done to eliminatethe possibility of the trapping of positrons and quenching ofpositronium (the metastable bound state of the positron andan electron) by the air and gases otherwise getting trappedwithin the powdered sample [13] The positron lifetimemeasurements were carried out using a slow-fast coincidencespectrometer having a time resolution (FWHM) of 200 psfor the gamma rays from 60Co source under experimentalconditions For Doppler broadening measurements a highpure germanium (HPGe) detector with resolution 114 keV

International Journal of Spectroscopy 3

at 511 keV was used This and another detector with identicalresolution were used on either side of the source-sampleassembly for coincidence Doppler broadening spectroscopy(CDBS) studies The CDBS experiments helped to identifythe elemental environment around the positron trapping sitesin materials by virtue of the ability to record the gammaray energy distribution spectra of positron annihilation withthe high momentum core electrons without the nuclearbackground The details about these measurements and themethods of data analysis to extract the relevant informationcan be seen in the paper by Asoka-Kumar et al [14]

3 Results and Discussion

31 Characterization of the Samples After obtaining the finalproducts it was essential to check the chemical compositionof each sample The reason for making EDAX characteri-zation was to ratify the purity and surety of the chemicalcomposition Two representative EDAX patterns namely ofx = 03 and 18 compositions are shown in Figure 1 Theresults of EDAX confirm the expected stoichiometry withsmall deficiencies of Cr3+ ions Further the incorporation ofCr3+ in the place of Fe3+ was indicated by the intensities of therespective peaks in the two patterns No traces of impuritieswere found The EDAX results suggested that the precursorshad fully undergone the chemical reaction to form ferritematerial of the expected composition

The particle size and morphology of all the compositionswere studied by means of transmission electron microscopyTypical images for x=00 10 and 16 compositions are shownin Figure 2 It can be seen that the average particle size is inthe range 60ndash65 nm

TheCoCrxFe2minusxO4 sampleswere characterized byXRD toascertain the single-phase structure formation and to deducecell edgeparameter and cation distribution and grain sizeverification Typical XRD patterns of CoCrxFe2minusxO4 sampleswith x = 00 07 11 13 17 and 20 are shown in Figure 3The background noise and the broadness of the peaks arecharacteristics of particles with nanometer dimensions sincethere is not a sufficient number of crystallographic planesto result in sharp diffraction lines The XRD patterns alsoshowed that all the samples have the monophase spinelstructure No extra lines corresponding to any other phaseor nonreacted ingredients were detected The diffractionpatterns could be indexed for a face centered cubic (fcc)structure [15] The lattice constant for each compositionwas determined by using the ldquoPowder-Xrdquo software [16]The concentration dependence of the lattice constant (a)determined from the X-ray data is presented in Figure 4The lattice constant remains more or less constant initiallybut rapidly decreases for higher concentrations of Cr3+ Theobserved change in lattice constant value with Cr3+-content(x) is attributed to the small difference in the ionic radii of theconstituent cations Fe3+ (0640 A) and Cr3+ (0630 A) andthe change in the distribution of cations among the available(A-) and (B-) sites of the spinel lattice

Inte

nsity

(au

)

120 210 300 390 480 570 660 750 840 930

Fe

OCr

Co

119909 = 18

Energy (keV)

Fe

Cr

Co

(a)

120 210 300 390 480 570 660 750 840 930

Inte

nsity

(au

)

Energy (keV)

119909 = 03

Fe

O

Cr

Co

Fe

CrCo

(b)

Figure 1 The EDAX spectrum of the Cr3+-substituted samples atconcentrations 119909 = 03 and 18

In order to determine the cation distribution the XRDline intensity calculations were made using the formulasuggested by Buerger [17]

119868

ℎ119896119897=

1003816

1003816

1003816

1003816

119865

ℎ119896119897

1003816

1003816

1003816

1003816

2119875119871 (1)

Here 119868ℎ119896119897

is the relative integrated intensity 119865ℎ119896119897

is thestructure factor 119875 is the multiplicity factor and L = (1 +cos22120579)(sin2120579 cos 120579) is the Lorentz polarization factor

According to Ohnish and Teranishi [18] the intensityratios of planes I

220I440

and I400

I422

are considered tobe sensitive to the cation distribution There exists distinctcontrast in the atomic scattering factors of Cr3+ or Fe3+and Co2+ cations present in the system This makes thedetermination of the cation distribution quite reliable Anyalteration in the distribution of cations causes a significantchange in the XRD intensity ratios Therefore in the processof arriving at the final cation distribution the site occupancyof all the cationswas varied formany combinations and thosethat agreed with the experimental intensity ratios are shownin Table 1 The final cation distributions were deduced simul-taneously by considering the Bragg plane ratios the fittingof the magnetization data at 80K and the ion distribution

4 International Journal of Spectroscopy

(a)

(b)

(c)

Figure 2 Transmission electron micrographs of CoCrxFe2minusxO4system for (a) 119909 = 00 (b) 10 and (c) 16 compositions The insetsshow the enlarged images of grains in each case

parameters of Fe3+ among the (A-) and (B-) sites of spinellattice derived fromMossbauer spectral analysis [19]

32 Positron Lifetimes in the Unsubstituted Sample CoFe2O4

(x = 00) The positron lifetime spectra were analyzed usingthe PALSfit computer program developed by the Risoe group[20] The spectra of all the samples were fitted to obtainvariances of fit within satisfactory limits (107 plusmn 012) Thefits yielded three distinct lifetimes 120591

1 1205912 and 120591

3in all the

cases and their magnitudes as discussed below were thecharacteristics of positron trapping in specific sites withinthe spinel structure or positronium formation within thegrain boundaries In the CoFe

2O4(x = 00) sample the

intermediate lifetime 1205912was found as high as 356 ps with

relative intensity 1198682= 482 The normal interpretation

Table 1 The cationic distribution in the samples at different Cr3+concentrations (x)

119909 Cation distribution00 (Fe3+

09Co2+01)A [Co2+

09Fe3+11]B O4

2minus

01 (Fe3+08Co2+02)A [Co2+

08Cr3+01Fe3+11]B O4

2minus

03 (Fe3+06Co2+04)A [Co2+

06Cr3+03Fe3+11]B O4

2minus

07 (Fe3+03Co2+07)A [Co2+

03Cr3+07Fe3+10]B O4

2minus

11 (Fe3+03Co2+07)A [Co2+

03Cr3+11Fe3+06]B O4

2minus

13 (Fe3+03Co2+07)A [Co2+

03Cr3+13Fe3+04]B O4

2minus

15 (Fe3+03Co2+07)A [Co2+

03Cr3+15Fe3+02]B O4

2minus

17 (Fe3+03Co2+07)A [Co2+

03Cr3+17]B O4

2minus

18 (Co2+10)A [Cr3+

18Fe3+02]B O4

2minus

19 (Co2+10)A [Cr3+

19Fe3+01]B O4

2minus

20 (Co2+10)A [Cr3+

20]B O4

2minus

for the observation of such a well-resolved longer lifetimewith appreciable intensity is the presence of vacancy-typecrystalline defects within the material since positrons gettrapped in such lower-than-average-electron-density sitesThis is a reasonable assumption since it is nearly impossibleto synthesize ferrites with fully occupied crystalline structureBesides those positrons managing to diffuse out to thevacancies on the nanocrystalline grain interfaces may alsocontribute to this component The reason is that the thermaldiffusion lengths of positrons in oxide materials are typicallyabout 50ndash60 nm [21 22] Hence a small fraction of positronswould inevitably diffuse and migrate to the surfaces of thenanocrystals (which are of sizes about 60ndash65 nm) beforetheir annihilation Despite prolonged heating the graindimensions could not be increased to more than the abovelimit On the other hand the diffusion lengths in the presentcase could be shortened due to the trapping of positronsby vacancy clusters if present within the nanoparticles Thepositron lifetime in the perfect crystalline sample (120591

119891 for

which no theoretical value is available) can be calculatedusing the trapping model equation [23]

1

120591

119891

=

119868

1

120591

1

+

119868

2

120591

2

+

119868

3

120591

3

(2)

Substituting the experimental values of the positron life-times and their intensities of the CoFe

2O4sample in the

above equation we obtain 120591119891= 199 ps The shorter lifetime

component 1205911is obviously less than this value in all the

cases due to admixing with the Bloch state residence timeof trapped positrons [12] A small contribution coming to120591

1from parapositronium atoms of lifetime 125 ps is ignored

as the intensity of this component one-third that of theorthopositronium intensity 119868

3 is negligibly small

The annihilation characteristics of positrons diffusing outto the grain surfaces are also reflected in the variation of thelongest lifetime 120591

3and its intensity 119868

3 The magnitude of this

lifetime (18ndash21 ns) is typical of the ldquopick-offrdquo annihilationof orthopositronium atoms formed at the interfacial regionsof the grains [12] Although positronium formation is notsignificant enough to alter the interpretations in metallicoxides it has been found still relevant enough to force a

International Journal of Spectroscopy 5

minus111 minus220

minus311

minus400

minus420

minus422

minus333

minus440

minus620 minus533

minus444

119909 = 11

5 15 25 35 45 55 65 75 852120579 (deg)

Inte

nsity

(au

)

(a)

119909 = 20

minus111

minus220

minus311

minus400

minus420

minus422

minus333

minus440

minus620 minus533

Inte

nsity

(au

)

5 15 25 35 45 55 65 75 852120579 (deg)

(b)

119909 = 07

minus111 minus220

minus311

minus400

minus420 minus422

minus333

minus440

minus620

minus533

minus444

Inte

nsity

(au

)

5 15 25 35 45 55 65 75 852120579 (deg)

(c)

119909 = 17

minus111

minus220

minus311

minus400

minus420

minus422

minus333

minus440

minus620

minus533

minus444

Inte

nsity

(au

)

5 15 25 35 45 55 65 75 852120579 (deg)

(d)

119909 = 00

minus111

minus220

minus311

minus400

minus420

minus422 minus333

minus440

minus620

minus533

Inte

nsity

(au

)

5 15 25 35 45 55 65 75 852120579 (deg)

(e)

119909 = 13

minus111

minus220

minus311

minus400

minus420

minus422

minus333

minus440

minus620

minus533

minus444

Inte

nsity

(au

)

5 15 25 35 45 55 65 75 85

2120579 (deg)

(f)

Figure 3 Typical X-ray diffraction patterns of the CoCrxFe2minusxO4 system for different Cr3+ concentration (x)

three-component analysis of the positron lifetime spectraof nanocrystalline materials [24ndash26] and the intensity 119868

3

despite being relatively small (08ndash14) indicates the pres-ence of large free volume regions in the intergranular regionsof materials when composed by nanometer-sized particles orgrains

33 Results of Cr3+ Substitution Figures 5 and 6 describe thechanges occurring in the positron annihilation parameters as

a result of Cr3+ substitution for Fe3+ in CoFe2O4 A close look

into the trends of variation helps to identify three distinctstages of defects evolution andor structural variations In thefirst stage spreading over the concentration x = 01 to 07 thetwo positron lifetimes 120591

1and 1205912show remarkable increase in

the initial stage and attain saturation The longer lifetime 1205913

and its intensity 1198683show characteristic decrease that will be

discussed later The second stage of variation is marked forx gt 07 till 17 during which the lifetimes decrease and the

6 International Journal of Spectroscopy

00 05 10 15 20

0748

0750

0752

0754

119903B

(A)

Cr3+ concentration (119909)

(a)

00 05 10 15 20

0514

0516

0518

0520

119903A

(A)

Cr3+ concentration (119909)

(b)

00 04 08 12 16 20

834

835

836

119886(A

)

Cr3+ concentration (119909)

(c)

Figure 4 The lattice constant (a) and the radii of the tetrahedral(119903A) and octahedral (119903B) sites versus Cr

3+ concentration (x)

intensity sharply rises All these trends are just reversed onceagain in the last stage from x = 18 to 20

The variation of the different positron annihilationparameters with Cr3+-substitution is thus highly complex innature as the potential trapping centers might have changedduring the different stages of substitution due to not only thearrival of a new element but also the relative displacementthey may cause in the positioning of the other ions alreadypresent in the crystalline structure Certain information inthis direction is available from the results of CDBS measure-ments shown in Figures 7(a) and 7(b) The data has beenanalyzed using the usual quotient spectral method in whichthe projected one-dimensional spectra on the ((119864

1minus 119864

2)2)

axis of the counts in the window ((1198641+119864

2)2) = 511plusmn12 keV

are peak normalized and divided by that of a pure referencesample (Si single crystals) [14 27]The choice of Si to serve asa reference is not unjustified since it is not a constituent of thematerial at any stage in this investigation and the purpose isto magnify the differences in shapes of the momentum dis-tribution curves for easy understanding and interpretationIn Figure 7(a) the ratio curves of the samples with the twoextreme compositions CoFe

2O4(x = 00) and CoCr

2O4(x =

20) are shown togetherwith the identical curves obtained forthe constituent elemental samples Figure 7(b) illustrates thecurves obtained similarly for the Cr3+-substituted samples

of a few representative concentrations The ratio curves ofthe samples are found having a characteristic peaks at 119901

119871=

103 times 10minus3m0c (where m

0is the electron mass and c is the

velocity of light) The peaks of the ratio curves of the threeconstituent metals that is Co Fe and Cr appear at 150 times10minus3m

0c 122times 10minus3m

0c and 113times 10minus3m

0c respectively but

with decreasing amplitudes This observation is consistentwith the decreasing number of d-electrons and decreasingradius of the 3d-shell Thus the peak of the ratio curveof either the pure or any of the Cr3+-substituted samplesdoes not coincide with those of the elemental curves as anample proof to suggest that positrons are not trapped inoxygen vacancies a fact even otherwise vindicated by theirpositive charge that will repel positrons On the other handtrapping takes place in the cationic vacancies and the peakat 119901119871= 103 times 10minus3m

0c common to all the samples and

irrespective of the Cr+ concentration (x) indicates the encir-clement of the defects by oxygen ions In several of our recentstudies on nanocrystalline oxide semiconductors we havesimilarly obtained the peak due to annihilation with oxygenelectrons at 119901

119871= 103 times 10minus3m

0c [28ndash30]

The identical elemental environment around the positrontrapping sites at all concentrations of Cr3+ substitutionis further verified from the 119878 versus 119882 plot shown inFigure 8 that is normally used to identify the changes in thepredominant type of positron trapping defects at differentstages of variation of the experimental parameters The 119878and119882 parameters have been derived from the CDB spectraas the counts falling under segments respectively from0 to 375 times 10minus3m

0c and from 75 times 10minus3m

0c to 1225 times

10minus3m0c normalized by the total counts accumulated under

0 to 375 times 10minus3m0c The 119878-119882 plot is linear and all the

points lie essentially on a straight line This indicates thatpositrons essentially encounter similar elemental environ-ments irrespective of the cationic redistribution This is afurther credence to the argument that the defects which trappositrons are surrounded by oxygen ions and therefore thetraps are none other than the cationic vacancies But thereare variations in the intensity of annihilation with the oxygenelectrons as indicated by the individual variations of the 119878and 119882 parameters with Cr3+ concentration (x) shown inFigures 9(a) and 9(b) (For the sake of clarity the curves ofnot all the samples are shown in Figure 7(a) or Figure 7(b)but the peak coordinates of all the curves are shown inFigure 8) FromFigures 9(a) and 9(b) also we can distinguishfrom one another basically three regions the demarcationbeing identical to that mentioned in the case of the positronlifetime results The first two regions (x = 00ndash07 and x =07ndash17) are characterized by a fall and a rise of the peaksof the curves and the last stage is marked by again a fall Itcan be argued that although the annihilation environmentof positrons essentially remains identical they are trapped atdifferent stages of Cr3+ substitution by the defects situated atdifferent sites in the lattice structureThese points are furtherdiscussed in detail afterwards

The insensitivity of the CDB spectra to the oxygen vacan-cies can be explained on the basis of the results of positronlifetime measurements as well The difference between the

International Journal of Spectroscopy 7

lifetime characteristic of defects (ie 1205912) and the bulk lifetime

120591

119891is normally considered as an indication of the size of

the defect The enhancement in the positron lifetime due totrapping inmonovacancies is sim40ndash80 ps in typicalmetals andalloys [31] Assuming that 120591

2= 356 ps is an upper limit of

the positron lifetime in vacancy clusters in the unsubstitutedsample 120591

2minus120591

119891= 356 minus 199 = 157 ps will correspond to defects

much larger than monovacancies Theoretical estimationsin Fe which is normally bcc in structure but a constituentof the present samples have shown the enhancement ofpositron lifetime in a neutral vacancy cluster composed of4-5 neighboring monovacancies as 152 ps [32] Consideringthese facts the positron trapping site in the undoped alloycan be conceived to be a vacancy cluster composed of themonovacancy created by the absence of a doubly ionizedcation and four of its coordinated oxygen ions Based on X-ray diffraction magnetization andMossbauer results Mohanet al [10] have shown that the CoFe

2O4(x = 00) is a

nearly complete inverse spinel and the ionic distribution init is of the form (Fe3+

09Co2+01)

A[Co2+

09Fe3+11]

BO4

2minus Theabsence of a Co2+ ion with four oxygen neighbor ions willgive rise to a neutral pentavacancy cluster in which positronscan be trapped and annihilated The other possibility of theabsence of a trivalent cation with four neighboring oxygenions cannot be ruled out as it would have enhanced positrontrapping due to surplus negative charge and hence it isnecessary to point out whether the said vacancy cluster iscentered at the (A-) site or at the (B-) site This can beanswered by looking at the effects of Cr3+ substitution onthe cationic redistribution A schematic diagram showingthe two neighboring octants of a normal spinel structure isshown in Figure 10 At the very onset of substitution a drasticdrop in the intensity of the peak in the CDB spectra (rep-resented by the119882 parameter) is observed (Figures 7(b) and9(b)) and it indicates the diminishing positron annihilationprobability with oxygen electrons The ionic distributionsobtained from the X-ray diffraction peak intensity analysisfor samples with the different concentrations of the Cr3+ions are given in Table 1 Thus for example the distributionis (Fe3+

08Co2+02)

A[Co2+

08Cr3+01Fe3+11]

BO4

2minus for x = 01This implies that the substituted Cr3+ ions initially replacean equal number of Fe3+ ions from the (B-) sites butsimultaneously an equal number of Fe3+ ions from the (A-) sites move over to the (B-) sites in exchange of Co2+ ionsfrom the (B-) sites to the (A-) sites In effect an inversion ofthe spinel structure is prompted as a result of the substitutionprocess Hence as shown in Table 1 the number of Fe3+ions at the (A-) sites decreases whereas that at the (B-)sites remains unaltered till x = 07 compositions SinceCDB spectra indicate diminishing annihilation with oxygenelectrons and the positron lifetime 120591

2increases from 356 ps to

374 ps it is reasonable to argue that the defects in the samplewith 119909 = 01 are larger in size and increasingly deficientin oxygen ions than those in the unsubstituted (119909 = 00)sample In other words the defects were centered at the (A-)sites in the unsubstituted sample and at the (B-) sites in thesubstituted samplesWe attribute the second positron lifetimecomponent 120591

2to such large vacancy clusters

As already stated the substitution or doping resulted insharp rises in the two lifetimes 120591

1and 120591

2 The intensity

119868

2 however did not show any change It is therefore a

local effect in which the vacancy cluster has undergone anincrease in size From EDAX studies we have estimated theactual concentration of the Cr3+ ions effectively substitutedin the crystallites (Figure 1) It has been found that the(B-) sites suffered from nonstoichiometric deficiencies ofCr3+ ions and hence the Co2+ ions transferred to the (A-)sites are also less in number than that predicted by theformula (Fe3+

09minus119909Co2+01+119909)

A[Co2+

09minus119909Cr3+119909Fe3+11]

BO4

2minusThe result is that the vacancies so created will add to theexisting vacancy clusters resulting in further increase in theirsize and thereby enhancing the positron lifetimes Howeverthe deficiency decreases on subsequent doping and thereforethe lifetime 120591

2and intensity 119868

2remain rather unchanged in

the range of concentration from 01 to 07On the other hand the concentration of Fe3+ ions at

the (B-) sites does not change despite the substitution byCr3+ ions till x = 07 due to the simultaneous inversionprocess in which the Co2+ ions migrate to the (A-) sites[10] This means that the Cr3+ ions occupy the (B-) sitesThe Cr3+ substitution (x) above this value does not indicatefurther inversion and hence the effective number of Fe3+ions at the (B-) sites starts decreasing (Table 1) whereasat x = 07 the peak in the CDB spectrum appears thesharpest and the positron lifetimes start decreasing abovethis concentration indicating a reduction in the size of thevacancy clusters The intensity 119868

2increases (Figure 5) The

fact that positrons are now annihilating at sites which aresmaller in size can be understood as follows The latticeconstant of the CoCrxFe2minusxO4 samples steeply reduces insamples with increased Cr3+ concentration (Figure 4) Thecontraction of the lattice can be attributed to the slightlysmaller ionic radius of Cr3+ (0630 A) compared to that ofFe3+ (0640 A) Using the experimentally found values of thelattice constant (119886) and the oxygen positional parameter (119906)[1] it is possible to calculate the radii of the tetrahedral andoctahedral sites 119903A and 119903B respectively using the relations [1]

119903A = radic3(119906 minus1

4

) 119886 minus 119877

0 (3a)

119903B = (5

8

minus 119906) 119886 minus 119877

0 (3b)

Here 1198770is the radius of the oxygen ion (taken as 132 A) and

119906 is taken as = 0379 considering that CoCrxFe2minusxO4 is fullyinverse at 119909 = 00 composition [1 2] The site radii for thedifferent compositions estimated from the above equationsare also shown in Figure 4 Increasing the concentration ofCr3+ will result in filling the vacancies at the octahedralsites and owing to the positive charge positrons are trappedin a reducing number in the Cr3+-vacancy complexes soformed On the other hand while the octahedral sites werelarge enough to accommodate the Co2+ ions the radii of thetetrahedral sites are magnitude-wise smaller than its ionicradius (0740 A) and hence it is likely that a fraction of

8 International Journal of Spectroscopy

00 04 08 12 16 20

46

48

50

52

54

1198682

()

Cr3+ concentration (119909)

(a)

00 04 08 12 16 20350

360

370

380

1205912

(ps)

Cr3+ concentration (119909)

(b)

00 04 08 12 16 20140

150

160

170

Cr3+ concentration (119909)

1205911

(ps)

(c)

Figure 5 The positron lifetimes 1205911and 120591

2 and intensity 119868

2versus

Cr3+ concentration (x)

the tetrahedral sites are unoccupied during the inversion ofthe spinel structure at concentrations x lt 07 As a resultpositrons now that are no more trapped at the octahedralvacancies will move over to the vacancies at the tetrahedralsites and get trapped there The fast decreasing positronlifetimes support this argument since the tetrahedral vacancyclusters are smaller than the octahedral ones The availabilityof additional trapping sites expectedly increases the intensity119868

2as well Considering that positrons are now getting trapped

in the vacancy clusters present at the tetrahedral sites thelattice contraction would directly influence their lifetimeand the observations in the range mentioned above arein accordance with the same A careful study of the ionic

00 04 08 12 16 2006

08

10

12

14

16

1198683

()

Cr3+ concentration (119909)

(a)

00 04 08 12 16 201600

1800

2000

2200

1205913

(ps)

Cr3+ concentration (119909)

(b)

Figure 6 The orthopositronium lifetime 1205913and intensity 119868

3versus

Cr3+ content (x)

distribution shown in Table 1 makes us realize that fromx = 07 onwards till x = 17 the occupancies of Fe3+ and Co2+at the (A-) sites remain unchanged whereas the Cr3+ ionsdirectly replace the Fe3+ ions at the (B-) sites At 119909 = 17all the Fe3+ ions at the (B-) sites are replaced by Cr3+ ions(Table 1)

The last stage in the variation of the positron anni-hilation parameters versus the Cr3+ concentration (x) isobserved between x = 18 and 20 During x = 07 to 17the cation distribution at the (A-) sites remained unalteredas Fe3+

03Co2+07 while Cr3+ ions monotonically replaced

the Fe3+ ions at the (B-) sites with the Co2+ concentrationat the (B-) sites remaining unaltered as 03 As the latticecontraction continues the transformation of the spinel struc-ture from the inverse to the normal configuration that hadstarted during x = 01ndash07 and discontinued during x = 07ndash17 gets completed From octahedral site stabilization energyconsiderations it is known that cobaltchromite (CoCr

2O4) is

a normal spinel [10] In earlier positron annihilation studies

International Journal of Spectroscopy 9

0 5 10 15 20 25 30 35075

100

125

150

175

200

225

250

275

Co

Cr

FeCoFe2O4 CoCr2O4

Ratio

of n

orm

aliz

ed co

unts

pL (10minus3 m0 c)

(a)

0 5 10 15 20 25 300875

1000

1125

1250

1375

1500

1625

1750

119909 = 00

119909 = 03

119909 = 07

119909 = 17

119909 = 19

Ratio

of n

orm

aliz

ed co

unts

pL (10minus3 m0 c)

(b)

Figure 7The ratio curves generated from the coincidence Dopplerbroadening spectra of the different samplesmdash(a) elements Co Feand Cr besides CoCrxFe2minusxO4 of 119909 = 00 and 20 (b) CoCrxFe2minusxO4of x = 00 03 07 17 and 19 All the spectra had been peaknormalized and then divided by that of a pure reference Si sample inorder to generate the ratio curves

of nanocrystalline ZnFe2O4[24] and NiFe

2O4[25 33] the

positron lifetimes had been observed to decrease when anormal spinel ferrite transforms to an inverse spinel andconversely they increase when the transformation is justthe opposite These observations had been verified throughMossbauer spectroscopic studies too [25 33] Since the twopositron lifetimes 120591

1and 120591

2 drastically increase during

014 015 016 017057

058

059

060

061

062

063

119909 = 18

119909 = 07

119909 = 19

119909 = 00

119909 = 17

119909 = 01119909 = 11

119909 = 13119909 = 03

119909 = 20

119909 = 15

119878

119882

Figure 8 The 119878-119882 plot of the Cr3+-substituted samples

119909 = 18 and 20 (Figure 5) this stage is attributed to thetotal transformation of the partly inverse CoCrxFe2minusxO4 tothe fully normal CoCr

2O4 Note further that unlike during

x = 01ndash07 when the intensity 1198682did not show any change

it decreases in the final stage of inversion indicating the fulloccupancy of the (A-) sites byCo2+ ionsThe fact remains thatthe spinel structures normally suffer from nonstoichiometricdisorders and therefore vacancy clusters are inherently in-built in the structure The large value of 120591

2with still an

appreciable intensity 1198682supports this argument

As has been already pointed out the longest lifetime 1205913

and its intensity 1198683are due to the nanocrystalline dimensions

of the samples and they result from positronium atomsannihilating at the intergranular region Hence they need notnecessarily reflect the effects of any change in the vacancycluster dynamics within the grains Yet 120591

3shows a sudden

decrease during the initial stage 119909 = 01 to 07 and thenremains constant (Figure 6) The intensity 119868

3gradually falls

during this stage but shows a characteristic rise during thesecond stage 119909 = 07 to 17 and then remains constant(Figure 6) The initial fall can be attributed to small tracesof Cr3+ ions unsuccessful in being incorporated into thespinel structure and hence left to remain in trace amountswithin the intergranular region EDAXanalysis also indicatedfrustration even within the lattice due to the failure in thecomplete substitution of Fe3+ by the Cr3+ ions in the systemIn the latter stage (ie 119909 gt 07) the lattice contraction hasexpectedly resulted in a decrease by 03 in the grain size andthereby the number of positrons reaching out on the grainsurfaces has slightly increased

Although it is known that the magnetic properties of thesamples undergo rapid and interesting changes during theCr3+ substitution correlating such changes to the behaviorof positron annihilation parameters is never straightforward

10 International Journal of Spectroscopy

00 05 10 15 20057

058

059

060

061

062

063

119878

Cr3+ concentration (119909)

(a)

00 05 10 15 200135

0140

0145

0150

0155

0160

0165

0170

Cr3+ concentration (119909)

119882

(b)

Figure 9 The 119878 and119882 parameters versus Cr3+ concentration (x)

and is not attempted hereThe structural properties and theirchanges as depicted by the positron annihilation parametersand their variations may influence the magnetic propertieswhich need to be investigated by appropriate experimentalmethods

4 Conclusions

In understanding the effects of Cr3+-substitution in placeof Fe3+ in CoFe

2O4studied by positron annihilation spec-

troscopy we have offered the physical interpretation of theresults in terms of three distinct stages of defect evolutionand interaction First the unsubstituted ferrite (CoFe

2O4

119909 = 00) itself was found to contain large vacancy clustersThese clusters are identified as being present at the (A-) siteswith the divalent Co2+ ion and four of its coordinated oxygenions making way for such very strong trapping centers forpositrons At the onset of Cr3+-substitution (x) the positronlifetimes increased due to the transfer of positron trapping

Figure 10 Schematic representation of the two neighboring octantsof a normal spinel structure The large circles represent oxygenions The small solid and open circles represent the cations at thetetrahedral and octahedral sites respectively as illustrated in [33]

into defects to the (B-) sites In the second stage from 119909 =07 to 119909 = 17 a concomitant lattice contraction influencedthe positron annihilation characteristics This contraction isattributed to the slightly smaller ionic radius of Cr3+ thanthat of Fe3+ There is also a change in the positron trappingsites from the vacancy clusters at the (B-) sites back to thoseat the (A-) sites The last stage is marked by a full inversionof the structure to that of a normal spinel chromite and thepositron annihilation parameters depicted this stage with acharacteristic reversal of the trend of variation with the Cr3+concentration

Finally we conclude that the positron lifetime measure-ments complemented by results from coincidence Dopplerbroadening spectroscopy can be a viable alternative experi-mental tool to monitor the generation and evolution of struc-tural disorders in XY

2O4(where X and Y are divalent and

trivalent metals resp) systems during physical treatmentslike doping and grain size reduction Positron annihilationparameters are seen to sense directly or indirectly physicalphenomena of different kinds and implications like theredistribution of cations lattice contraction or expansionand structural transformations in certain cases [24ndash26]

Acknowledgments

The authors are grateful to Professor Indranil Das of SahaInstitute ofNuclear Physics (SINP) Kolkata for providing thereference Co and Cr samples for CDBS experiments Someuseful discussions with Dr N N Mondal and a graphicalillustration (Figure 10) prepared by Mr Pradipta K Das arealso gratefully acknowledged One of the authors (Kunal BModi) is thankful to Professor H H Joshi for providing someof the ferrite samples

References

[1] J Smit and H P J Wijn FerritesmdashPhysical Properties of Ferro-magnetic Oxides in Relation to Their Technical Applications NV Philips Gloeilampenfabrieken Eindhoven The Netherlands1959

[2] F Scordari Fundamentals of Crystallography edited by CGiacovazzo OxfordUniversity Press NewYork NYUSA 1992

International Journal of Spectroscopy 11

[3] W B Cross L Affleck M V Kuznetsov and I P Parkin ldquoSelf-propagating high-temperature synthesis of ferrites MFe

2O4

(M=Mg Ba Co Ni Cu Zn) reactions in an external magneticfieldrdquo Journal ofMaterials Chemistry vol 9 no 10 p 2545 1999

[4] K Tomiyasu J Fukunaga and H Suzuki ldquoMagnetic short-range order and reentrant-spin-glass-like behavior in CoCr

2O4

and MnCr2O4by means of neutron scattering and magnetiza-

tion measurementsrdquo Physical Review B vol 70 no 21 ArticleID 214434 12 pages 2004

[5] V T Thanki N N Jani U V Chhaya H H Joshi and R GKulkarni ldquoMagnetic properties of CoFe

2minus119909Cr119909O4synthesized

by co-precipitation methodrdquo AsianJournal of Physics vol 6 no1-2 p 222 1997

[6] G A Sawatzky F van der Woude and A H Morrish ldquoCationdistributions in octahedral and tetrahedral sites of the ferrimag-netic spinel CoFe

2O4rdquo Journal of Applied Physics vol 39 no 2

pp 1204ndash1205 1968[7] G Lawes B Melot K Page et al ldquoDielectric anomalies and

spiral magnetic order in CoCr2O4rdquo Physical Review B vol 74

no 2 Article ID 024413 6 pages 2006[8] N Menyuk K Dwight and A Wold ldquoFerrimagnetic spiral

configurations in cobalt chromiterdquo Journal de Physique vol 25pp 528ndash536 1964

[9] A Hauet J Teillet B Hannoyer and M Lenglet ldquoMossbauerstudy of Co and Ni ferrichromitesrdquo Physica Status Solidi (A)vol 103 no 1 pp 257ndash261 1987

[10] H Mohan I A Shaikh and R G Kulkarni ldquoMagnetic proper-ties of the mixed spinel CoFe

2minus119909CrxO

4rdquo Physica B vol 217 no

3-4 pp 292ndash298 1996[11] B G Tolesha S E Shrisath M L Mane S M Patange S S

Jadhav and K M Jadhav ldquoAutocombustion high-temperaturesynthesis structural andmagnetic properties of CoCr

119909Fe2minus119909

O4

(0 le 119909 le 10)rdquoThe Journal of Physical Chemistry C vol 115 no43 pp 20905ndash20912 2011

[12] R W Siegel ldquoPositron annihilation spectroscopyrdquo AnnualReview of Materials Science vol 10 pp 393ndash425 1980

[13] S Biswas S Kar S Chaudhuri and PM G Nambissan ldquoMn2+-induced substitutional structural changes in ZnS nanoparticlesas observed from positron annihilation studiesrdquo Journal ofPhysics Condensed Matter vol 20 no 23 Article ID 2352262008

[14] P Asoka-Kumar M Alatalo V J Ghosh A C Kruseman BNielsen and K G Lynn ldquoIncreased elemental specificity ofpositron annihilation spectrardquo Physical Review Letters vol 77no 10 pp 2097ndash2100 1996

[15] B D Cullity Elements of X-Ray Diffraction Addison-WesleyReading Mass USA 1978

[16] C Dong ldquoPowderX windows-95-based program for powder X-ray diffraction data processingrdquo Journal of Applied Crystallogra-phy vol 32 p 838 1999

[17] M J Buerger Crystal Structure Analysis Wiley New York NYUSA 1960

[18] H Ohnish and T Teranishi ldquoCrystal distortion in copperferrite-chromite seriesrdquo Journal of the Physical Society of Japanvol 16 pp 35ndash43 1961

[19] V T Thanki ldquoStudy on magnetic properties of oxide materi-alsrdquo [PhD thesis] Saurashtra University Rajkot India 1996(unpublished)

[20] J V Olsen P Kirkegaard N J Pedersen and M EldrupldquoPALSfit a new program for the evaluation of positron lifetimespectrardquo Physica Status Solidi (C) vol 4 no 10 pp 4004ndash40062007

[21] T Koida S F Chichibu A Uedono et al ldquoCorrelation betweenthe photoluminescence lifetime and defect density in bulk andepitaxial ZnOrdquo Applied Physics Letters vol 82 no 4 p 5322003

[22] A Zubiaga F Tuomisto F Plazaola et al ldquoZinc vacancies in theheteroepitaxy of ZnO on sapphire influence of the substrateorientation and layer thicknessrdquo Applied Physics Letters vol 86no 4 Article ID 042103 3 pages 2005

[23] P Hautojarvi and C Corbel ldquoFor a detailed discussion ondifferent cases of positron trapping in solidsrdquo in PositronSpectroscopy of Solids pp 491ndash532 IOS Press Amsterdam TheNetherlands 1995

[24] P M G Nambissan C Upadhyay and H C Verma ldquoPositronlifetime spectroscopic studies of nanocrystalline ZnFe

2O4rdquo

Journal of Applied Physics vol 93 no 10 pp 6320ndash6326 2003[25] S Chakraverty S Mitra K Mandal P M G Nambissan

and S Chattopadhyay ldquoPositron annihiliation studies of someanomalous features of NiFe

2O4nanocrystals grown in SiO

2rdquo

Physical Review B vol 71 Article ID 024115 8 pages 2005[26] S Chakrabarti S Chaudhuri and P M G Nambissan

ldquoPositron annihilation lifetime changes across the structuralphase transition in nanocrystalline Fe

2O3rdquo Physical Review B

vol 71 no 6 Article ID 064105 6 pages 2005[27] Y Nagai T Nonaka M Hasegawa et al ldquoDirect evidence of

positron trapping at polar groups in a polymer-blend systemrdquoPhysical Review B vol 60 no 17 pp 11863ndash11866 1999

[28] T Ghoshal S Biswas S Kar S Chaudhuri and P M GNambissan ldquoPositron annihilation spectroscopic studies ofsolvothermally synthesized ZnOnanobipyramids and nanopar-ticlesrdquo Journal of Chemical Physics vol 128 no 7 ArticleID 074702 2008 Virtual Journal of Nanoscale Science andTechnology vol 17 no 8 2008

[29] T Ghoshal S Kar S Biswas S K De and P M G NambissanldquoVacancy-type defects and their evolution under Mn substitu-tion in single crystalline ZnO nanocones studied by positronannihilationrdquo Journal of Physical Chemistry C vol 113 no 9 pp3419ndash3425 2009

[30] B Roy B Karmakar P M G Nambissan and M Pal ldquoMnsubstitution effects and associated defects in ZnO nanoparticlesstudied by positron annihilationrdquoNano vol 6 no 2 p 173 2011

[31] I K MacKenzie Positron Solid State Physics edited by WBrandt and A Dupasquier North Holland Amsterdam TheNetherlands 1983

[32] M J Puska and R M Nieminen ldquoDefect spectroscopy withpositrons a general calculational methodrdquo Journal of Physics Fvol 13 no 2 pp 333ndash346 1983

[33] S Mitra K Mandal S Sinha P M G Nambissan and SKumar ldquoSize and temperature dependent cationic redistribu-tion in NiFe

2O4(SiO2) nanocomposites positron annihilation

and Mossbauer studiesrdquo Journal of Physics D vol 39 no 19 pp4228ndash4235 2006

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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