Journal of Alloys and Compounds 492 (2010) 496–499

Journal of Alloys and Compounds 492 (2010) 496–499Contents lists available at ScienceDirectJournal of Alloys andCompoundsj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j a l l c omMixing of iron and molybdenum and photo-doping effect on Sr 2FeMoO 6Y.C.Hu a ,Q.Ji a ,J.J.Ge a ,R.B.Xie a ,Z.S.Jiang a ,X.S.Wu a ,∗,G.F.Cheng b ,Hairui Liu c ,Qingfeng Lu caNanjing National Laboratory of Microstructures,Key Lab of Solid State Microstructures,Department of Physics,Nanjing University,Hankou Road,Nanjing 210093,China bShanghai Institute of Ceramics,CAS,1295Dingxi Road,Shanghai 200050,China cDepartment of Physics,Henan Normal University,Xinxiang 453007,Henan,Chinaa r t i c l e i n f o Article history:Received 17August 2009Received in revised form 19November 2009Accepted 20November 2009Available online 26 November 2009Keywords:Magnetically ordered materials X-ray diffraction Crystal structurePositron spectroscopya b s t r a c tThe structure and photo-doping effect on Sr 2FeMoO 6are investigated using the X-ray powder diffraction and positrons annihilation technique,respectively.Anti-site defect is about 8.8%,can be estimated directly from the XRD pattern,which is well consistent with that of obtained from the XRD refinements.The integral intensity ratio of the reflection (101)and the reflections (200)and (112)may used to estimate the concentration of the anti-site defect.The positron annihilation lifetime in Sr 2FeMoO 6is sensitive to photo-doping.The average lifetime and the electron density n e vary with photo-doping.© 2009 Elsevier B.V. All rights reserved.1.IntroductionThe double perovskite compound Sr 2FeMoO 6(SFMO),with a high magnetic transition temperature (T c ≈410K)[1]and a remarkable magneto-resistance around room temperature,has become one of the most promising candidates in magnetic storage materials [2].SFMO is a typical ordered double perovskite structure A 2BB O 6,where A is the alkali-earth ion,B and B are the transition metals.As reported in [3,4],the crystal structure of SFMO is cubic or tetragonal,where the alternating FeO 6and MoO 6octahedral are arranged regularly in a rock salt superlattice with the voluminous Sr cation occupying the voids among the octahedral.The existence of anti-site defects (Mo on Fe site and vice versa)has great influence on the magnetic and transport properties of Sr 2FeMoO 6[5,6].Defects are important structural information in understanding the properties of material [7].Positron annihilation technique (PAT)can provide unique information about defects and has many advan-tages in structural characteristic.It is a nondestructive and effective method to detect the information of defects [8],whereas there are few studies about SFMO using this technique recently.Photo-doping,which remains the component and structure of material unchanged,is an effective way to improve the property [9,10].It has been extensively used in semiconductor material [11,12],but∗Corresponding author.Tel.:+862583594402;fax:+862583595535.E-mail address:xswu@ (X.S.Wu).seldom studies are performed on magnetic materials.In order to understand the photo-doping effect on defects in SFMO,we synthe-size the compounds using standard solid-state reaction.The crystal structure and photo-doping effect on defects have been studied by means of X-ray powder diffraction and the positrons annihilation technique,respectively.2.ExperimentalSample of Sr 2FeMoO 6is prepared by standard solid-state reaction.Stoichiomet-ric powders of SrCO 3,Fe 2O 3and MoO 3are mixed,ground and heated at 900◦C for 10h in air.The pre-reacted mixture is then finely ground,pressed into pellets and sintered at 1280◦C in a stream of 5%H 2/Ar gas for 15h with several intermediate grindings.The samples are heated and cooled at a rate of 5◦C/min under the same atmosphere.Structure of the sample is examined by X-ray powder diffraction (XRD)using a Rigaku D/max 2500diffractometer with CuK ␣radiation (50kV 250mA)and a graphite monochromator.The XRD pattern shows that the sample crystallizes in sin-gle phase.The XRD data are analyzed by means of the Rietveld refinement program GSAS [13].Sample is illuminated by halogen lamp (60mW/cm 2)with varying illumina-tion time from 0min to 40min in a space of 10min.Power of the lamp is 100W and distance between sample and lamp is 7.9cm.The sample temperature values reached at 28±1◦C after 40min irradiation.The heating rate of the samples is about 2±1◦C/10min through the illumination.We use pieces of the samples at the same time.After each illumination,the positron lifetime measurements are carried out using a conventional fast–fast coincidence ORTEC-100U system with a prompt time resolution of 228ps (FWHM,full width at half maximum)and 10␮Ci 22Na source in sandwich geometry with the pellets.The total count for spectrum is 1million with the counting rate of 1000cps.Lifetime spectrum is analyzed by the PATFIT computer program with necessary source corrections.The measurements of positron lifetime are all performed at room temperature (20±1◦C).0925-8388/$–see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.jallcom.2009.11.148Y.C.Hu et al./Journal of Alloys and Compounds492 (2010) 496–499497Fig. 1.Observed(circles)and calculated(continuous line)XRD pattern for Sr2FeMoO6.The lowest curve is the difference between the observed and the calcu-lated XRD patterns.The vertical bars at the bottom indicate the Bragg reflection positions.(101)is the Bragg reflection at2Â≈19◦and(200)+(112)is that at 2Â≈32◦.3.Results and discussionFig.1presents the XRD pattern for Sr2FeMoO6.The excellent crystalline quality of sample used in this work can be appreci-ated.The XRD data can be used to refine the structure of the sample and abundance of information could be revealed due to precise counting statistics.Rietveld refinements of XRD diffraction pattern are carried out using the I4/m and the corresponding Wyck-off positions:Sr,4d(1/2,0,1/4);Fe1,2a(0,0,0);Fe2,2b(0,0,1/2); Mo1,2b(0,0,1/2);Mo2,2a(0,0,0);O1,8h(x,y,0);O2,4e(0,0,z). The refinement is performed according to the following group order[14,15]:(1)scale factor;background;zero point shift/sample displacement,transparency coefficient;(2)cell parameters;(3) peak shape;half width;asymmetry parameter and preferred ori-entation;(4)atom position parameter;(5)site occupancies;(6) overall thermal parameters;(7)isotropic thermal parameters.The Gaussian function is considered to refine the profile.No absorp-tion correction is taken into account.The wavelengths of CuK␣1, CuK␣2and the intensity ratio arefixed as1.5406A,1.5444A,and 0.497A,respectively.Full occupancy at every site in the unit cell is assumed during the refinements,i.e.the occupancies of Fe and Mo at2a and2b in total are1,respectively.The preferred orien-tations are very small and the temperature factors B Sr,B Fe1,B Fe2,B Mo1,B Mo2,B O1,and B O2are0.01304Å2,0.00611Å2,0.00611Å2,0.00392Å2,0.00392Å2,0.02301Å2,and0.01271Å2,respectively. The refinement process is smooth and leads to good quality fac-tors Rwp=10.00%and Rp=7.26%.All reflections can be indexed and a=b=5.58235(1)Å,c=7.87965(4)Å.The occupancy of Fe on Mo site (which is the occupancy of Fe2,equivalent in value to that of Mo on Fe site)is8.8%,which means the order concentrationÁis82.4% due toÁ=1−2x,where x is fraction of Fe on Mo site.Superstructure reflection(101)at2Â≈19◦can be seen clearly,which may indicate the existence of cation order-ing between Fe/Mo.Fig.2is the sketch of the unit cell of Sr2FeMoO6,in which Fe and Mo are completely ordered.Each unit cell is composed of10ions and the coordinates ofthe Fig.2.The unit cell of Sr2FeMoO6,in which Fe and Mo is completely ordered. ions are as following:Sr(1/2,1/2,1/4),(1/2,1/2,3/4),Fe(0,0,1/2), Mo(0,0,0),and O(1/2,0,0),(0,1/2,0),(0,0,1/4),(0,0,3/4),(1/2,0,1/2), (0,1/2,1/2).Based on the structure factor formula F(h k l)= nj=1f j e2 i(hx j+ky j+lz j),we can calculate the F(h k l)of Sr2FeMoO6as:F(h k l)=f Sre2 i(h/2+k/2+l/4)+e2 i(h/2+k/2+(3/4)l)+f Oe2 i(h/2)+e2 i(k/2)+e2 i(l/2)+e2 i(3/4)l+e2 i(h/2+l/2)+e2 i(k/2+l/2)+f Fe e2 i(l/2)+f Mo e2 i0when(h k l)=(101),F(101)=f Sr(e(3/2) i+e(5/2) i)+f O(e i+e0i+e(1/2) i+e(3/2) i+e2 i+e i)+f Mo−f Fe =f Mo−f FeSo we can obtain:I101∝P101|F(101)|2=P101|f Mo−f Fe|2,whereP101=(1+cos22Â)/(4sin2ÂcosÂ)is the Lorentz polarization fac-tor.The reflection of(101)is indeed observed at2Â≈19◦.Infact,as reported in Ref.[16],there is a fraction of Mo atoms onthe B site,equivalent in value to that of Fe atoms on the B site,which is defined as the anti-site defect.Suppose there are x Moions on Fe sites in the unit cell.The F(101)can be modified as:F(101)=(1−x)f Mo+xf Fe−(1−x)f Fe−xf Mo=(1−2x)f Mo−(1−2x)f Fe=(1−2x)(f Mo−f Fe)Four interesting results can be seen from above consequences.Firstly,the intensity of(101)reflection decreases with the increaseof the anti-site defect.Secondly,the integral intensity ratio betweenthe reflection peak of about2Â≈19◦and the reflection peak ofabout2Â≈32◦,I19◦/I32◦,which is used to indicate the order of Fe/Mo[17],can be interpreted as following:I32◦=I200+I112I200∝P200F2002,I112∝P112F1122,Thus,I32◦∝P200F2002+P112F1122Due to F(200)=(f Fe+f Mo)+2(f Sr+3f O),F(112)=(f Fe+f Mo)−2(f Sr+3f O).So I32◦∝P200|(f Fe+f Mo)+2(f Sr+3f O)|2+P112|(f Fe+f Mo)−2(f Sr+3f O)|2,which has no relation to Fe/Mo order.I19◦/I32◦can498Y.C.Hu et al./Journal of Alloys and Compounds 492 (2010) 496–499indicate the Fe/Mo order concentration as I 19◦is induced by Fe/Mo order.The larger I 19◦/I 32◦,the higher order of Fe/Mo.Thirdly,we define R is the integral intensity ratio of I 19◦/I 32◦for ideal sample without anti-site defect and R e as that for our sample which is prepared by standard solid-state reaction.R =I 19◦I 32◦∝P 101 F (101)2P 200F 2002+P112F1122∝P 101 f Mo −f Fe2P 200 (f Fe +f Mo )+2(f Sr +3f O ) 2+P 112 (f Fe +f Mo )−2(f Sr +f O )2When consider the fraction of anti-site defect x ,R e ∝(1−2x )2P 101 f Mo −f Fe2P 200(f Fe +f Mo )+2(f Sr +3f O ) 2+P 112(f Fe +f Mo )−2(f Sr +f O )2,soR e /R =1−2x .Due to Á=1−2x ,thus Á=R e /R .R e =I 19◦/I 32◦=2.56%from our experiment and we cancalculate R =3.76%[18,19]as sin Â(101)/ =0.11and sin Â(200)/ ≈sin Â(112)/ =0.18.ThusR e /R =82.5%,which is also similar with the order concentration Áfrom Rietveldrefinement.So we think Á=R e /R is a convenient method to calculate the order concentration approximately.Fourthly,based on the third result,we can define the disorder concentration as:=⎧⎨⎩2x,0<x ≤12(1−x )+(1−x ),12≤x <1.When x =1/2,F (101)=0, =100%.This means that the peak at 19◦will disappear as Fe/Mo complete disorder.When x =0or 1,the Fe/Mo is perfect ordered.Analysis on the positron lifetime spectra usually tells us three meaningful components of 1, 2and 3in the ratio I 1+I 2+I 3=100%.The second lifetime component 2with the typ-ical value of 278–280ps and a relative intensity of 1.5–2%,is due to the partial trapping of positrons at residual extrinsic vacancy-type defects. 2suggests that photo-doping has little effect on vacancy-type defect,according to our experiments [20],we here do not consider for further discussion.Fig.3(a)left shows the short lifetime component 1against different illumination time in the Sr 2FeMoO 6.The short lifetime component 1,having a value of 180–200ps,is due to the free annihilation of positrons.Anti-site defect is not open volume defect and contributes to 1.Electrons in the sample are activated by photo-doping and the concentration of electrons at the site of free annihilation increases,which improves the annihilation rate 1. 1decreases because of 1=1/ 1.The increase of 1is due to the decrease of the electron concentration at the site of free annihilation when t ≥30min.The longest lifetime component 3against different illumina-tion time in SFMO is shown in Fig.3(a)right.The longest component 3,with the typical value >0.5ns,results from positronium formed and annihilated through the ‘pick-off’process.The three main mod-els that describe the positronium formation in condensed matter are the Ore model (OM)[21],the spur model (SM)[22]and the free volume model (FVM)[23].Our work can be better explained according to FVM.Illumination may make electrons transit into the free volume and improve the annihilation rate 3. 3decreases because of 3=1/ 3. 3continually decreases because more elec-trons might locate the free volume.This is similar with Ref.[24],which suggests that two or more electrons can locate one vacancy.The concentration of electron at the free volume is saturated when t =30min due to Coulomb compel potential betweenelectrons.Fig.3.Positron lifetime parameters (a)left 1,right 3;(b)left I 1,right I 3;and (c)left AV ,right n e against different illumination time in the Sr 2FeMoO 6.The solid lines are guides for the eyes.Fig.3(b)shows I 1(left)and I 3(right)against different illu-mination time in the Sr 2FeMoO 6.I 1is found first to increase (0min ≤t ≤30min)and then decrease (t >30min)with a relative intensity of 85–88%,which is the main part of the positron lifetime spectra.This can be explained in light of the following scenario:electrons at the site of free annihilation is easier to active by illu-mination than at the free volume.When t =30min,electrons at the site of free annihilation is completely activated.When t >30min,electrons at the free volume is continually activated but at the site of free annihilation has already saturated,thus the intensity of 1decreases.Fig. 4.Observed (circles)and calculated (continuous line)XRD pattern for Sr 2FeMoO 6after 30min illumination,which is the representative XRD pattern for illuminated samples.The lowest curve is the difference between the observed and the calculated XRD patterns.The vertical bars at the bottom indicate the Bragg reflection positions.Y.C.Hu et al./Journal of Alloys and Compounds 492 (2010) 496–499499Table 1Unit cell parameters for Sr 2FeMoO 6after irradiation obtained from Rietveld refinement of XRD data.0min10min 20min 30min 40mina 5.58235(1) 5.58246(9) 5.58236(5) 5.58250(5) 5.58261(9)c7.87965(4)7.87968(8)7.87982(4)7.87985(6)7.87990(6)v245.55084(2)245.56228(3)245.55737(1)245.57068(5)245.58227(3)Rwp (%)10.0010.4110.289.9210.12Rp (%)7.267.596.866.767.41We have calculated the average positron lifetime AV ,defined as AV =I i i ,where I i is the relative intensity of the i th lifetime com-ponent. AV is better to describe the mean size of the defects in the samples.As shown in Fig.3(c)left, AV of the photo-doped is smaller than that of photo-free.According to Navarro et al.[25],Sr 2FeMoO 6may be unstable when heated,and that may have important oxida-tion effects if exposure it to air for several days.We measured the structure of the samples after PALS measurements.Fig.4shows the XRD pattern for Sr 2FeMoO 6after 30min illumination,which is the representative XRD pattern for illuminated samples.There is no trace of SrMoO 4is detected,due to that 40min might be a very short time for oxidation procedure.Structure parameters after irra-diation obtained from Rietveld refinement are shown in Table 1.The increase of unit cell makes the size of defects small.This might be important reason for decrease of average lifetime.Ions may absorb photons and occupy part of defects,thus also reduce the mean size of defects.We calculated the electron density n e ,defined asn e =1/(¯ r 20c ),where c is the speed of light and r 0is classical elec-tron radius.The variation of n e against illumination time is shown in Fig.3(c)right.The minimum value of AV suggests the maximum of electrons density when t =30min.4.ConclusionTo investigate the structure and photo-doping effect on defect in SFMO,we performed X-ray powder diffraction and the positrons annihilation technique.The result of X-ray powder diffraction sug-gests that the superstructure reflection (101)appearing due to the Fe/Mo order on the B/B site can index the concentration of anti-defect.Positron lifetime measurements after illumination suggest that the positron annihilation lifetime is sensitive to photo-doping.The minimum of defects size and the maximum of electrons den-sity when t =30min indicate that photo-doping may be an effective way to improve the property of the material.AcknowledgementsThis work is supported by NNSFC (10774065,10523001)and NKPBRC (2006CB921802,2010CB923404).Professor QFL thanks for the financial support from Henan Education foundation (No.2007140008).References[1]K.-I.Kobayashi,T.Kimura,H.Sawada,K.Terakura,Y.Tokura,Nature 395(1998)677.[2]A.Gary,Prinz,J.Magn.Magn.Mater.200(1999)57.[3]F.K.Patterson,C.W.Moeller,R.Ward,Inorg.Chem.2(1963)196.[4]F.S.Galasso,F.C.Douglas,R.J.Kasper,J.Chem.Phys.44(1966)1672.[5]D.Stoeffler,S.Colis,Mater.Sci.Eng.B 126(2006)133.[6]M.F.LÜ,J.P.Wang,J.F.Liu,W.Song,X.F.Hao,D.F.Zhou,X.J.Liu,Z.J.Wu,J.Meng,J.Alloy Compd.428(2007)214.[7]Z.Bajnok,Zs.Simon,Nucl.Phys.B 802(2008)307.[8]X.J.Hu,J.S.Ye,H.J.Liu,S.Mariazzi,R.S.Brusa,Thin Solid Films 516(2008)1699.[9]J.H.Hao,W.D.Si,X.X.Xi,Appl.Phys.Lett.76(2000)3100.[10]J.H.Hao,X.T.Zeng,H.K.Wong,J.Appl.Phys.79(1996)1810.[11]M.Idrish Miah,Mater.Chem.Phys.111(2008)249.[12]O.V.Prezhdo,Chem.Phys.Lett.460(2008)1.[13]B.H.Toby,J.Appl.Crystallogr.34(2001)210.[14]X.S.Wu,W.M.Chen,X.Jin,S.S.Jiang,Physica C 273(1996)99–106.[15]X.S.Wu,S.S.Jiang,N.Xu,F.M.Pan,X.R.Huang,W.Ji,Z.Q.Mao,G.J.Xu,Y.H.Zhang,Physica C 266(1996)296–302.[16]M.T.Anderson,K.B.Greenwood,G.A.Taylor,K.R.Poppelmeier,Prog.Solid StateChem.22(1993)197.[17]Ll.Balcells,J.Navarro,M.Bibes,A.Roig,B.Martínez,J.Fontcuberta,Appl.Phys.Lett.78(2001)781.[18]/xray/comp/scatfac.htm .[19]A.Guinier,X-ray Diffraction,W.H.Freeman and Company,San Francisco,1963.[20]Y.C.Hu,P.F.Wang,B.Lv,Q.Ji,X.S.Wu,Q.F.Lu,J.Appl.Phys.105(2009)07D726.[21]A.Ore,Univ.Bergen Arbok,Naturvet Rekke,1949,p.9.[22]O.E.Mogensen,J.Chem.Phys.60(1974)998.[23]W.Brandt,S.Berko,W.W.Walker,Phys.Rev.120(1960)1289.[24]P.Hautojarvt,Positrons in Solid,Springer-Verlag,Berlin,Heidelberg/New York,1979.[25]J.Navarro,C.Frontera,D.Rubi,N.Mestres,J.Fontcuberta,Mater.Res.Bull.38(2003)1477–1486.。