*Corresponding author. Fax: #880-2-8613051.E-mail address: inst@bangla.net (A.K. Azad)

Journal of Magnetism and Magnetic Materials 214 (2000) 251}257

Room temperature, low temperature and polarized neutronstudies of YSr

2Fe

3O

8A.K. Azad!,*, A.K.M. Zakaria!, F.U. Ahmed!, S.K. Paranjpe", A. Das"

!Institute of Nuclear Science and Technology, Bangladesh Atomic Energy Commission, GPO Box 3787, Dhaka 1000, Bangladesh"Condensed Matter Physics Division, Bhabha Atomic Research Centre, Mumbai-400085, India

Received 25 August 1999; received in revised form 5 January 2000

Abstract

The nuclear and magnetic structures of YSr2Fe

3O

8have been studied by using X-ray and neutron powder di!raction.

Neutron di!raction data have been collected at room temperature (300 K) and low temperature (12 K). The tetragonalspace group P4/mmm was used for nuclear re"nements and the orthorhombic space group I

#mm@m was used for

magnetic re"nements. All possible oxygen sites in the unit cell are found to be fully occupied resulting in an oxygenoccupancy of &8.00 per formula unit. The magnetic structure is based on a unit cell related to the nuclear cell by thetransformation of matrix (1 11 0/1 1 0/ 0 0 2). The magnetic origin of the extra peaks, found in the di!raction patterns, wascon"rmed by polarization analysis method. The iron moments are coupled antiferromagnetically within each FeO

2-

layer, as well as along the c-axis. The magnetic moments are found to be 3.45(2) lB

and 3.54(3) lB

at 300 and 12 K,respectively. ( 2000 Elsevier Science B.V. All rights reserved.

Keywords: Neutron di!raction; Polarized neutron scattering; Crystal structure; Magnetic structure

1. Introduction

The modi"cation of structure and superconduct-ing property of YBa

2Cu

3O

yinduced by substitu-

tion of copper by metal or metalloid ions has beenextensively studied with a variety of techniques[1}6]. Dopants like Mg, Zn, and Ni which causea sharp depression in ¹

#are found to occupy the

planar Cu sites, while trivalent ions like Fe, Co, Aland Ga, which cause a somewhat less drastic de-crease in ¹

#, are found to replace linear chain

copper sites for low concentration. Ni and Zn sub-

stitution do not change the room temperature lat-tice parameters very much but Co and Fe dopingresults in a structural phase transition from anorthorhombic to tetragonal lattice at approxim-ately 3 at% doping level for fully oxygenatedmaterials [7].

The total replacement of Cu(1) and Cu(2) by Fe[8] results in a con"guration similar to that of theTa- and Nb-substituted compounds [9}11], inwhich Fe at the Cu(2) site has a "ve-fold pyramidalcoordination and at the Cu(1) site has an octahed-ral coordination. Similarly, the substitution of Cuby Co and Ba by K leads to the YBa

1.5K

0.5Co

3O

8compound [12] with Co(1) octahedrally coor-dinated and Co(2) in a "ve-fold pyramidal coord-ination.

0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 0 0 9 - 3

Replacement of Ba by strontium leads to a largevariety of compounds. YSr

2Cu

3O

6`yis a bulk

superconductor with ¹#"60 K at a high oxygen

pressure of 7 Gpa [13]. This compound can bestabilized in ambient conditions by partial substitu-tion of Cu by Al, Ga, Fe, Co, or Pb [14]. Like in theBa-based 123, in Sr-based 123 compounds also,Cu(1) can be totally replaced by Al, Co and Ga.Since the Y3` ion has a stronger preference for theeight-fold coordinated site than Sr2`, for smallsubstitution of isovalent Sr2` ion for Ba2`, alllattice parameters decrease monotonically [15,16].In YSr

2AlCu

2O

7the Al ions substitute for the

copper ions at the chain sites of the 1 : 2 : 3 parentstructure and have tetrahedral coordination. Thetetrahedra form chains which are propagatingalong the a- and b-axis on the basal plane [17].

The compound YSr2(Cu

1~xFe

x)3O

yhas also

been extensively studied with MoK ssbauer spectro-scopy, X-ray di!raction and magnetizationmeasurements [18,19]. Iron is found to be equallydistributed among the Cu(1) and Cu(2) sites. Themagnetic properties of iron at the two sites dependon oxygen content. In the case of oxygen-saturatedYSr

2Cu

3~xFe

xO

ysystem, the structure is tetrag-

onal for all x and among the three iron sites one isassigned to the Cu(2) planes and the other two tothe Cu(1) chains.

To our knowledge, no other fully substitutedcompounds for the Ba and Cu sites have beenprepared and characterized so far. The interest inthe present study is to know the precise structure ofthe Ba- and Cu-free compound with replacementby Sr and Fe, respectively, and to understand themagnetic ordering of Fe ions.

2. Experimental details

Samples with nominal composition YSr2Fe

3O

ywere prepared by thoroughly mixing high-puritystoichiometric amounts of Y

2O

3, SrCO

3and

Fe2O

3. The powders, mixed in appropriate cation

ratios, were ground in an agate mortar for half anhour. The mixture was "red in air at 9603C for 20 h.The product was ground again and sintered at9403C for 8 h, slowly cooled to 4003C at a rate of13C/min, annealed at 4003C for 14 h in oxygen

atmosphere and "nally cooled down to room tem-perature at a rate of 13C/min. Characterization andinitial structure study was made by X-ray powderdi!raction in the Bragg}Brentano geometry usingcopper monochromator (Rigaku D-Max/RC). Thesample was found in single phase with tetragonalstructure. Neutron powder di!raction measure-ments were carried out at room temperature(300 K) and low temperature (12 K) using (j"1.09 As ) position-sensitive detector (PSD)-basedpowder di!ractometer, at DHRUVA reactor atBhabha Atomic Research Center (BARC), Mum-bai. Polarized neutron measurements (j"1.205 As )were performed at the polarization analysis spec-trometer at DHRUVA [20]. A Cu

2MnAl (1 1 1)

crystal was used as the polarizing monochromatoras well as analyzer. An RF #ipper was used to #ipthe incident neutron beam polarization.

3. Results and discussion

3.1. Neutron powder diwraction

Room-temperature and low-temperature di!rac-tion data were collected using neutrons ofwavelength 1.094 As . Room temperature data werecollected in three steps of the detector setting,covering 2h, 10}453, 30}603 and 50}853. The col-lected data were "rst re"ned individually for thethree steps using the FullProf [21] program for thenuclear phase alone. The re"nements were made inthe space group P4/mmm and using the initialparameters from the values reported forYBa

2Fe

3O

8[8]. The 2nd and 3rd sets were then

grouped together and re"ned using the same pro-cedure. The possible presence of extra oxygen in theYttrium layer of the structure, or the existence ofextra oxygen or oxygen vacancies in the SrO andFeO

2layers, is not considered during the re"ne-

ments because re"nements of these defective mod-els does not give better occupancy factors orRietveld reliability factors (R-factors). Due to thelarge correlation between the occupancy and iso-tropic temperature factors, these parameters couldnot be re"ned simultaneously. The scale factor,half-width parameters, positions of the atoms, oc-cupancies and temperature factors were varied in

252 A.K. Azad et al. / Journal of Magnetism and Magnetic Materials 214 (2000) 251}257

Fig. 1. Observed (dots) and calculated (continuous line) Rietveld re"nement patterns of YSr2Fe

3O

8at the region 303)2h)793. The

bottom line indicates the intensity di!erence between the experimental and the re"ned patterns.

di!erent cycles of the re"nement. These re"nementsresulted in fairly good R-factors (R

1"6.60,

R81

"8.24, R%91

"6.46). It has been reported that[1,19] the lattice parameter c decreases continuous-ly while a increases monotonically with increasingFe concentration in 1 : 2 : 3-type structures, and forhigher concentration a'c/3. For the presentsample, (full substitution of Cu by Fe), we "nd thata(c/3. The di!erence between the lattice para-meters a and c of YBa

2Fe

3O

8[8] and YSr

2Fe

3O

8is almost 0.032 and 0.150 As , respectively. The de-crease in lattice parameters in relation to Ba 1 : 2 : 3can be attributed to the fact that the ionic radius ofSr`2 [22] in dodecahedral coordination is smallerthan that of Ba2` by 0.17 As . Fitted, observed, anddi!erence patterns of YSr

2Fe

3O

8in the region

313)2h)793 are shown in Fig. 1.Some strong peaks, mainly in the low-angle re-

gion of 2h, not accounted for by the adopted model,were observed in neutron di!raction pattern atroom temperature. These peaks are not seen inXRD pattern also, which is suggestive of their mag-netic origin. These extra re#ections could be in-dexed in terms of a larger unit cell of parametersa"b"a

0J2 and c"2c

0, where a

0and c

0are

the lattice parameters corresponding to the chem-ical cell. This is similar to what happens in theisomorphous Ba compound [8]. All the observedmagnetic re#ections have half integral indices h, k, lwhen referred to the crystal axes of the nuclearstructure and all three axes of the magnetic unit cellare larger than those of nuclear cell. A neutrondi!raction pattern was taken at 12 K to observestructural changes, if any, in the sample. No signi"-cant changes were observed at this temperature.However, the lower-angle magnetic (1 0 3) peakshape was found to be better than that of roomtemperature where the integrated intensity in-creased by about 10%. A small decrease in latticeparameters and an increase in magnetic momentwere also observed.

The data in the angular range 103)2h)413were re"ned with the initial parameters found fromhigher angle re"nements and including both thenuclear and magnetic phases at 300 and 12 K. Themagnetic phase was re"ned using the orthorhombicspace group I

#mm@m. These calculations resulted in

fairly good R-factors. Table 1 shows the resultsobtained from 300 K data. Observed and "ttedpatterns in the region 103)2h)413 at 300 and

A.K. Azad et al. / Journal of Magnetism and Magnetic Materials 214 (2000) 251}257 253

Fig. 2. Rietveld re"nement patterns for (a) 300 K (b) 12 K at the region 103)2h)413. Observed intensities are shown by dots and thecalculated intensities by the solid line.

Table 1Re"ned structural parameters of YSr

2Fe

3O

7.986at 300 K.

Chemical cell: Tetragonal, Space group: P4/mmma"b"3.8750(5) As , c"11.675(4) As , v"175.309 As 3. Magneticunit cell: Orthorhombic, Space group: I

#mm@m a"5.472(1) As ,

b"5.472(1) As , c"23.097(7) As , v"722.42 As 3. kx"3.45 l

B, k

y"

kz"0. R

1"5.32, R

81"6.81, R

%91"6.09, s2"1.25, R

B"8.95

and R."9.47

Atom Position x y z B (As )2 Occupancy

Fe(1) 1a 0.0 0.0 0.0 0.76(7) 1.0Fe(2) 2g 0.0 0.0 0.3276(1) 0.79(5) 2.0Y 1d 0.5 0.5 0.5 0.76(2) 1.0Sr 2h 0.5 0.5 0.1721(3) 2.57(3) 2.0O(1) 2g 0.0 0.0 0.1670(3) 1.07(4) 2.0O(2) 4I 0.5 0.0 0.3523(1) 4.05(2) 3.986(3)O(3) 2f 0.0 0.5 0.0 4.46(6) 2.0

12 K, are shown in Fig. 2. The re"ned structuralparameters and R-factors of room-temperature andlow-temperature patterns are compared with para-meters for three reference samples having similarstructure [8,18] in Table 2. The behavior of peaksat low-angle region 2h&163 was studied usingpolarization analysis method. This experimentalcon"guration can establish unambiguously

whether the additional peaks are magnetic in ori-gin, as will be discussed in detail later.

The total oxygen stoichiometry is found to be&7.986(3) i.e. the chemical and structural resultsyield a composition corresponding to the formulaYSr

2Fe

3O

7.986. This indicates that the average

oxidation number of Fe is close to #3. This aver-age valence is distributed over two crystallographi-cally inequivalent Fe(1) sites with octahedralcoordination, and Fe(2) sites with square pyramidalcoordination. The Fe(2) atoms are displaced fromthe oxygen plane by 0.0247 As towards the apicaloxygen. As a result, a dimpled FeO

2is observed

rather than the planar and corrugated iron}oxygenlayers (Fig. 3). The structure is the block of threelayers (FeO

2)0(Y)

#(FeO

2)0

where the subscriptso and c indicate whether the cation is at the originor at the center, respectively. The structure se-quence is also represented schematically forYSr

2Fe

3O

8in Fig. 3.

3.2. Polarized neutron scattering (PNS)

Polarized neutrons have been used to verify theadditional peaks which are magnetic in origin andto separate the magnetic and nuclear scattering at

254 A.K. Azad et al. / Journal of Magnetism and Magnetic Materials 214 (2000) 251}257

Table 2Fractional atomic coordinates, cell constants and Rietveld re"nement reliability factors of YSr

2Fe

3O

7.986at room temperature and low

temperature (12 K) as compared to three reference sample parameters, YBa2Fe

3O

8[8], YSr

2Cu

2FeO

7.5B0.1[18] and

YSr2Cu

2FeO

6.8B0.1[18]. For all samples, re"nements were done in the tetragonal space group P4/mmm

Parameters YSr2Fe

3O

7.986YBa

2Fe

3O

8YSr

2Cu

2FeO

7.5B0.1YSr

2Cu

2FeO

6.8B0.1

300 K 12 K RT RT RT

a/"b

/(chem) 3.8750(1) 3.8682(4) 3.9170(1) 3.8198(7) 3.8395(7)

c/

(chem) 11.675(4) 11.652(3) 11.8252(4) 11.3430(1) 11.5410(5)a."b

.(mag) 5.472(1) 5.4624(6) 5.5395(1) * *

c.

(mag) 23.097(7) 23.065(6) 23.6503(7) * *

</

175.307 174.353 181.43(2) 165.50 168.80<

.681.588 688.221 725.73(4) * *

kx

3.45(2) 3.54(3) 3.49(2) * *

z of Fe(2)/Cu(2) 0.3277(5) 0.3335(2) 0.3388(1) 0.3498(8) 0.3500(7)z of Sr/Ba 0.1704(9) 0.1683(3) 0.1654(2) 0.1791(5) 0.1926(5)z of O(1) 0.1670(7) 0.183(2) 0.1804(2) 0.163(4) 0.164(4)z of O(2) 0.3522(8) 0.3582(1) 0.3783(1) 0.373(1) 0.374(2)R

15.32 5.76 5.91 7.56 9.91

R81

6.81 7.35 8.05 7.05 6.97R

%916.09 5.35 5.48 1.99 2.12

R.

9.47 9.57 7.70 * *

Fig. 3. Nuclear and magnetic structures of YSr2Fe

3O

8. For

simplicity, only nuclear cell is shown in the "gure.

angles where the magnetic and nuclear peaks over-lap. The technique to separate the magnetic re-sponse using polarized neutrons has been describedin the literature [23]. The basic principle employedhere is that, when a magnetic "eld H within thesample is parallel to the scattering vector Q, i.e.HEQ, all of the magnetic scattering is associatedwith a reversal of the neutron spin (spin-#ip scatter-ing) while for HoQ, the spin-#ip and non-spin-#ipare equally probable. The nuclear scattering is in-dependent of the orientation of H and Q. For thecon"guration HoQ (vertical "eld), for a purelymagnetic re#ection, the spin-#ip (!#) and non-spin-#ip (##) intensities should be equal. On theother hand, for a purely nuclear re#ection, only thenon-spin-#ip (##) intensities will be observedand there will be virtually no spin-#ip (!#) inten-sities because nuclear coherent Bragg scatteringnever causes a reversal of the neutron spin directionupon scattering.

Polarized neutron scattering data have beencollected for two peaks at scattering angles of 15.93and 18.23 at room temperature for HoQcon"guration as shown in Fig. 4. The intensity forboth spin-#ip and non-spin-#ip scattering is the

A.K. Azad et al. / Journal of Magnetism and Magnetic Materials 214 (2000) 251}257 255

Fig. 4. Polarized neutron scattering observed at the range143)2h)203 in the con"guration HoQ.

same for the peak at 2h&15.93. We canconclude that this scattering is purely magnetic inorigin and the compound is magnetically long-range ordered. The peak at 18.23, on theother hand, has strong intensity for non-spin-#ip,while the intensity for spin-#ip is smaller by a factorof 7. We can conclude that there is no spin-#ipintensity other than that caused by the "nite #ip-ping ratio, and this peak is identi"ed as a purenuclear re#ection.

4. Conclusions

We re"ned the crystal structure of YSr2Fe

3O

8by neutron powder di!raction at 300 and 12 K.The Rietveld re"nements were done using the tet-ragonal space group P4/mmm for the nuclearstructure and the orthorhombic space groupI#mm@m for the magnetic structure. The oxygen

occupancy was found to be&8.00 per unit formula.The magnetic origin of the sample has been con-"rmed by polarized neutron scattering. The mag-netic moments of all the iron ions have the samevalues of 3.45(2) l

Band 3.54(3) l

Bat 300 and 12 K,

respectively.

Acknowledgements

Financial support for A.K. Azad and A.K.M.Zakaria was provided by the International AtomicEnergy Agency (IAEA) under project no. BGD/004and BGD/018 to do this work at Bhabha AtomicResearch Centre under IAEA fellowship program.We wish to acknowledge Dr. C.T. Ye, Head, Neu-tron Scattering group, China Institute of AtomicEnergy, for his helpful suggestions and supplyingthe sample during his expert visit to BangladeshAtomic Energy Commission. We thank Dr. A.S.Sequeira, Head (Rtd.), Solid State Physics Division,BARC for many helpful discussions. We also thankR. Chitra, BARC for the X-ray measurements. Weare grateful to BARC for giving us the permissionto use the experimental facilities.

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