PHYSICAL REVIEW B VOLUME 48, NUMBER 5 1 AUGUST 1993-I

Mossbauer study of (Fe& „Ni„)7Ses

Hang Nam Ok, Kyung Seon Back, and Eng Chan KimDepartment ofPhysics, Yonsei University, Seoul 120-749, Korea

Chul Sung KimDepartment ofPhysics, Kookmin University, Seoul 136-702, Korea

(Received 19 January 1993; revised manuscript received 5 April 1993)

(Fe& Ni )7Se8 (x=0.02,0.05,0.08) has been studied with Mossbauer spectroscopy and x-raydiffraction. The crystal structure is found to be a triclinic superstructure of the NiAs structure while

Fe7Se8 has a hexagonal structure. Abrupt change of quadrupole shifts near 122 K suggests that thespin-rotation transition proceeds abruptly, in contrast with the gradual transition reported for Fe7Se8with a triclinic superstructure. The iron ions at all four sites are found to be in a highly covalent ferrousstate. Both Neel and Debye temperatures are found to decrease with increasing nickel concentration.

I. INTRODUCTION

The fundamental crystal structure of Fe7Se8 is knownto be of the NiAs type. Since one-eighth of the iron sitesare vacant, the composition can be represented asFe7VSe8, where V stands for an iron vacancy. Two typesof superstructure due to ordering of the vacancies areknown. ' One is the hexagonal superstructure (3c struc-ture) that has the following unit-cell constantsA =2a=7.234 A, and C=3c =17.65 A, where a and cdesignate the unit-cell lengths of the fundamental NiAsstructure; more recent measurement shows thatA =7.2613 A and C =17.675 A. The other is a triclinicsuperstructure (4c structure) that has the following unit-cell lengths: A =&3B, B=2a, and C =4c. Quenchingfrom about 400'C produces the 3c structure whereas slowcooling yields the 4c structure.

Ferrimagnetic ordering in Fe7Se8 was studied with theneutron-diffraction method. According to this measure-ment, the magnetic moments of iron atoms are parallel tothose of the atoms on the same layer parallel to the (001)plane, while they are antiparallel to those of the atoms onthe next layer. When the temperature of the crystal islowered from the Neel temperature, the direction of themoment, which is at first on the (001) plane, changes to-ward the [001] direction. The behavior of this spin-rotation process depends on the type of crystallographicsuperstructure in the specimen. For the 3c structure thespin-rotation process takes place abruptly at 130 K.However, in the case of the 4c structure the process startsat about 220 K and shows a gradual shift with decreasingtemperature. The spin-rotation shift toward the c axis isnever completed, and even at 4.2 K, a tilt of about 20' hasbeen observed.

There are three main types of iron sites, A, B, and C.An A site is in the c plane with Fe vacancies, whereas theB and C sites are in the c plane with no Fe vacancies. AnA site ion has two adjacent ferromagnetic cation sites va-cant, while B and C sites have four and three adjacent an-tiferrornagnetic cations, respectively, absent. The A sitescan be further subdivided into A& and A2 sites, takinginto account the next-nearest-neighbor ferromagnetic c

planes. The relative ratio of the numbers of A ] A2 andB, and C sites in the superstructure are 1:2:2:2.

In the present study, the effects of the cationic substitu-tion of Ni for Fe on the crystallographic and magneticproperties have been investigated by preparing the mixedsystem (Fe, ,Ni )7Se8 and by using Mossbauer and x-ray techniques.

II. KXPKRIMKNT

Synthesis of the (Fe, Ni„)7Ses (x=0.02, 0.05, 0.08)samples was accomplished by the following direct-reaction method. The starting materials were Fe, Ni, andSe powders of 99.995%%uo, 99.99%%uo, 99.999%%uo purity, respec-tively. Mixtures of proper proportions of the elementssealed in evacuated quartz ampoules were heated to600'C for 24 h, at 900'C for 24 h, at 1100 C for 1 h, at950'C for 7 days, and then cooled down to room temper-ature. Extra precautions have been exercised to preventoxygen from diffusing into the quartz ampoule and seleni-urn vapors from escaping from the initial mix during thesealing process, as described elsewhere. ' The sampleswere ground and pressed into pellets before being sealedinto evacuated quartz arnpoules for another firing. Thesecond firing proceeded at 950'C for 7 days, and then thesamples were cooled down to 400'C. After being kept at400'C for 24 h, they were quenched to room tempera-ture. The samples were Fe enriched to 6.7 at. %%uoof th emetal atoms in the samples for Mossbauer measurements.

X-ray-diffraction patterns of the samples were obtainedwith Cu Ka radiation. A slow scanning speed of 0.25advance in 28/min, was used in order to optimize resolu-tion of the closely spaced reAections. A Mossbauer spec-trorneter of the electrochemical type was used in theconstant-acceleration mode. A Co single-line source ina rhodium matrix was used at room temperature.

III. RESULTS

Figure 1 shows x-ray-diffraction patterns for(Feo 9sNio o2)7Ses and Fe7Ses at room temperature. Everypeak in the patterns for (Fe, Ni )7Ses can be indexed

0163-1829/93/48(5)/3212(4)/$06. 00 48 3212 1993 The American Physical Society

MOSSBAUER STUDY OF (Fe& Ni )7Se8 3213

0 oIa (~ N1„,),So,

0~ P P ~ ~ ~ ~~ a e- ~h —A -~% a+a a~

+%~%~ g %~ ~v 4~@

4-

'= w. ~ ~.~ Q+ ~ ~ ~~P+P ~ ~ ~ ~ ~ ~0+

~P

441 K

Isa eaao ollg ol

DOeD

oryy I& ooeo

8-10-0--c.e.W~ a h ' gg

~ ~ PP ~Pho ~ ~(e 'L ~ ~a E%o

~ 4 ~

292 K

30~0Dcv

I

40I

SOI

eoI

TO

4-P S. Pa&+ ~ J ~P ~0, P

~ ~ ~ hP woP+~ ~

0 ~~ ~ Ph ~ ~ ~

~ ~ P~0~ ~

180 K

Fe&See

I

30I

40 SO

2e ( Deg r ees)

I

eoI

70I

eo

P~ ~A P ~ ~

O C &',~&a~ ~ ~

CL2C)

CACG

4

C. -'--—~ P ++ ~ aE 4 ~

W+ %ma~ ~ ~

I

'I

II

B G

~ V PI a ~ ~

PPh P ~ ~ P ~~ 0

120 K

~ ~ ~-sP P~ h ~

~ ~ 0

FICx. 1. X-ray-diffraction patterns for (Feo 98Nioo2)7Se8 andFe7Se8 at room temperature.

2-

4-

60 K

on a triclinic unit cell while those in the patterns forFe7Se8 can be indexed on a hexagonal unit cell. Close ex-amination and analysis of the observed peak positionsand intensities show that the (Fei „Ni )7Ses (x=0.02,0.05, 0.08) samples have the triclinic 4c structure. Thelattice constants are listed in Table I. The unit-cell con-stant A, B, and C decrease with increasing nickel concen-tration. This can be expected in view of the fact that theionic radius of 0.72 A for Ni + ions is smaller than 0.76A for Fe + ions. On the other hand, analysis of the pat-terns for Fe7Ses shows that the specimen has the hexago-nal 3c structure with the following unit-cell constants:3 =7.233+0.005 A and C=17.55+0.01 A which areclose to those of Okazaki and Hirakawa' but somewhatsmaHer than those of Parise et al.

Figure 2 shows some of the Mossbauer spectra of(Fei „Ni„)7Se8 measured at various absorber tempera-tures. Using a least-squares computer program, four setsof six Lorentzian lines corresponding to the A „Az, B,and C sites were 6tted to the Mossbauer spectra belowNeel temperature under the mell-known restraints,which are valid when the quadrupole interaction is muchweaker than the magnetic hyperfine interaction.

Figure 3 shows the temperature dependence of thequadrupole shift defined by b,E& = ( V6 —V, +Vi —Vz)/4, where V; represents the position of the ithabsorption line. The quadrupole shift value for

-10 -8 -6 -4 -2 0 2

VELOL'ITY (mm/s)

I

10

FKJ. 2. Mossbauer spectra of (Feoo98Nioo2)7Se8 at varioustemperatures.

(Fe0 98Ni00&)7Se8 jumps abruptly at 123+1 K correspond-ing to the spin rotation from lying in the (001) planeabove the transition to pointing along the [001] directionbelow, which was observed for Fe7Se8. ' However, it isnoteworthy that the spin rotation for (Fe, «Ni„)7Sesproceeds abruptly while that for Fe7Se8 with the sametriclinic 4c structure proceeds gradually over a widerange of temperatures exceeding 200 K. It is also foundthat the spin-rotation temperature for (Fei Ni )&Se8 de-creases slowly with increasing nickel concentration to be-come 121+1 K for (Feo.92Ni0. 08)7Se8 ~

Figure 4 shows the temperature dependence of magnet-ic hyperfine fields for (Fe0 98Ni0 02)7Ses. There seem to besome bumps near 123 K related to the spin rotation,which are similar to those observed for Fe7Ses with 3cstructure. A, B, and C sites have 18, 14, and 13 interpla-nar superexchange links, and may be expected to havemagnetic hyperfine 6eld magnitudes in the proportion of18:14:13,neglecting all intraplanar and all cation-cation

TABLE I. Lattice constants of triclinic unit cell for (Fe& Ni )7Se8.

0.020.050.08

A (A)

12.62l'12.61112.51

B(A)

7.21357+21 157.1825

C(A)

22.98'22.93'22.912

a(deg)

89.52589.53589.535

P(deg)

89.54589.54'89.545

X(deg)

89.55'89.55589.54'

'Subscript below each number indicates estimated error in the last digit.

3214 OK, BAEK, KIM, AND KIM 48

0.1 —.

A) 0—

TABLE II. Isomer shifts at room temperature relative toiron metal for (Fe& Ni )7Se8.

EE

-0.1—

0.1—

A2 p-

-p1—

1.0—

B 0— ~ a

-1.0—

03—

0.02

0.05

0.08

Sub spectrum

A2BC

A2BC

A2BC

Isomer shift (mm/s)

0.610.580.510.670.610.590.500.690.620.580.510.68

(+0.01)

0.2—

C 0.1—

0—-0.1

0 100 200 300

T (K)

FIG. 3. Temperature dependence of the quadrupole shiftsAE& for (Feo.98Nio. oz)7Se8.

interactions which are supposed to be much weaker thaninterplanar interactions. ' The three experimental mag-netic hyperfine fields of A, 8, and C sites at 12 K arefound to be 273 (the weighted average value of thehyperfine fields at A

&and A 2 sites), 210, and 195 kOe, re-

spectively. These values are in the ratio of 18.0:13.8:12.9,which is in agreement with the above-mentioned ratio18:14:13.

The isomer shifts at room temperature for(Fe, „Ni )7Ses are listed in Table II. The isomer-shiftvalues of 0.50 to 0.69 mm/s relative to iron metal. areconsistent with irons in either a high-spin Fe + or a high-ly covalent ferrous state. " The magnitudes of the mag-netic hyperfine fields, as shown in Fig. 4, exclude the first

lnf =—3' 4T2 ezT ~ d~1+2k~0" O~ o

where Ez is the recoil energy of Fe for the 14.4 keV

500-

alternative. Selenium is known to form strongly covalentbonds and since the iron ions are bound only to selenium,a large covalency is not unexpected.

Figure 5 shows the Neel temperature as a function ofnickel concentration x. It is noted that the Neel tempera-ture decreases linearly with increasing nickel concentra-tion. This implies that Fe -Se-Ni + superexchange in-teraction in (Fe, Ni„)7Ses is weaker than Fe +-Se-Fe +

superexchange interaction.Figure 6 shows 1nF versus T for (Fep9sNippp)7Se8,

where F stands for the total resonance absorption area ofa Mossbauer spectrum at T. F is proportional to therecoil-free fraction f. The Debye model gives the follow-ing expression' for the natural logarithm of f:

300-4QO-

200=-A~A~

BC

300—

l

100l

200 300

I

0.0 2 0.0 41

0 06I

o-o 8

T (K) Nickel Concentrat ion x

FIG. 4. Temperature dependence of the magnetic hyperfinefields H for (Feo 98Nio o2)7Se8.

FICx. 5. Neel temperature as a function of nickel concentra-tion x for (Fe& Ni„)7Se8.

48 MOSSBAUER STUDY OF (Fe, „Ni„)7Se, 3215

250 I

0-CO

—10

-2.0-

2300*

210

0I

4

T' ( )OOOO K )

10 0I I I I I I

0.02 0.04 0 06 0.08Nicke I Concentration x

FIG. 6. Natural logarithm of the resonance absorption areaI' vs T for (Feo.9sNi0. 02)vSe8.

FIG. 7. Debye temperature 0 as a function of nickel concen-tration x for (Fe& „Ni„)7Se8.

gamma ray. 0 and k& represent the Debye temperatureand Boltztnann constant, respectively. Equation (I) witha proper additive constant was fitted to the data in Fig. 6,using a least-squares computer program to get the Debyetemperature O. Figure 7 shows the Debye temperatureas a function of the nickel concentration x, for(Fe, „Ni„)7Ses. It can be seen in Fig. 7 that the Debyetemperature decreases with increasing nickel concentra-

tion x. Ni ions replacing Fe ions seem to weaken the in-teratomic binding force between Fe and Se ions.

ACKNOWLEDGMENTS

This work was supported by the Basic ScienceResearch Institute Program of the Ministry of Education,the Republic of Korea, 1993, and by the Maeji Instituteof Academic Research, Yonsei University.

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9L. M. Levinson and D. Treves, J. Phys. Chem. Solids 29, 2227(1968).J. B. Goodenough, Magnetism and Chemical Bond (Intersci-ence, New York, 1963).

~~R. Ingalls, F. van der Woude, and G. A. Sawatzky, inMossbauer Isomer Shifts, edited by G. K. Shenoy and F. E.Wagner (North-Holland, Amsterdam, 1978).R. L. Mossbauer and W. H. Wiedermann, Z. Phys. 159, 33{1960);B. Kaufmann and H. J. Lipkin, Ann. Phys. (N.Y.) 18,294 (1962).