Mössbauer studies of NaFes2 : magnetic hyperfine fields and covalency in MFeS2 compounds (m=Na, k, Rb, Cs)

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Mössbauer studies of NaFes2 : magnetic hyperfine fields and covalency in MFeS2 compounds (m=Na, k, Rb, Cs)

C.A. Taft

To cite this version:

C.A. Taft. Mössbauer studies of NaFes2 : magnetic hyperfine fields and covalency in MFeS2 compounds (m=Na, k, Rb, Cs). Journal de Physique, 1977, 38 (9), pp.1161-1162.

�10.1051/jphys:019770038090116100�. �jpa-00208682�

1161

MÖSSBAUER STUDIES OF NaFeS2 : MAGNETIC HYPERFINE FIELDS AND COVALENCY IN MFeS2 ^COMPOUNDS (M=Na, K, Rb, Cs) (*)

C. A. TAFT

Centro Brasileiro de

Pesquisas Fisicas,

Av. Wenceslau

Braz, 71,

^{Rio de}

Janeiro,

^Brasil

(Reçu

^{le 2 novembre}

1976,

^{révisé le 5 mai}

1977, accepté

^{le 23 mai}

1977)

Résumé. ²⁰¹⁴ L’étude Mössbauer de NaFeS2, ^{de la} température ambiante à celle de l’hélium liquide,

révèle la présence ^d’ordre magnétique ^en dessous de 77 K. Le champ hyperfin ^à 4,2 ^{K est} de 270 kOe.

Les paramètres hyperfins du fer dans NaFeS2 indiquent ^la

configuration

^à spin ^{fort de Fe3+}

(6S5/2),

Nous montrons que les valeurs réduites du champ hyperfin ^dans NaFeS2 ^{comme dans les} composés MFeS2 (M=K, Rb, Cs) peuvent ^se corréler à l’aide de notions simples : champ hyperfin ^transféré

par les électrons 4s,

électronégativité

des cations.

Abstract. ²⁰¹⁴ Mössbauer studies of NaFeS2 ^{from room} temperature ^{down to} liquid ^{helium tem-} perature ^{indicate a} magnetic

ordering

below 77 K. The magnitude ^{of the} hyperfine field is 270 kOe at 4.2 K. The measured

hyperfine

parameters characterize iron in NaFeS2 ^to be in the

high

spin ^Fe3+

6S5/2

configuration. ^We show that in NaFeS2 2014 ^as in the other MFeS2 compounds (M=K, Rb, Cs) - the reduced values of hyperfine ^{fields (150-270} kOe) ^can be correlated with simple concepts ^{such as} transferred hyperfine fields via 4s contribution and cationic electronegativity.

LE JOURNAL DE PHYSIQUE ^TOME 38, ^SEPTEMBRE 1977,

Classification Physics ^Abstracts

76.80

1. Introduction. ^- In our

previous

^{work we} report- ed on Mossbauer studies of

covalency

effects in alcali-dithioferrates

AFeS2 (A=K, Rb, Cs).

^These

effects cause a

large

reduction of the observed

magnetic hyperfine

fields from the

expected

ionic value

[1, 2].

In this work I report ^on Mossbauer studies of

NaFeS2

^{in the}

temperature

range (300-4.4

K)

^indi-

cating

^{also a}

large

deviation of the measured

magnetic

field from the ionic value.

It is

interesting

^{to note that the}

magnitude

^{of the}

magnetic

^{field in the} alcali-dithioferrate series can be correlated with a

covalency

scale based on 4s contri- bution and cationic

electronegativity.

2.

Experimental.

^{- The}

samples

^were

investigated using

^a 1024 multichannel

analyser, operating

^{in the}

multiscaler mode to a

velocity

transducer of the Kankeleit

type.

^{The source was a 10 mCi of}

" Co

in a Pd matrix

having

^{a width of 0.28}

mm/s

^measured

against

^a natural iron absorber. The _{gamma rays} were

detected with a thin Nal

(Tl)

scintillator mounted in a

photo-multiplier

^{tube. The}

liquid

^{helium measu-}

rement was taken with both source and absorber in

liquid

helium. The low temperature measurements

were recorded

using

^{an Elscint} cryostat ^with

tempe-

(*) ^This paper ^was presented ^at the International Conference

on the Applications of the Mossbauer Effect, ^Corfu (Greece) ^1976.

rature controller. The

temperature stability

^was

± 2 °C. ^The

samples

^were

prepared by fusing

^iron

powder

with sodium carbonate and

sulphur

^and

leaching

^{the cold}

product

^{in water.}

3. Results and discussion. ^-

Figure

^{1 shows the}

transmission

spectra

^of

NaFeS2

^{at several} tempera-

tures. From 300 K down. to 77 K a

quadrupole

doublet with a temperature

independent splitting

^of

0.58

mm/s

is observed. The room

temperature

^isomer shift value is 0.36

mm/s (w.r.t. Fe).

^These parameters

are

typical

for iron in the

high spin ^Fe3+ 6S5J2

^confi-

guration [1-4].

^{At 4.4 K the}

spectrum

^was

additionally split by

^a

strong magnetic hyperfine

interaction and the

magnitude

^{of the}

hyperfine

^{field was}

I Hhf

^{= 270 k0152}

( ±

⁵

kOe).

^The

sign

^of

Hhf

^is

assumed to be

negative

^{as this is}

usually

^{the case}

in

Fe3+ high-spin compounds.

Electric field

gradient (EFG)

calculations

[1, 2]

in this series of

compounds

^indicated

negligible

^values

of asymmetry parameter

(q).

^{In the} present ^{case of}

a small

quadrupole

interaction

superimposed

^{on a}

large magnetic hyperfine

interaction in the

absorber,

the observation of an

unchanged quadrupole split- ting (1)

indicates that the main axis of the EFG coin- (’) (= shift of the inner lines pairs ^{versus the outermost lines}

of the six-line pattern). ^,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019770038090116100

1162

FIG. 1. ^- Absorption spectra ^for NaFeS2 ^{at various} temperatures.

cides with the axis of the

magnetic hyperfine

^field.

In this case a

negative sign

of the EFG can be

simply

derived.

The

magnitude

of the internal field in

NaFeS2 (270 k0e)

which is assumed to be the saturation

field,

is

unusually

^{small for a} ferric ion. There are appa-

rently

considerable

covalency

effects from

neigh- boring spins.

Molecular orbital calculations

suggest

that there should be a linear

relationship

^between

the covalent induced

hyperfine

field and the

charge

transfer to the 4s state.

Assuming

^{such a relation-}

ship

^{we can write}

[1]

where Hionic = - 630 kOe is the ionic field and

H4s

^{= +} 1 348 kG is the _average

hyperfine

^field

due to one 4s electron and

Q4,

is the number of 4s electrons at the

Fe3 +

ion

originating

^{from the}

covalent

ligands.

^{From the}

Modified

^{WWJ Plot}

[1]

one obtains

Q4s

^{= 0.22.} This estimate

yields Hhf

^{= - 330}

k0e,

^{which is a}

reasonably good

^esti-

mate.

Various attempts ^{have been made to} correlate the extensive Mossbauer data with available

electronega- tivity

^scales

[5].

^{In this} isostructural series

MFeS2 (M=Na, K, Rb, Cs)

in which the cation M is the

only varying element,

it appears reasonable to

attempt

such a correlation. The

increasing electronegativity

of the cation will decrease the 4s electron

density

^at

the Fe nucleus and hence reduce the

proposed

^trans-

ferred

hyperfine

^{fields. One would thus}

expect

^the

compounds

^{with the}

larger electronegativity

^values

to be more

ionic, i.e.,

^have

larger

^{values of}

hyperfine

fields. The fields measured at 4.4 K are considered

as saturation values

(for

^{Na and}

CsFeS2)

^{and as}

negative.

^In

figure 2,

^{I have}

plotted

^{the measured}

FIG. 2. ^- Magnetic hyperfine ^{fields in} MFeS2 (M ⁼ K, Rb, Cs, Na)

vs cationic (M) electronegativity.

values of

magnetic

^{field vs cationic}

electronegati- vity [6, 7],

^which

suggests

that the cationic electrone-

gativity

^is

effectively

^a

good covalency parameter

ⁱⁿ this series of

compounds.

Acknowledgments.

^{- I am}

grateful

^{to OEA and}

CNPq (Brasil)

for financial

support.

References

[1] TAFT, ^C. A., RAJ, D. and DANON, J., J. Physique Colloq. ³⁵ (1974) ^{C 6-241.}

[2] TAFT, ^C. A., RAJ, ^{D. and} DANON, J., ^J. Phys. & Chem. Solids 36

(1975)283.

[3] TAFT, C. A., RAJ, ^{D. and} DANON, J., Phys. Stat. Sol. 64 (1974)

111.

[4] TAFT, ^C. A., Proceedings of the International Conference on

Mössbauer Spectroscopy, Cracow, ^Poland (1975).

[5] GONSER, U., Ed. Mössbauer Spectroscopy (Springer-Verlag,

Berlin Heidelberg, ^New York) ^1967.

[6] PRITCHARD, ^{H. O. and} SKINNER, ^H. H., Chem. Rev. 55 (1955)

745.

[7] PAULING, L., The Nature of the Chemical Bond (Cornell ^Univ.

Press, 3rd Ed., New York) ^1960.