A neutron diffraction study of Fe7Se8

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A neutron diffraction study of Fe7Se8

A.F. Andresen, J. Leciejewicz

To cite this version:

A.F. Andresen, J. Leciejewicz. A neutron diffraction study of Fe7Se8. Journal de Physique, 1964, 25

(5), pp.574-578. �10.1051/jphys:01964002505057401�. �jpa-00205832�

meeting ^{of the} Physical Society ^of Japan, 1957, p. ^76.

[9] ^HIRONE (T.) and ADACHI (K.), ^J. Phys. Soc., Japan, 1957, 12, ^156.

[10] DOROSHENKO (A. V.), ^KLYUSHIN (V. V.), ^LOSHMANOV (A. A.) and GOMAN’KOV (V. I.), ^The Physics of

Metals and Metallography, 1961, 12, ^119.

[11] ^KUNITOMI (N.), ^HAMAGUCHI (Y.) and ANZAI (S.),

J. Phys. Soc., Japan, 1963, 18, 744.

[12] ^FURBERG (S.), ^{Acta Chem.} Scand., 1953, 7, 693.

[13] ^KUNITOMI (N.), ^KOMURA (S.) ^{and ONDA} (T.), ^JAERI-

Memo No. 1157, 1963, ⁱⁿ Japanese.

[14] ^HIRONE (T.), ^MAEDA (S.) ^{and TSUYA} (N.), ^{Rev. Sc.}

Instr., 1954, 25, ^516.

[15] ^KUNITOMI (N.), ^HAMAGUCHI (Y.), ^SAKAMOTO (M.)

and KOMURA (S.), ^J. Phys. Soc., Japan, 1962, ¹⁷ (Supplement B-II) ^354.

[96] ^KELLEY (K. K.), J. Amer. Chem. Soc., 1939, 61, 203.

[17] ^HASTINGS (J. M.), ^ELLIOTT (N.) and CORLISS (L. M.), Phys Rev., 1959, 115, 13.

[18] ^WATSON (R. E.) and FREEMAN (A. J.). ^Acta Cryst., 1961, 14, 27.

[19] ^PEARSON (W. B.), ^{Canad. J.} Phys., 1957, 35, 886.

[20] ^TAKEI (W. J.), ^Cox (D. E.) ^{and SHIRANE} (G.), Phys.

Rev., 1963, 129, ^2008.

[21] KOMATSUBARA (T.), ^MURAKAMI (M.) et HIRAHARA (E.), J. Phys. ^Soc. Japan, 1963, 18, ^1106.

A NEUTRON DIFFRACTION STUDY OF Fe7Se8 By ^A. F. ANDRESEN and J. LECIEJEWICZ (1)

Institutt for Atomenergi, Kjeller, Norway.

Résumé. 2014 Une étude de Fe7Se8 ^{a été} entreprise afin de déterminer l’ordre magnétique ^{et les}

transitions de phase ^{dans ce} composé. ^{Sur un} échantillon refroidi lentement depuis ⁶⁰⁰ °C, ^une

surstructure correspondant ^{à la} structure 3c trouvée par Okazaki [4] ^{a été mise en} évidence. Celle- ci est obtenue à partir ^{de la maille de NiAs en} doublant l’axe a et en triplant ^{l’axe c. Le} diagramme neutronique ^{a été fait} à différentes températures ^{entre 4,2 °K et 493} °K. Les dimensions de la maille sont :

Les groupes d’espace C46 ^{2014P62 et} C562014 ^{P64 proposés} par Okazaki ^et Hirakawa [3] ^ne sont pas

applicables. Un groupe possible ^serait P31, ^{avec des lacunes en} (1/2, 0), (1/2, ^{0, 1/3),} (0, 1/2, 2/3).

Les moments magnétiques ^sont alignés ferromagnétiquement ^dans chaque plan (0001) ^et antiferromagnétiquement entre deux plans ^{voisins. A basse} température, la direction des moments est perpendiculaire ^{à l’axe} c, mais entre 120 °K et 130 °K, ^{les moments tournent en se} rapprochant

de l’axe c. L’intensité de la réflexion (0003) indique ^une inclinaison des spins par rapport ^{à cet axe.}

Aucune indication n’a pu être trouvée ^sur le changement progressif ^{de l’axe de} facile aimantation

qui ^avait été mesuré par Hirakawa [5].

Abstract.

An investigation ^of Fe7Se8 ^was undertaken to study ^the magnetic ordering ^and phase transitions in this compound. ^{On a} sample slowly cooled from 600 °C a superstructure ^was

identified corresponding to the 3c structure found by ^Okazaki [4]. This is derived from the small

Ni-As-type ^cell by doubling ^{of the a-axis and} tripling ^of the c-axis. Neutron diffraction diagrams

were made at several temperatures between 4.2 °K and 493 °K. The unit cell dimensions found

were :

The space groups C46

P62 ^and C56

^P64 proposed by ^{Okazaki and Hirakawa} [3] ^{are not} appli-

cable. A possible space group is P31, ^{with vacancies at} (1/2, 1/2, 0) (1/2, 0, 1/3) (0, 1/2, 2/3).

The magnetic ^{moments are} ferromagnetically aligned ^{within each} (0001) ^sheet and antiferro-

magnetically aligned ⁱⁿ neighbouring ^{sheets. At low} temperatures the moment direction _{is per-}

pendicular ^{to the c-axis, but} between 120 °K and 130 °K the moments turn to a direction closer to

the ^{c-axis. The} _intensity ^{of the} (0003) reflection indicates a tilt of the spin ^with respect ^{to this}

axis. No indication has been found of the gradual change ^{in the axis} of easy magnetization ^which

was measured by ^Hirakawa [5] ^{on a} torque magnetometer.

LE JOURNAL DE PHYSIQUE ^TOME 25, ^MAI 1964,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01964002505057401

Introduction.

It has been shown by Harald- sen and Gr0l1 voId [1] that the stoichiometric compo- sition FeSe does not exist at low temperatures.

Hirone and Chiba [2] found that by annealing ^a sample ^{of this} composition ^{at a} temperature ^below 350 °C ^a mixture of two phases, ^{« and} ~, ^was

formed. The a.-phase, ^{with a} PbO-type ^structure,

exists up ^to 49 % ^{Se, and the} P-phase ^{with a} NiAs-type ^structure exists down to 53.5 % ^Se.

The last composition corresponds approximately

to the compound Fe7Se8. ^The deviation from

stoichiometry ^{is connected with the} formation of metal vacancies, and Okazaki and Hirakawa [3]

and Okazaki [4] have found that at low tempera-

tures ordering of these leads to the formation of several superstructures depending ^{on the} annealing temperature.

Fe7Se8 ^{shows some} interesting magnetic pro-

perties. Ferrimagnetism ^is associated with the

ordering ^{of the vacancies and} according ^{to Hirone}

and Chiba [2] ^{the Curie} point ^{is 174 OC. While at}

room temperature ^the easy ^{axis of} magnetization

is perpendicular ^{to the} c-axis, ^Hirakawa [5] ^finds

that this changes ^{into the} c-direction below about -120 °C. The change ^is spread ^{over a} large temperature interval. Such a gradual change

was also observed by ^Pauthenet [6] ⁱⁿ pyrrhotite, Fe7S., ^{and Néel} [7] suggested ^{this to be due to}

local inhomogeneities ^{in an} impure specimen.

Hirakawa has, however, repeated ^{his measure-}

ments on a high purity sample ^{and still finds a} gradual transition.

The nature of this transition could easily ^be

checked by ^neutron diffraction. As part ^{of a} series of investigations ^{on the} crystallographic ^and magnetic ordering ⁱⁿ NiAs-type compounds ^it

was therefore found worth-while to investigate

also this compound.

Preparation ^and X-ray examination.

Calcu- lated amounts of iron and selenium of spectros- copic purity (Johnson, Matthey ^et Co) ^{were sealed}

in double-walled evacuated quartz tubes. This

was necessary since the inner tube would often crack on cooling. ^{The tubes were fired at 1050 °C}

for 6 hours and then kept ^{at 8000 for 3} days. ^The sample ^{was then crushed} and examined by X-rays.

As it was still found to contain unreacted iron, ^it

was again ^{sealed in vacuum and heated to 900 aC}

for 2 days. Finally ^{it was} kept ^{at 600 °C for one}

week and then cooled slowly ^{down to 200 °C in}

the course of 2 days.

The sample ^{was found to be well} crystallized

and the X-ray pattern could be indexed on the 3c-cell of Okazaki [4]. This cell is derived from the small NiAs-type ^cell by doubling of the a-axis and tripling of the c-axis (fag. ^{1). The unit}

cell dimensions at room temperature ^are

a ~ 7.2~ A and c ⁼ 17.60 A. The appearance

FIG. 1.

Unit cell of Fe,Se,. ^{Se-atoms not shown.}

of a (0003) reflection indicates ordering ^{of the}

vacancies on every second Fe-layer ^{as found} by

Okazaki and Hirakawa [3]. However, ^the space groups P6, ^and P~4 given by ^{them can not accom-}

modate the Se-atoms and possible space groups

were found to be P3, ^and .P32.

TABLE 1

ATOMIC POSITIONS IN Fe7Ses

SPACE GROUP C3

^P31

Using P31, ^and assuming tlm vacancies to occupy the positions (1, 1, 0 ; 1, 0, 1; 0, 1, ^{2 2 0; 2 ’ 3 " 2} 2) ^{3 the ato-}

mic positions ^{are as} given ^{in Table 1. There are}

2’I Fe-atoms and 24 Se-atoms in the unit cell. For each metal vacancy introduced 2 of the ferrous ions will transfer to ferric ions. One formulae unit can therefore be written L See

where L denotes a vacancy. To obtain equal charge ^on every ^metal layer ^the ferric ions should be accommodated in the same layer ^{as the va-}

cancies. Such an arrangement ^was also found by

Hircne and Chiba [2] ^to explain their observed

magnetic moment. ^This assu.mption will therefore be used in the following calculations.

Neutron diffraction examination.

Neutron diffraction diagrams ^{were run at 4.2} °K, ⁹⁰ °K,

293 °K, and 493 using ^{neutrons of} wavelength

1.146 A from the reactor JEEP I. The low tem-

perature diagrams ^were obtained with a Hoffman

FIG. ^2.

Neutron diffraction diagrams ^of Fe7Se8 ^at

4.2 oK, ²⁹³ OK, ^{and 493} OK, instrumental background

subtracted. X= t .146 A,

liquid helium research dewar and the high tempe-

rature diagram using ^a quartz ^{furnace. Room} temperature diagrams ^{were run in both c ases to} put the intensities ^{on a} comparable ^{scale. The} diagrams ^{run at 4.2} oK, ²⁹³ OK, ^{and 493 OK are}

shown in figure ^2.

A change ^{in the} unit cell dimensions is imme-

diately apparent ^{and the} changes ^{above room} temperature ^{are in} agreement with those observed

by ^Okazaki [4]. ^{The a-axis decreases from 7.33} A

at 493 OK to 7.21 A at 293 oK and continues to decrease until 7 .17 A at liquid ^helium temperature.

The c-axis increases from 17.43 Å at 493 ° K to 17.60 A at 293 -K but then decreases again ^to 17.46 A at 4.2 OK.

At 493 OK no magnetic contribution is found and the observed intensities agree well with those calculated for the atomic arrangement ^{of Table 1.}

In all the diagrams ^some intensity disagreement,

in particular ^{a rather} strong (1125) super-reflection

seemed to require ^atomic displacements. Impro-

vement was brought ^about by displacing ^{in the}

c-direction the Fe-atoms adjacent ^{to a} vacancy

(see ^Table 1).

Suitable temperature ^{factors for 493} oK,

BFe = 2 . 7 A2, ^{Bse = 1. 9} A2, ^{and for room}

temperature, ^{Bye = 1.6} A2 and Bse = 1.1 A2,

were calculated using ^a Debye temperature ^of

194 This is lower than the Debye tempera-

ture 225 ~K derivable from specific ^{heat data} (Gronvold [8]).

In the rom ternperature diagram the appea-

rance of a strong (0003) reflection indicates a

ferrimagnetic ordering ^with spins parallel ^{in each} Fe-layer ^and antiparallel ⁱⁿ neighbouring layers,

the spin ^direction being perpendicular ^{to the}

c-axis. By normalizing ^{the observed} intensities

matching reflections with no magnetic ^contribu- tion, ^the magnetic intensities were found to be smaller than those corresponding ^{to 4 and 5} unpaired electrons in Fe2+ and Fe3+ considering spin only contributions. Assuming ^{the same de-}

crease in moments for both ions magnetic ^moments

of 3.6 _~B and 4.5 _yB were deduced.

At liquid ^helium temperature ^the (0003) peak

has decreased considerably, ^{which can be ascribed}

to a turning ^{of the} magnetic ^{moments in the}

direction of the c-axis. The fact that the inten-

sity ^{has not decreased to} its value in the para-

magnetic ^{state indicates that the moments are not} pointing ^{in the} c-axis direction but have stopped

in an intermediate position. Again normalizing

the observed intensities for the purely ^nuclear

reflections and assuming ^{the same} values for the moments as at room temperature ^{a tilt of 180 with} respect ^to the c-axis was deduced. It is impossible

to say from ^our low intensity powder ^{data whether}

this is connected with ^a spiral spin arrangement ^or

just ^{a canted} arrangement. ^In the calculations

temperature factors with BF,.~ z:-- 0.7 -I 2 and B’6e ^{= 0.5} A 2were used. The magneto ^form-

f actor used was that of Nathans et al. [9].

The magnetic transitions.

In order to inves-

tigate ^more closely ^the magnetic transitions the

temperature dependence ^{of the} (0003) ^reflection

was measured from 4.2 ~K to 510 OK. Below

room temperature ^{was used a} liquid nitrogen cryostat ^{with a} heating ^coil surrounding ^the sample,

and above ^room temperature ^a quartz ^furnace.

During measurements the temperature ^{was con-} trolled to £ ^{20. The observed} integrated ^inten- sity ^is plotted ⁱⁿ figure ^{3 as a function of} tempera-

FIG. 3.

Integrated intensity ^{of the} (0003) reflection ^{as a} function of temperature, fully ^{drawn curve} representing B2(2).

ture. As can be seen the change ^{in the} easy axis of magnetization ^{is rather} abrupt ^and taking place

in the temperature ^interval 120 oK - 130 oK.

This is contrary ^{to the} results obtained by ^Hira-

kawa with a torque magnetometer. ^No hyste-

resis in this transition was found.

The decrease in intensity up ^to the Curie point

could be well approximated by the square of the Brillouin function for S ⁼ 2. This is shown as the

fully ^{drawn curve in} figure ^{3. The Curie} point according ^{to this curve is 210 ~C which is} higher

than the Curie point previously ^obtained by suscep-

tibility measurements [2].

Conclusions.

According ^{to our neutron dif-}

fraction measurements the change ^{in the} easy axis of magnetization is rather abrupt, ^{and is not} spread ^{out over a} large temperature ^{interval as} reported by ^Hirakawa [5]. ~ ^The transition tempe-

rature 120 130 OK is lower than reported by him, ^{but the Curie} point ^{210 ~C is} higher ^than previously reported [2]. ^{The moments do not}

turn to point along ^the hexagonal axis, ^but stop

in an intermediate position approximately ¹⁸⁰

from this. Intensity disagreements ^{indicate a} slight displacement in the c-direction of the Fe- atoms adjacent ^{to a} vacancy. A more exact

determination of the atomic displacements ^and

details about the magnetic structure, i.e. whether

a canted spin arrangement ^{or a} spiral, will require single crystal ^data.

Discussion

Dr WEIK. - Avez-vous mesuré la densité de la substance et si oui, ^{est-elle en accord avec la struc-}

ture lacunaire suggérée ?

Dr ANDRESEN. - Nous n’avons pas fait de telles mesures, mais il ^a été établi par d’autres recherches que 1’ecart a la composition ^stoechio- metrique ^est lie a la formation de lacunes.

Pr BERTAUT. - Que pensez-vous d’un d6sordre de spins comme cause possible ^de la diminution du spin ^{de Fe ? Un} tel d6sordre peut 6tre eaus6 tr6s indirectement _par un certain d6sordre des lacunes.

Dr ANDRESEN. - C’est possible.

Pr WALLACE Y-a-t-il un effet d’ordre-d6s- ordre _{pour les} ions Fe3’, Fe2+, ^dans

semblable a celui dans FegO 4 ^{a basse} temp6rature ?

Dr ANDRESEN. - Dans notre cas, nous avons une couche occup6e par ^les ions Fe2+ seuls et 1’autre _par deux ions Fe3+, ^{un ion Fe2+ et une}

lacune. On ne peut ^{donc avoir une} transition d’or- dre-d6sordre _que pour les trois derniers ions. Cet effet est trop ^{f aible} pour 6tre observe.

Pr UEDA. - Ma question ^se rapporte ^{aux difl/-}

rences entre les donn6es de rayons ^X et neutrons.

Est-ce que la variation quelque ^peu 6trange ^le long ^{de I’axe c a 293 °K est 1]6e a la} distribution des lacunes.

Avez-vous fait une analyse chimique apr6s ^{I’ [la-}

boration du cristal ?

Dr ANDRESEN. - 11 n’y ^{a aucun d6saccord entre}

les donn6es de rayons ^X et neutrons. Je suppose que la raison _pour laquelle ^{Okazaki et Hirakawa}

donnent pour groupe d’espace P62 ^est qu’ils ^n’ont

pas ^tenu compte ^{des atomes de Se.}

11 n’y ^{a aucun} changement dans la distribution des lacunes pendant ^{les mesures} reportees ^ici.

Nous n’avons _pas fait d’analyse chimique.

La composition ^des 6chantillons est voisine de

53,5 % ^Se. Pour des concentrations ini£rieures en

Se, ^la phase ^« serait visible et avec des concentra-

tions superieures , ^une deformation monoclinique

apparaîtrait.

REFERENCES [1] ^HARALDSEN (H.) ^{and GRØNVOLD} (F.), ^Tidsskr. Kjemi,

Bergvesen, Met., 1944, 10, 98.

[2] ^HIRONE (T.) and CHIBA (S.), ^J. Phys. Soc., Japan, 1956, 11, ^666.

[3] ^OKAZAKI (A.) and HIRAKAWA (K.), ^J. Phys. Soc., Japan,1956, 11, 930.

[4] ^OKAZAKI (A.), ^J. Phys. Soc., Japan, 1961, 16, ^1162.

[5] ^HIRAKAWA (K.), ^J. Phys. Soc., Japan, 1957, 12, ^929.

[6] ^PAUTHENET (R.), C. R. Acad. Sc., 1952, 234, 2261.

[7] ^NÉEL (L.), ^{Rev. Mod.} Physics, 1953, 25, 58.

[8] ^GRØNVOLD (F.), Private communication.

[9] ^NATHANS (R.), ^SHULL (C. G.), ^SHIRANE (G.) ^and

ANDRESEN (A.), ^J. Phys. ^Chem. Solids, 1959, 10, 138.

L’INTERACTION MAGNÉTIQUE ^{DANS LES} STRUCTURES DE K2NiF4

Par E. LEGRAND et M. VERSCHUEREN,

Département Physique, État Solide, C. E. N.-S. C. K., Mol, Belgique.

Résumé. 2014 Dans la structure quadratique ^{à couches} K2NiF4, il y ^a de très fortes interactions entre atomes appartenant ^{au même} plan, alors que l’interaction entre ^{deux plans est bien plus}

faible. La diffraction neutronique démontre l’existence d’une interaction antiferromagnétique

dans les plans, la direction des moments étant celle de l’axe c. Les composés La0,5Sr1,5MnO4 ^et La2NiO4 ^{ont la même} structure cristalline. Aux basses températures, ^leur susceptibilité magnétique

dévie de la loi de Curie-Weiss. Néanmoins, ^{aucun ordre} magnétique ^{n’a été} trouvé par diffraction

neutronique ^{du moins} jusqu’aux températures ^de l’hydrogène liquide. ^{Des résultats} préliminaires

de Cs2MnCl4 ^sont présentés.

Abstract.

In the tetragonal layer ^structure K2NiF4, strong magnetic interactions between atoms belonging ^{to the same} plane ^are present, ^{while the} interaction between two layers ^{is much}

weaker. Neutron diffraction work revealed an antiferromagnetic interaction in the planes, ^the

direction of the moments being ^{that of the c-axis.}

The compounds La0,5Sr1,5MnO4 ^and La2NiO4 ^{have the same} crystal structure. At low tempe-

ratures their magnetic susceptibility also deviates from the Curie-Weiss behaviour. Nevertheless

no magnetic long range order has been found with neutron diffraction, ^{at least at} temperatures

down to that of liquid hydrogen. Preliminary ^{results on} Cs2MnCl4 ^are presented.

LE JOURNAL DE PHYSIQUE TOME 25, ^MAI 1964,

1. Introduction.

Les propriétés magnétiques

des composés ^{à structure} perovskite ^{ont été étu-}

diées intensivement par diffraction ^{des neutrons}

[1] [2]. Il existe une structure fort similaire à la structure perovskite, que ^nous appellerons ^struc-

ture K2NiF 4. ^{Cette structure} quadratique, ^diffère

de la structure cubique perovskite KNiF, ^{en ce}

que les couches d’atomes de nickel, perpendi-

culaires à l’axe _{c, y} sont séparées par deux couches d’atomes fluor-potassium ~ fig. 1). ^Les paramètres

de la maille sont : c ⁼ 13,07 A ^{et a =} 4,00 A [3].

Il était intéressant de voir comment les inte- ractions magnétiques, qui existent dans le KNiF 3

sont modifiées quand ^{on passe à la structure}

modifiée K2NiF4, ^et quelle ^{est la structure} magné- tique ordonnée dans des composées ^{de ce} type.

2. Expériences.

^En premier ^{lieu on a fait des}

mesures de diffraction de neutrons sur le K2NiF4

lui-même [4]. ^Le spectre ^de diffraction d’une poudre

de ce composé ^montre des raies supplémentaires

de surstructure, ^aux températures ^{d’azote et}

d’hydrogène-liquide. ^{Il est} possible ^{d’indicer ces}

raies en supposant ^{une structure} antiferromagné- tique ^{à deux} sous-réseaux quadratiques, ^{dans les} plans ^{de base 2). En} plus ^{on a fait une esti-}

mation de la température de transition en mesu-

rant la variation du maximum de la raie magné- tique (100) ^{en fonction de la} température. ^La température de transition se trouve vers 190 OK.

De l’intensité des raies magnétiques, ^on déduit que les moments magnétiques ^sont orientés suivant l’axe c.

Des mesures de susceptibilités ^ont également ^été

effectuées sur une poudre ^{et sur un} monocristal [5].

L’inverse de la susceptibilité ^en fonction de la

température ^{montre un} large ^{minimum vers}

200 OK. Les mesures sur le monocristal montrent le même minimum, ^{mais en} plus ^{on trouve} que ^la

susceptibilité parallèle ^{à l’axe c, et} celle perpen- diculaire à cet axe diffèrent à partir ^d’environ

110 °K (fig. 3), ^ce qui ^{montre un eff et d’aniso-} tropie.

D’autres composés qui ^{ont la structure} K2NiF4