STOICHIOMETRY DEPENDENCE OF ZEEMAN HYPERFINE INTERACTIONS IN Fe1+x Ti2(1+x)S4 (x = 0, 0.030, 0.052)

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STOICHIOMETRY DEPENDENCE OF ZEEMAN

HYPERFINE INTERACTIONS IN Fe1+x Ti2(1+x)S4

(x = 0, 0.030, 0.052)

G. Fatseas, J. Dormann, M. Danot

To cite this version:

JOURNAL DE PHYSIQUE Colloque C6, suppldment au no 12, Tome 37, DPcembre 1976, page C6-579

STOICHIOMETRY DEPENDENCE OF ZEEMAN HYPERFINE

INTERACTIONS IN

Fe,

+,

Ti,,,

+,,S,

(x

=

0, 0.030,

0.052)

G. A. FATSEAS and J. L. DORMANN

Laboratoire de Magnbtisme, C. N. R. S. 1, place Aristide-Briand, 92190 Meudon, France and

M. DANOT

Laboratoire de Chimie MinCrale A ERA no 472, U. E. R. de Chimie B. P. 1044, 44037 Nantes Cedex, France

RBsum6.

-

Le rale de la mkthode de prkparation du traitement thermique et de la non stoechio- mktrie sur l'ordre cristallographique et les propriktks magnktiques du chalcogknure ternaire FeTizS4, a kt6 le sujet de contradictions ces dernieres anntes.

A6n de contribuer

A

l'ktude de ces questions, nous avons pensk voir comment les parametres mentionnks ci-dessus se reflktent dans les spectres Mossbauer.

Les rksultats obtenus entre 4,2 K et 300 K sont discutks par comparaison aux rksultats cristallo- graphiques et magnktiques antkrieurs.

Abstract. -The role of the method of preparation, of the heat treatment and of the non stoichio- metry, on the crystallographic and magnetic ordering of the ternay chalcogenide FeTizS4 is not well inderstood to date.

In order to clarify these questions, or, at least, to give supplementary data for future discussions, we have looked how the parameters mentioned above are reflected in the Mossbauer spectra. The results obtained between 4.2 K and 300 K showed marked differences between stoichio- metric and non stoichiometric samples, and small or no differences between the different heat treatments and the different methods of preparation, respectively, and are discussed in terms of crystallographic and magnetic ordering.

I. Introduction.

-

It is known that 3d-transition metal atoms can occupy the octahedral interstices, between neighbouring sulfur layers, of TiS,, to form ternary compounds M,TiS, with NiAs-like type structures [I-81. Among them, the FeTi2S4 was found to have the monoclinic Cr,S4-type structure

[l, 3,4] and it has been the subject of crystallographic, magnetic and electrical-resistivity studies.

We summerise here some of these results, with particular emphasis on the conflicting ones which need further discussion. These results are :

1.1 CRYSTALLOGRAPHIC ORDER.

-

Although Hahn et al. [9] found, in their pioneer work, that FeTi,S4 is trigonally disordered, more recent studies [l-5, 101 have shown that it is monoclinic with ordered arrange- ments of vacancies (space group 12/m). The vacancy ordering seems, however, to be well established for slowly cooled samples only, and not for quenched materials for which conflicting results were reported. Thus, Plovnic etal. [I], studying samplesprepared by the direct reaction Fe

+

Ti

+

S and quenched at 1 300 OC, concluded that it is not possible to disorder the vacancies thermally, the disorder taking place only by loss of stoichiometry. This result is in conflict with those of Muranaka [I 11 who found a gradual increasing of

vacancy-disordering with increasing quenching tempe- rature up to 450 OC, this, last temperature being the order-disorder transition point.

Another question is the mode of distribution of Fe and Ti atoms in the cation-sites.

Both Hahn [9] and Plovnic et al. [I, 31 have consider- ed that the Fe and Ti atoms are ordered in alternate metal layers ; that is, full titanium layers alternating with layers containing the Fe atoms and vacancies (Fe

+

O)Ti,S4. This fact, generally considered as obvious, is very difficult to prove as X-ray can not see wether titanium and iron exchange some of their positions or not. Furthermore, Takahashi and Yamada [4] have reported that the existence of the monoclinic superstructure cell does not necessarily imply the ordering of Fe and Ti atoms in alternate layers. The occurence of a superstructure, only implies that vacancies are ordered. Iron and titanium may be located, each, in every second metallic layer or not, this fact not affecting the X-ray measurements for which Fe and Ti are nearly indistinguishable. These two situations are however different from a Mossbauer spectroscopic point of view. In the case of an ideal order, only one Fe-site will be observable (see Fig. in ref. [l] or [3]). For a random distribution of Fe and Ti in the cation sites, two, at least, iron sites could

C6-580 G. A. FATSEAS, J. L. DORMANN AND M. DANOT exist corresponding to the two crystallographic cation

sites of the monoclinic (12/m) structure 141. 1.2 MAGNETIC PROPERTIES. - Conflicting results have also been obtained for the magnetic properties of FeTi,S4 from the interpretation of magnetic- susceptibility and magnetization curves.

All authors [I, 2,4, 5, 111 have obtained Curie-Weiss law behaviour above approximately 140 K for the x-'(T) curves ; but they reported different results below the magnetic ordering temperature. The various non-linearities or anomalies observed below 140 K lead the authors to conflicting interpretations : parti- cularly, Morris et al. [2] and Muranaka [ I l l have

reported quite similar results of a mixed antiferro- magnetic and ferromagnetic behaviour, the importance of each component depending on the degree of vacancy- ordering. More precisely : The ordered part of the material would be antiferromagnetic with paramagne- tic N6el temperatures TN = 132 K [2], 138 K [11] or about 150 K [3]. The ferromagnetic component, with a Curie temperature

--

60 K [2], would be due to the existence of small clustures of iron, arising as a result of crystallographic disorder.

This result is in conflict with those of Takahashi and Yamada [4] who reported a ferromagnetic beha- viour with a Curie temperature

--

11 1 K for ordered monoclinic FeTi,S, samples and a short range magne- tic ordering near the Curie point. A ferromagnetic behaviour was dso found by recent magnetization measurements [12-131 (T,

--

125 K) and a neutron diffraction study [14].

At the present point of understanding of these materials it is important, in our opinion, to obtain supplementary informations. We report here Moss- bauer spectroscopy results, between 4.2 K and 300 K on both stoichiometric (x = 0) and non stoichio- metric

(x

= 0.030 and 0.052) compounds, all of them

prepared by the two methods of preparation reported in the litterature and submitted to two different heat treatments namely quenching and slow cooling.

2. Experimentals. - 2.1 SPECTROMETER AND FITT-

ING PROCEDURE. - All our spectra were obtained

with a

-

25 mCFe57(Rh) source and a Mossbauer spectrometer using a symmetrical saw-tooth vibrator. All samples contain 10 mg natural iron per cm2 corresponding to a thin Mossbauer absorber. The least-squares fitting routine uses Lorentzian line-shapes with equal linewidth for all peaks and all sites and equal Mossbauer absorption coefficient for dl sites. All fits were obtained with the minimum possible number of sites compatible with a good reproduction. 2.2 SAMPLE PREPARATION. - Both stoichiometric (x = 0) and not stoichiometric ( x = 0.030 and 0.052) polycrystalline compounds were prepared by two methods :

1) From direct reaction of high purity iron and

TiS, [4, 51. After heating the mixture at 550 OC for five days in evacuated pyrex tubes, the material was ground into fine powder and heated again at 900 OC in an evacuated silica tube for one week. From this starting compound (I) two groups of samples were obtained as follows :

a) rapid quenching (r. q.) from 900 0C into ice

water,

b) slow cooling (s. c.) samples from 900 OC to 500 OC at 60/h, from 500 OC to 200 OC at 1.5 O/h and €rom 200 0C to 20 0C at 100/h.

2) From direct reaction of iron, titanium and sul- fur [l, 111. The same as above heat treatments have been used respectively for the starting compound and the quenched and slowly cooled materials.

2.3 X-RAY ANALYSIS.

-

All samples in powder

form were checked with a Guinier Camera using Cu

radiation.

The Guinier photographs were interpreted by comparison with TiS, used as reference (latice para- meters a', c').

The transition from the Cd1,-type structure of TiS, to the monoclinic Cr,S4-type structure implies two crystallographic modifications - - :

1) Appearance of a superstructure with lattice parameters c = 2 c'. This is manifested by new lines whose intensities depend on the degree of the crystallo- graphic order.

2) Crystallographic deformation implying the disap- pearance of the 3-fold symmetry of TiS, and the presence of a pseudo orthohexagonal Cr,S,-type unit cell (a = a'J3) with a slight monoclinic distortion

@ ci900).

This is manifested in the Guinier spectra by the splitting of some lines of TiS,. For example, the (1 10) of TiS, (-- 1.7

A)

is split into two lines for FeTi,S4 ;

the (310) (1.716

A)

and the (020) (1.708

A).

Similarly the line (102) is split into four lines ; the (204), (204), (1 14) and (1 la). These splittings depend on the crystal- lographic deformation, which is closely related to the degree of crystallographic order :

- the larger the distance between (20 1) and (1 1 I) (for ex. 204 and 114) the more the basal hexagone is deformed ;

-

the larger the distance between (hkl) and (hkf) (for ex. 204 and 202) the larger is the B-angle of the unit cell.

On the basis of the above observations the following results were obtained :

Slowly cooled samples : very well defined super- structures and net crystallographic structures of Cr,S4- type were obtained in both stoichiometric and not

STOICHIOMETRY DEPENDENCE OF ZEEMAN HYPERFINE INTERACTIONS IN Fel+zTizcl+z,S4 C6;581, stoichiometric samples (net splitting of the 110 line).

For the crystallographic order, no difference. was observed between the samples prepared by the two different methods.

Quenched samples : compared with the slowly cooled samples, all quenched samples show weaker superstructure lines and less pronounced line splittings. This shows a smaller degree of ordering and implies the existence of slightly inequivalent unit cells (thick lines).

This situation, of partly perturbed order, becomes more pronounced in the non stoichiometric samples, increasing with increasing non stoichiometry. However our samples never reach the completely disordered state. The most non stoichiometric (x = 0.052) samples show still some order as indicated by the presence of some of the weak superstructure lines.

3. Mossbauer results and discussion. - The Moss- bauer spectra obtained are shown (2) in figures 1 and 2. They were computer-analysed in the following components :

counts

1

r103 _{Fe Ti2 SL (5.C)}

FIG. 1. -Computer analysed Mossbauer spectra of slowly cooled (s. c.) FeTi2S4 for different temperatures. The zero velocity is the center of gravity of a metallic iron-spectrum at

300 K.

(2) Because of the limited space, only some of the spectra are given here. Detailed spectra are available on request.

counts

1

xlo3 4 2 K

FIG. 2. -Spectra at 4.2K of the stoichiometric quenched sample (r. q.) FeTi2S4 and of the non stoichiometric x = 0.052

slowly cooled (s. c.) and quenched samples (r. q.).

- in one (for x = 0) or two (for x # 0) para- magnetic doublets above 180 K ;

-

in one (for x = 0) or more (for x # 0) 8 line patterns of combined electric and magnetic hyperfine interactions below 100 K ;

- in mixed spectra containing both components between these temperatures. The computer results are illustrated in figures from 3 and 4 and can be discussed,

in relation with the questions raised in the introduction, as follows :

3.1 DEPENDENCE OF THE RESULTS ON THE METHODS OF PREPARATION. - The same spectra and results were obtained (within the experimental error) for any pair of samples having the same composition and the same heat treatment but prepared by the two different methods (Fe

+

Ti f S or Fe

+

TiS,). This result suggests the absence of any notable difference on the degree of vacancies-ordering and on the cation- distribution between samples prepared by the two different methods. This result is to be compared with the result of Takahashi and Yamada [4] who proposed the method of preparation as possible explanation of some discrepancies between them and other authors 141.

3.2 DEPENDENCE OF THE RESULTS ON THE HEAT TREATMENT.

-

3.2.1 Stoichiometric samples (x = 0).

-

Both, slowly cooled and quenched samples show one iron-site only this site being a 8-line pattern below approximately, 150 K (h-field, Fig. 3) and a paramagne- tic doublet above this temperature ( E = 0. 22 mm/s). This site has, for both samples, the same hyperfine parameters ; hyperfine field (H), isomer shift (6) and quadrupole splitting E = 114 e2 qQ. The only observed differences between these two samples are a slightly larger linewidth for the spectra of the quenched samples at low temperatures and also a different temperature dependence of the assymetry parameter y.

C6-582 G. A. FATSEAS, J. L. DORMANN AND M. DANOT a) The result of only one iron-site with a nearly

normal experimental linewidth

re',,,

[15] in the slowly cooled FeTi2S4, gives some information on the order- ing of vacancies and of the mettallic atoms.

The vacancy-ordering is a well known experimental result for this compound [I-5, 101 and has led t o the generally accepted ordered distribution (Fe

+

n)Ti2S4 meaning that full titanium layers alternate with well ordered Fe

+

9

layers.

However this cation-order is not experimentally supported and was considered just obvious in the past. Our result of one-site-spectrum gives some experimental evidence that iron atoms are present only in the vacancy-layers. Any disordering between Fe and Ti atoms in the cation positions would lead to at least two sites corresponding to the two distinct crystallographic cation-sites of the Cr,S4-type structure [4]. It is worth pointing out that from Mossbauer spectra point of view, there is another also distribution compatible with a one site-spectrum. This would have fully ordered Fe

+

Ti layers alternating with layers of Ti

+

;

that is : (Ti

+

0)

(Fe

+

Ti)S4. We think however that the Fe and Ti ordering in the Fe

+

Ti layers is less probable because the Fe and Ti atoms are quite similar.

b) The result of line-broadening on the one-site- spectrum of the quenched FeTi2S, sample [16] suggests the existence of slightly inequivalent iron-sites due to a smaller degree of vacancies-ordering compared to the high order (Fe

+

3)Ti2S4 of the slowly cooled mate- rial. The smaller order could be interpreted as a possible occupation of some vacancies in the Fe

+

layers by titanium atoms of the full Ti layers. This would lead to slightly inequivalent unit cells resulting from inequi- valent environments in vacancies and titanium, around the iron atoms. Such a situation is compatible with the broadened one-site-spectrum and with our X-ray results (thick X-lines). It is however evident that this descrip- tion is one of the existing possibilities and other distri- butions might be proposed. But what is important, is that any of the different possible distributions must lead to only small crystallographic (or vacancies and cations-neighbouring) modifications to be compatible with our results.

This result of slight decrease of order occuring in the quenched samples is in agreement with the result of Morris et al. [2]. It is also in agreement with the result of Plovnick et al. [I] who reported that vacancy- disordering does not take place in quenched FeTi2S4 samples even for quenching temperatures up to 1 300 OC. This result is, however, completely different from those of Muranaka [11] who reported that vacancy-disordering exists in samples quenched a t above about 450 O C . Our results show that a complete vacancy-disordering is absent in our quenched FeTi2S, samples and it has not been observed even in our non stoichiometric compounds as reported below.

c) The polar angle 8 [17] of the hyperfine field (H)

with respect to the principal axis of the electric-field gradient tensor is temperature dependent and was determined to be c 370 from the C-axis, the easy magnetization axis [14, 161 at 4.2 K. As this axis is the easy magnetic direction at both 78 K [18] and 4.2 K 1141 temperatures, we can reasonably suppose that this direction does not change in the intermediate region and conclude that the temperature dependence of 8 reflects a continuous rotation of V,, from 37O

(at 4.2 K) to

--

650 (at 130 K) for both quenched and slowly cooled samples. These 8 values are to be compared with the Mossbauer result of 0 = 900 obtained at 77 K on the isomorphous FeCr,S4 compound [I91 having the same easy magnetic direc- tion (c-axis).

h -r X = 0 (S.C) and (r.q)

Ht,H~,hth2+x; 0.052 (S.0

STOICHIOMETRY DEPENDENCE OF ZEEMAN HYPERFINE INTERACTIONS IN Fe1+xTizcl+x)S4 C6-583

of the stoichiometric compounds.

-

The second paramagnetic component is a particular feature of the non stoichiometric samples and gives rise, below

-

150 K, to the H, and H2 strong field

-

six lines patterns (with E

-

- 0.08 mm/s).

FIG. 4. -Iron sites and their intensities in different tempera- tures for the non stoichiometric ( x = 0.052) slowly cooled (s. c.) sample. The broken lines give the total intensities for each

group of H and h-sites. Preparation method : Fe

+

TiS2. For the x = 0.30 sample the number of sites and the temperature dependence of the intensities are quite similar to those of x = 0.052 and the results can be presented as follows (Fig. 4) : The two doublets with intensities 0.66 and 0.34 change their intensities, respectively to 0.94 and 0.06 for the sample with x = 0.030. The first doublet gives rise at low tempera- tures, to the h, and hz-sites with respective intensities 0.62 and 0.33. The other doublet gives rise to the HI-site with 0.05 intensity. The Hz-site, very weak even in x = 0.052, does not exist here.

The above results can be discussed as follows :

a) Crystallographic order. - In the non stoichio- metric Fe,+,Ti2~,+,,S4 compounds of the present work (3), having constant populations-ratio of Ti and Fe atoms, the supplementary x-iron and 2 x-tita- nium atoms must necessarly decrease the number of vacancies, from 1 vacancy in the stoichiome- tric (Fe

+

Q)Ti2S4 to (1

-

3 x) vacancies in Fe, +,Ti,(, +,,S4. Starting from the stoichiometric ideal ordered distribution (Fe

+

O)Ti,S,, the occupa- tion of 3 x-vacancies may be made in two ways :

(3) Other non stoichiometric possibilities exist : for ex. Fel+%TizS4, Fel+zTiz-zS4, Fe~-,Tiz+~s+ etc., not discussed here.

-

either the 3 x supplementary cations occupy vacancies-positions, leaving unchanged the titanium and hence modifying the neighbours of the iron within the vacancy-layers only : (Fe, +,Ti,,a,-,,)Ti2S4,

-

or the cations are randomly distributed in all the cation positions, modifying the neighbours of the iron within the vacancy-layers and along the c-axis as well :

(Fei ,,-,Ti,,

+,gl

-3x) (Tiz-,Fey)S4 with O<y< 1 +x.

Although these two distributions are different (the second one corresponding to a stronger perturbation), both situations must result in a strong vacancy-ordering perturbation and in the formation of different distinct iron sites corresponding to distinct environments in iron vacancies and in Fe and Ti atoms.

Such situations could explain the decrease of super- structure lines observed by our x-ray measurements and also the two different paramagnetic doublets in the Mossbauer spectra. We think however that any choice between these distributions needs the study of much more non stoichiometric compositions and of different types of non stoichiometry.

b) Hyperfine magnetic jields.

-

Two interesting features were found in the low temperature region :

-

A field-value h in the stoichiometric samples, abnormally small compared to the reported value of magnetic moment per iron atom at low temperatures :

1.35 pB [4], 1.55 pB [12] or 3.1 pB [14]. Further results and theoretical estimations are needed to known if a possible field compensation by opposite field contribu- tions may exist.

The h-field disappears at 155

+

5 K (Fig. 3), a much higher temperature than the magnetic order-disorder temperatures reported in the litterature (see introduc- tion). This suggests a short range magnetic order-

ing

141.

-

A strong field H-site appearing in the non stoi- chiometric samples, with a composition-independent H-value but with an intensity increasing with increasing non stoichiometry.

The strong field H ( H I in x = 0.030 and HI, H2

in x = 0.052) is a specific property of the non stoichio- metric samples and, from intensity arguments, it seems to be attributed to iron atoms which are manifested in a separate component even in the paramagnetic region (Fig. 4). These atoms represent 34

%

of the iron atoms and they result from the presence of 5

%

supplemen- tary iron atoms. It would be interesting to known how a 5

%

non stoichiometry, results in the apparition of 34

%

new iron atoms. This could allow to understand what the new iron atoms are and how the strong field H is related with the perturbed order in the non stoichio- metric samples.

C6-584 G. A. FATSEAS, J. L. DORMANN AND M. DANOT the crystallographic order is strongly perturbed, show

that the magnetic properties, as they are reflected in the hyperfine fields, are strongly related to the degree of order. This is manifested not only by the appearance of the strong field-site but also by the dispersed magnetic-paramagnetic sites transition-temperatures between

-

110 K and 155 K (Fig. 4). These results are not incompatible with the existence of inhomogeneous clusters. Such inhomogeneities are not, however, manifested in the quenched stoichiometric sample used in the work of the above authors [2, 111.

3.3 ISOMER SHIFTS (8). - The isomer shift, relative to the matallic iron, is for all sites and all samples equal to 0.78

rt

0.02 mm/s at 300 K and increases linearly with decreasing temperature, reaching, at first,

0.89 mm/s at 120 K and then, with a slower rate, 0.94 at 4.2 K.

These values give raise to the following remarks :

moment of susceptibility X - ' ( T ) measurements. Although 6 = 0.79 mm/s at 300 K lies in the region of the reported Mossbauer data for ~ e compounds, ~ + we think that this calibration must be used very care- fully for FeTi2S,, because this compound has a metallic type conductivity [3, 51 which has been attributed to direct cation-cation interactions from the Goodenough critical distance concept [3]. A similar problem exist in Fe,Ses of NiAs-type structure.

-

For a given temperature, the isomer shift is composition (x)-independent. This was verified not only from our non stoichiometric samples but also in Fe,.,Ti2S,. This last compound was chosen because it was reported to be trivalent (Fe3') from x-l(T) measurements [4].

Our Mossbauer spectrum at 300 K revealed a

6 = 0.79 mm/s and did not confirm the proposed valency Fe3 +

.

-

The iron atoms in FeTi,S, have been reported to Acknowledgements.

-

The authors wish to thank be bivalent (Fe2+) [4, 111 from the paramagnetic Dr. A. Marais for helpful discussions.

References

111 PLOVNIK, R. H., VLASSE, M. and WOLD, A., Znorg. Chem. 7 (1968) 127.

[2] MORRIS, B. L., JOHNSON, V., PLOVNICK, R. H. and WOLD, A., J, Appl. Phys. 40 (1969) 1299.

[3] PLOVNICK, R. H., PERLOFF, D. S., VLASSE, M. and WOLD, A.,

J. Phys. Chem. Solids 29 (1968) 1935.

[4] TAKAHASHI, T. and YAMADA, O., J. Solid State Chem. 7 (1973) 25.

[5] DANOT, M., ROUXEL, J. and GOROCHOV, O., Muter. Res.

Bull. 9 (1974) 1383.

161 The isomorphous selenides have been systematically studied by BERODIAS and CHEVRETON (rkf. [lo]).

[7] DANOT, M., BICHON, J. and ROUXEL, J., Bull. Soc. Chim.

8 (1972) 3063.

[8] DANOT, M. and BREC, R., Acta cryst. B 31 (1975) 1647. [9] HAHN, H., HARDER, B. and BROCKMULLER, W., 2. Anorg.

Allgem. Chem. 288 (1956) 260.

[lo] BBRODIAS, G. and CHEVRETON, M., C . R. Hebd. Sean. Acad.

Sci. Paris 261 (1965) 2202.

[ l l ] MURANAKA, S., Mad. Res. Bull. 8 (1973) 679.

[12] SENOUCI, B., Etude de quelques propriktks structurales et magnktiques de FeTizS4, these de 3e cycle, Universitk Paris XI, Juin 1976.

[13] DANOT, M., VILLERS, G., SENOUCI, B., ROUXEL, J., FAT- SEAS, G . A. and MARAIS, A., Intern. Cod. on (( Solids Compounds of transition elements P. Uppsala, Sweden, June 21-25, 1976 (extended abstracts).

[I41 VILLERS, G., DANOT M., SENOUCI, B., MERIEL, P., ROUXEL, J. and MARAIS, A., Phys. Stat. Solids, to be published.

[IS] r e x p / r p e 1:1.4 at 4.2 K where T p e is the experimental

linewidth of a thin natural metallic iron absorber at 300 K.

[16]

re,lr,,

1:2 at 4.2 K.

[17] The fit is very sensitive to the 0-angle values : only a few degree change of 8, results in a noticeable deterioration of the fit. Contrarily, the fit was fairly insensitive to the azimuthal angle cp.

1181 MURANAKA, S., J. Phys. Soc. Japon 35 (1973) 61 6.

[19] HONG, S. R. and NAM, 0. K. H., Phys. Rev. B 11 (1975) 4176.

[20] T e , / r p e = 2.3 at 4.2 K for slowly cooled and 2.8 for