A 57Fe Mössbauer study of four pyrite compounds: CoS2, NiS2, CoSe2 and NiSe2

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992

V. H. MCCANNand J. B. WARD

460 hr. NiSe,: 3 cycles as above (T = MO-550 C): total time at this temperature = 481 hr.

(b) M~ssbauer specrroscopy ~~ssbauer spectra were taken in the constant acceleration mode using a “CofRh source at room temperature. The absorbers were either (i) mixed with vacuum grease and pressed between Mylar sheets or (ii) mixed with sucrose, ground, and pressed into a flat disc

inside a steel washer. Spectra below room temperature were taken in an Andonian dewar. All MGssbauer spectra were fitted to lorentzian lines using a computer programme similar to that of Bent et af. [ 16). The spectra showing hype&e magnetic splitting were fitted using a subroutine which diagonalized the hamiltonian &p= [eZq@(4Z(2Z - 1))][3Z,‘- Z(Z+ l)] -

gpnz3(z,

cos

e +

Z, sin

e)

to a system with axial EFG, and with H at an angle B to the z axis (principal EFG axis). In all cases the computer programme minimised x2 by an iterative method and calculated best values of baseline, fractional absorption, isomer shift IS, quadrupole splitting QS(=fe’&), linewidth I’, also H and B where appropriate. Derivatives for the parameters were found an~ytically except for QS, H and B where spectrum changes were calculated for smait changes in these parameters. The programme also provided estimated standard deviations for these quantities. The spectrometer channels were converted to velocity using a National Bureau of Standards natural iron absorber. All isomer shifts reported in this paper refer to the centre of the natural iron spectrum as zero velocity, and take the separation of the inner Fe lines (lines 3 and 4) as 1.6807mm s-‘. appropriate

3. EXPERIMENTAL.

RE!3UJ,Ts:

COMMON

FEATURES

(a) Room temperature values of IS Table 1 summarises room temperature values of IS and QS for the pyrite structure Fe, Co and Ni disulphides and diselenides reported to the present. Signs of QS, where determined, are also included. All IS values are quoted with respect to the centre of the natural Fe spectrum at 300 K, those in Refs. 117,181 having been corrected to this point by the addition of the IS of a Cu source, 0.225 (2) mm s-’ [ 191. Table 1. Room temperature values of IS and QS, in mm s-‘, for pyrite structure compounds. Error estimates given in brackets are 22 standard deviations on the last figure, e.g. 0.376(6)= 0.376~0.~. IS values refer to the centre of the natural iron spectrum as zero velocity

QS Substance

(mm SK’) (rnZ_‘)

Fe& +0.614(6) +0.323(6) Fe, &oO.& FeOO,NiO&, -0.274(6) Fe,,,Ni, .A2 -0.27(l) FeSe, 0.~7(~) FeO.OZCoo.seSel 0.376(6) 0. i 12(IS) Fe, 02Ni0.&2

&

Reference

0.313(6) -300 U, 17,201 0.359(7) 293 [ZO],this paper 0.440(7) 293 [20],this paper 0.442(S) 295 I91 0.447 Room r1g1 0.447(5) 293 This paper 293 0.482(8) This paper

The University of Oslo group[21] have reported a linear relation between IS and unit cell volume V for marcasites and similar relations have also been reported between IS and V-‘1171 and between IS and V in the series Fe,_,Co,S,[22]. The IS of the three pyrite disulphides reported in this paper obey a similar relation, lying on a line parallel to those of the marcasite compounds. This is shown in Fig. I. To compare marcasite and pyrite compounds, the IS values have been plotted vs volume per metal atom v where v = Vi4 for pyrite compounds and V/2 for marcasites. The IS vdues of the three diseienides are not so well correlated however.

O!LO_

0.30 _

60 Volum p8r “:: atom

1m-JO)

Fig. 1. Room temperature Isomer Shift (IS) values of pyrite and marcasite compounds plotted vs volume per Fe atom (0). The marcasite lines are taken from Ref. [21].

993

A “Fe Mijssbauer study of four pyrite compounds (b) Room temperature values of QS Table 1 shows that QS in the diselenides decreases in magnitude in the same order as in the disulphides, i.e. FeSerCoSerNiSe,. (c) Temperature variations of QS Fe in the pyrite structure compounds is low-spin Fe” with t& configuration and thus does not exhibit the large temperature variation in QS shown by the Fe’+ compounds. The changes in QS between 300K and the temperature of onset of magnetic order are shown in Fig. 2 and are small enough to be accounted for by the contraction and associated small changes in the u positional parameter [9,17]. In the three compounds CoSe*, Co& and NiS2 which show magnetic ordering, QS decreased in magnitude below the transition temperature. This effect appeared in the data of Nishihara et al.[9] for Ni,,.995F%.W5S2 but was more pronounced in our sample and thus may be related to the “Fe content. This trend also appears in Co& and CoSe,. The total change in QS appears to be correlated with the magnitude of the supertransferred hf field. Co& shows an anomaly in the thermal expansion coefficient at Tc [23]. Binloss [24] has measured the variation of lattice parameter with temperature for CoS2, and also for Fe& which shows normal non-magnetic behaviour. He uses a thermodynamic argument to conclude that the expected magnetostriction in Co& is mostly confined to a change in the crystallographic u parameter. QS is quite sensitive to the value of u [9], and the changes in QS may therefore result from atomic rearrangements below Tc. A -AL.

served below Tc. One group[22] did not recognize this broadening as being due to magnetic hypertine structure, the other group[5] analysed the spectra for the magnetic hypefine field. Because of the method of analysis used (comparison of simulated spectra with experimental results) the latter group could not distinguish between the possibilities of there being either a positive or a negative value of QS. By analysing the results in the way described in Section 2 we were able to decide that QS is positive. In Table 2 results taken at a temperature of 100K are compared with the two alternatives of Woodhams et al. [5]. In Fig. 3 is the spectrum taken at 100K with the best fit to each of the choices of negative and positive values of QS superimposed upon the spectrum. These results show that the angle, 0, between the z axis of the EFG tensor and the local magnetic field, H, is the same at all “Fe sites. Although evidence[23,25-271 has been given on the itinerant nature of the d electrons the analysis of polarized neutron diffraction data shows that the magnetic moments are, nevertheless, fairly well localized at the metal sites (about 2% of the magnetic moment is delocalised)[28]. Thus, in a localised electron model the results of our experiments can be interpreted in terms of a canted spin structure with the resultant field at each metal site being at an angle of about 75” to the local trigonal axis. At low temperatures H tends to a value of lo? 2 kG. At 19 K the spectrum is more complex and cannot be Table 2. Quadrupole and magnetic hypertine parameters for Co,_,Fe,S1. A comparisonwith the two alternativesof Ref. [5] ?I,

REWL’IXMAGNJD’IC BEHAVIOUR

(a) Cobalt disulphide Co& In previous Mijssbauer experiments on “Fe in Co,_,Fe,S2 broadened asymmetrical spectra were ob-

I

99(l) 91 91

I

QS (mms-l)

x 0.01 0.02 0.02

H 0 (kG) (degrees) Reference

+0.32q3) 8.3(2) +0.330(30) 7(l) -0.340(30) 6(l)

74(2) 75(H) 30(U)

1

This paper

1

4

0.40

P------+

c%aF’02S’2

”

Q

0.36 1 l0.34 +0.30

1

0.14 0.10

r

-0.22 -0.26 -0.30

'I ' 0

1

1

100

200 T(

I, 300

K)

Fig. 2. Quadrupole splitting (QS) vs temperature for four “Fe-doped pyrite compounds. Signs of QS, where determined, are given.

PI

PI

V. H. MCCANN and J. B. WARD

994

(c) Nickel disulphide NiSz

_%I,

I

-0.5

I

OQ

0.5 Source velocity tmm s-1)

II 1.0

Fig. 3. Miissbauer spectra of Co0.99Feo.o,S2 at 1OOK.The lines show computed best fits for two alternative signs of the quadrupole splitting (QS).

fitted to a single set of values of the Hamiltonian parameters. This spectrum and others taken above and below Tc are shown in Fig. 4. We have not yet been able to decide whether this could be explained by a rearrangement of the magnetic moments. An unexplained broadening of the neutron diffraction line at 4 K has also been observed by Andresen et al.[6]. (b) Cobalt diselenide CoSez Mijssbauer spectra of Co,_,Fe,Se, were taken at room temperature and 100K for x equal to 0.1, and also at 82 and 19K for x equal to 0.02. In the latter two cases a broadened and asymmetric spectrum was observed which can be attributed to a local magnetic field at the “Fe sites of 2? 1.5 kG. However because of the smallness of the field it was not possible to decide on an angle between H and the local trigonal axis, or to say whether these parameters are unique at each site. For the same reason it is possible to measure only the magnitude of

QS.

The “Fe Miissbauer spectrum of Ni,,Feo.o,S, shows magnetic broadening at low temperatures and spectra were taken at a few selected temperatures (see Figs. 1 and 2). At temperatures from Tc to 35 K the spectra were fitted by a single value of 0 for the four iron sites per unit cell. This value of 0 (26”2 3”) is consistent with the work of Nishihara et al.[9]. From 6’Ni Mijssbauer spectra Czjzek et al. [IO] determined that 0 is unique but they could not give the value of 8 as in their case the magnetic hyperfine interaction is much larger than the electric quadrupole interaction and so they can determine only an effective value of quadrupole splitting [(3 cos2 0 - l)(e*qQ/2)1. Spin directions have not been determined in neutron diffraction experiments; just the relative orientation of the magnetic moments at the four nickel atoms per unit cell. In general the angle B between the local trigonal axis at each site and the direction of the effective magnetic field could be different at each site but the single value of 0 is consistent with the neutron diffraction data, as previously analysed[29]. At low temperatures the asymptotic value of H is (20 2 1)kG. NiS undergoes a ferromagnetic transition at 31 K. Spectra taken below this temperature are complex but at present we have not been able to obtain sufficient low temperature data to resolve the spectrum unambiguously. Conflicting interpretations have been presented in the literature[9, IO]. (d) Nickel diselenide NiSe,! Results are shown in Figs. 1 and 2. As NiSe, does not show magnetic ordering, we found the expected spectrum, both at room temperature and 83 K, of a pair of lines split by the electric quadrupole interaction. Only the magnitude of QS could be measured. In this case the interesting feature is that this is the smallest quadrupole splitting measured in these compounds. 5. CONCkUSION We have shown that in these pyrite compounds if the internal magnetic field is at least 5 kG then it is possible to measure the sign of QS. Measurements of the hyperfine parameters have led to a more explicit description of the spin structure of Co& and NiS2, and we have been able to show that the spectra of CoSe, exhibit broadening which correlates with its measured magnetic transition [8]. Acknowledgement-Some of the experimental work reported here was carried out by K. C. Ewans as part of an honours project in Physics.

Hulliger F. and Mooser E., I. Phys. Chem. Solids 26, 429 (1%5).

00 -0.5

Source Vehty

(mm 5-l)

Fig. 4. Mossbauer spectra of Co,,Fe,.,S,.

Bither T. A., Bouchard R. J., Cloud W. H., Donohue P. C. and Siemons W. J., Inorg. Chem. 7, 2208 (1%8). Ogawa S., Waki S. and Teranishi T., Int. J. Magn. 5, 349 (1974). Goodenough J. B., J. Solid State Chem. 5, 144 (1972).

A “‘Fe Mijssbauer study of four pyrite compounds 5. Woodhams F. W. D., White P. S. and Knop O., I. Solid State Chem. 5, 334 (1972). 6. Andresen A. F.. Furuseth S. and Kjekshus A., Acta. Chem. Stand. 21, 16 (1%7). 7. Furuseth S. and Kjekshus A., Acta Chem. Stand. 23, 2325 (i%9). 8. Adachi K., Sato K. and Takeda M., 1. Pftys. Sot. Japan 26, 631 (1%9). 9. Nishihara Y., Ogawa S. and Waki S., 1. Phys. Sac. Japan 39, 63 (1975). 10. Czjzek G., Fink J., Schmidt H., Krill G., Gautier F., Lapierre M. F. and Robert C.. J. Phvs. Colloaue 35, C6 (1974). 11. Hastings J. M. and Codiss i. M., fiM J. Res. bev. 14, 227 (1970). 12. Miyadai T., Miyahara S., Takizawa K. and Uchino K., Phys. Letters 44A, 529 (1973). 13. Miyadai T., Takizawa K., Nagata H., Ito H., Miyahata S. and Hirakawa K., J. Phys. Sot. Japan 38, 115 (1975). 14. A.E.R.E., Harwell, Betks., U.K. 15. Koch-Light Laboratories Ltd., Colnbrook, Bucks., U.K. 16. Bent M. F., Persson B. 1. and Agresti D. G., Comput. Phys. Comm. 1, 67 (1969).

995

17. Temperley A. A. and Lefevre H. W., J. Phys. C/tern. Solids 27, 85 (1966). 18. T. A. Bither, ptivate communication. 19. Stephens J. G. and Stephens V. A., Miissbauer E#ect Data Index Covering the 1974Literature. Plenum Press, New York (1975). 20. Ward’J. B. and Howard 13.G., 1. Appt. Phys. 47,388 (1976). 21. Kiekshus A. and Rakke T.. Acta Chem. Scand. ?&A, 1001 (i974). 22. Gallagher P. K., MacChesney J. B. and Sherwood, R. C., I: Chem. Phys. 50, 4417 (1969). 23. Kasai N.. I. Phys. Sot. Japan 35, 1552(1973). 24. Binloss W., I. &pl. Phys.42, 1474(1971). 25. Jibu M., Ishikawa Y. and Tajima K., Phys. Letters 45A, 235 (1973). 26. Ogawa S. and Yamadaya T., Phys. Letters d7A, 213 (1974). 27. Adachi K.. Sato K.. Okimoti M.. Yamauchi G.. Yasuoka H. and Nakamuta Y., i.-Phys. SK Japan 38, 81 (1975). 28. Ohsawa A., Yamaguchi Y., Watanabe H. and Itoh H., 1: Phys. Sot. Japan 40, 986 (1976).

29. Krill G., Lapierre M. F., Gautiet F., Robert C., Czjzek G., Fink J. and Schmidt H., J. Phys. C. 9, 761 (1976).