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Spin flip and troilite-MnP structure transition in FeS as studied by Mössbauer spectroscopy
0022-3697193 S6.00 + 0.00 0 1993 Pergamon Press Ltd
1. Phys. Chem. Solids Vol. 54, No. 1 I, pp. 1.593-l 597. 1993 Printed in Great Britain.
SPIN FLIP AND TROILITE-MnP TRANSITION IN FeS AS STUDIED SPECTROSCOPY
STRUCTURE BY Mi)SSBAUER
OLOF KRUSE~ Department of Mineralogy and Petrology, Institute of Geology, Uppsala University, Box 555, S-751 22 Uppsala, Sweden (Received 2 March 1992; accepted in revised form 7 June 1993)
Abstract-Synthetic FeS (troilite) was analyzed by Miissbauer spectroscopy in the temperature range 283 Q T d 460 K. Spectra were analyzed applying the full Hamiltonian. The a transition between the troilite and MnP-type structures was completed at 413 K on heating. Compared with literature data of a meteoritic troilite, the behavior differed as follows: the transition occurred without hysteresis, with a steeper slope in an MnP structure concentration ([MnP]) vs temperature plot, with a lower [MnP] at room temperature, and with heating and cooling trends coinciding. The spin flip in the MnP structure occurred rapidly and is suggested to be coupled to the ease of the a transition. The absence of trace elements, e.g. Cr, in the synthetic sample is suggested to be of importance to the rapid spin flip and a transition. A reported increase in compressibility of troilite with increased pressure is suggested to be due to a successive weakening of the Fe, cluster bond strength. The relatively scarce occurrence of electron transfer enzymes with reaction centres with three Fe atoms is suggested to be due to an evolutionary drawback caused by the possibility of forming energetically stable Fe, clusters unsuitable for reaction. Keywords:
MBssbauer, full Hamiltonian, FeS, a transition, spin flip.
1. INTRODUCTION Depending interrelated at ambient
on the temperature, FeS assumes different crystallographic and magnetic structures pressure. At room temperature, it occurs
as hexagonal troilite with space group P62c [l]. The troilite structure, which has been reported only for FeS [2], may be regarded as a NiAs structure derivative (Fig. 1) with the cell axis lengths a and c being ,/3 and 2 times those of the NiAs subcell axes, respectively [3]. In troilite, all Fe atoms are in triangular three-atom clusters forming planes perpendicular to the c axis. These clusters are suggested to be due to Fe-Fe bonds formed by 3d electrons if the distances between the involved atoms are below a critical value [4]. The clusters, and hence the troilite structure, cease to exist at the tl transition. The cluster breakup may be accomplished either by increased temperature, increased pressure, or by vacancies at Fe sites. For FeS at ambient pressure, the temperature for a completed transition into the orthorhombic MnP-type structure (Prima, [2]) has often been given as 413 K. However, the transition is gradual as shown by a weakening of the troilite supercell reflections of synthetic FeS after heating to and measurement at 375 K [5]. In meteoritic FeS, the situation is not fully reversible. The c( TPresentaddress: SecoTools AB, S-737 82 Fagersta, Sweden.
transition on heating is complete at 413 K, while the MnP to troilite transition starts at 410 K; a residual 20% of the Fe atoms may still be observed in the MnP structure after slow cooling to 280 K [6]. The transition temperature T. decreases if the pressure increases [7] and equals room temperature at 3.4 GPa [2]. The latter investigation indicated a negative value of the pressure derivative of the isothermal bulk modulus (K+ = -5(4)) for troilite, indicating this phase becomes more compressible with an increase in pressure. At ambient conditions, quenched samples of Fe,_,S compositions made by dry synthesis show troilite superstructure X-ray reflections for x < 0.047, but not for larger x values [3,8]. The latter investigation also showed sharp cell-parameter discontinuities at x x 0.047. On heating at ambient pressure, MnP-structured FeS transforms into the NiAs structure with space group P6,/mmc [9] at _ 508 K [lo]. Below the magnetic ordering temperature (TN z 600 K) the spins of the Fe atoms are antiferromagnetically coupled. Below the spin flip temperature T, the spins point along the NiAs-subcell c axis (cNiAs);above T, they are oriented in planes perpendicular to cNiAs[l 11. The spin flips occur only in the MnP phase [6]. Various values for Ts have been given, e.g. 445 K for susceptibility measurements on a synthetic Fe0,W6S single crystal [l 11, 458 K for neutron
1593
OLOF KRUSE
1594
at
g 2~
0-s
.=Fe
Fig. 1. FeS structures represented by atomic configurations. The c axis of the NiAs subcell is vertical. Left: the NiAs structure. The line connecting the Fe atoms is straight; the planes of S atoms are perpendicular to this line. Middle: the MnP structure. Compared to the NiAs structure the Fe-Fe lines and S-planes are tilted. Right: the troilite structure. All Fe atoms are members of triangular clusters forming planes perpendicular to the c axis. Every other Fe-Fe line is tilted with respect to E.
room-temperature
for
20
months
prior
to
measurements. Mdssbauer measurements were made in a vertical transmission set-up with a constant acceleration drive using a 57CoRh y source. The powder sample, containing 7 mg Fe cm-*, was kept on a horizontal Be disc in an evacuated furnace. The temperature could be measured within an estimated accuracy of f2 K. A total of 37 measurements were made in the range 283 < T < 460 K, where the temperature was both increased and decreased stepwise. The velocity scale was calibrated with an Fe foil at room temperature. The 512 channel mirror spectra were folded and then analyzed using a full-Hamiltonian program [ 171. Due to the thin absorber the spectral lines were assumed to be of Lorentz type; thus, no thickness correction was made. The quadrupole splitting Qs was defined as QS = 1/2V,eQ(l +rl*/3)“* [18]. The sample was analyzed before and after Miissbauer measurements by X-ray diffractometry at room-temperature using KC1 as internal standard. Peaks were measured at a goniometer speed of 20 = 0.001” s-l. Five KC1 peaks were used to calculate second-degree polynomials for corrections of d values. The composition of the sample was computed from the d value of the 102 peak by an expression giving a standard error of 0.002 in x [19]. The obtained values of x in Fe, _,S before and after heating runs were +O.OOl and -0.004, respectively. The computed values of cell parameters, using eight troilite peaks before and after heating runs, were a = 5.969(l), c = 11.760(4) and a = 5.968(l) A, c = 11.756(4) A, respectively.
diffraction on synthetic FeS [12], and the temperature ranges -410-470 K on heating and -450-360 K on cooling for a meteoritic sample [6]. The sluggishness of both the MnP to troilite structure transition and the spin flip in the meteoritic sample is in stark contrast to the corresponding situation in synthetic FeS where both transitions occur reversibly in narrow temperature intervals [l 11. The presence in meteoritic troilite of trace elements in solid solution has been suggested as being important to this difference [6]. The present investigation was undertaken in order to compare the behavior of synthetic FeS with that reported for meteoritic troilite. 3. RESULTS AND DISCUSSION It has been shown [13] that a good interpretation of Mijssbauer spectra of troilite requires considerA Mijssbauer spectrum with the sextets correation of the full Hamiltonian. However, also in later sponding to troilite and MnP structure is shown in investigations (e.g. [l&16]) spectra of synthetic FeS Fig. 2. Parameters for the sextets were obtained were interpreted according to a perturbation model. following [6]. Thus, spectra were fitted with a total of Thus, the magnetic hyperfine interaction has been three sextets representing troilite and the two spin assumed to be much stronger than the electric one, configurations of the MnP phase, i.e. MnP r and which is considered as a mere perturbation. The MnP -. The intensities I of these sextets were free to present investigation was also intended to yield full vary. The crystallographic positions of all Fe atoms Hamiltonian Miissbauer data on synthetic FeS at were assumed to be equally well defined and hence the elevated temperatures. half-widths w were constrained to be equal. The QS values for troilite were set negative. The Fe atoms in 2. EXPERIMENTAL the MnP phase were all assumed to have the same environment, except for the spin directions. The Samples were obtained by dry synthesis of Fe powder (99.999% purity, reduced at -900 K in H, centroid shift (CS ) and hyperfine magnetic field strength (&) values were hence constrained to the gas for 90 min) and S (99.9999% purity). Fusion was same values, respectively. The QS value was fixed performed at 823 K for 23 h in an evacuated glass ampoule. The reaction products were then milled same values, respectively. The Qs value was fixed at -0.25 mm ss’ and the 0 values (which here under toluene and again heated at 843 K for 15 days. equals the angle between the cNiAsaxis and B,,) to 0 The ampoule was then slowly cooled and kept
Spin Sip and troilite_MnP structure transition in FeS
I
1595
I
I
I
I
I
I
b ,
-6
-4
-2
0
2
4
6
Fig. 2. Mtissbauer spectrum of synthetic FeS at 410 K. The transmission is given in per cent vs y source
velocity in mm s-‘. Dots indicate data points, the solid line a fitted function. The troilite structure is represented by the broader sextet, the MnP structure by the narrower one. The relative intensities of the sextets are 49 and 51%, respectively.
and 90”. No value of the asymmetry parameter q = (V,, - V,)/V, can be deduced for the troilite and MnP structures. However, for symmetry reasons q equals zero in the high-temperature NiAs structure. Since no significant discontinuities occur in the parameters CS, QS, or Bbl at the NiAs-MnP structure transition, q may be assumed to have the value zero also in the MnP structure [6]. In fitting troilite spectra, variations in q have been shown to have only minor influence on the obtained parameters [6]. Therefore, q was fixed to the value zero also for troilite. The small differences in composition and cell parameters obtained before and after Miissbauer measurements were assumed to be negligible. The obtained values for the parameters CS, B,, and QS are given in Table 1. The corresponding results for natural FeS given in [6] are almost identical. Hence these parameters cannot be the base for a discussion of the differences in reaction kinetics between natural and synthetic FeS. The obtained concentration of MnP phase, [MnP], as a function of temperature in the range 283415 K, is given in Fig. 3. A comparison with the corresponding plot for a natural sample with a composition close to Fe,.,,Cr,,,,S [6] shows the following differences: (1) The heating and cooling curves are indistinguishable for the synthetic sample, whereas natural FeS when heated for the first time shows [MnP] increasing linearly from zero at 405 K to 100% at 413 K and decreasing slowly to an apparently stable value of 20% on cooling. In the present case both starting and end concentrations are close to 10%. (2) For the natural samples, the troilite to MnP transition was completed at 413 K, whereas the reverse transition started at 410 K. In the present case, an extrapolation gives a completed transition at 413 K, but no hys-
teresis. (3) The change in [MnP] on cooling is more accentuated in synthetic FeS, where a 50% value is reached at 409 K. In the natural samples, the corresponding temperature was 396 K. The assumption that the measured difference in chemical composition is negligible is supported since the obtained [MnP] has the same starting and end values. However, the Miissbauer parameters for Fe atoms in the MnP structure are similar to those of troilite-structure Fe atoms with an Fe vacancy as a nearest neighbor [20). The obtained sextet may thus be assigned to Fe atoms in two different environments. The relative intensity due to vacancies, I,, can Table 1. Values of Miissbauer parameters CS, B,,r,and f&S for synthetic FeS in the temperature range 283 5 T 5 460 K. A total of 37 spectra were measured; data for the excluded spectra do not deviate from the trends of the listed ones. The obtained parameter values were seemingly unaffected by heating history. “tr” and “MnP” indicate sextets representing troilite and MnP-type structure, respectively. The sign ” !” indicates that the value has been a fixed input in the computations T(K)
Sextet
283 313
%nP tr MnP
335 352 368 387
GnP tr MnP tr MnP tr MnP
406 432 449 460
%nP MnP MnP MnP
CS (mm s-‘)
E,,,(T)
0.76 0.67 0.74 0.66 0.73 0.70 0.71 0.67 0.70 0.69 0.69 0.66 0.68 0.69 0.68 0.68 0.66
31.2 27.7 30.8 26.7 30.5 26.5 30.3 26.3 29.9 26.1 29.5 25.4 28.9 24.9 23.5 22.5 21.8
QS (mm s-r) -0.88 - 0.25! -0.88 -0.25! -0.87 -0.25! -0.86 -0.25! -0.86 -0.25! -0.85 -0.25! -0.85 -0.25! -0.25! -0.25! -0.25!
OLOF
1596 . . . . . . i
E ..
.Z OI
280 300 320 340 360 380 400 420 T(K)
Fig. 3. Relative MnP sextet concentration in synthetic FeS vs temperature. be computed as Z, = 100 x 2x/( 1 - x)% [20]. For the present starting value x = 0.001, Z, = 0.2%. It can hence be concluded that the influence of vacancies is negligible in the MnP sextet. The presence of the high-temperature MnP phase in the initial run at room temperature indicates that the sample had not attained equilibrium. Since [MnP] reaches the same value after measurements it can be assumed that the intended equilibration at room temperature for 20 months had negligible effect. For natural FeS the a transition and its connection to the spin relations has been discussed [lo]. The sluggishness of the MnP to troilite structure transition was suggested to be due to structural incoherence between the phases below 413 K and the preferred Fe spin orientation in the MnP structure being perpendicular to cNiAsand thus to the orientation necessary for the transition. In Table 2 the present obtained relative intensities of the three fitted subspectra are given. These correspond to the relative concentrations of troilite phase and to the two spin configurations, -+ and t, of the MnP structure. The Table 2. Relative intensities I of subsnectra for synthetic FeS in the temperature range 283 5 T 5 432 K. ltrl = I of troilite sex&t in per cent; [M&f] = I of MnP sextet for magnetic spins of Fe atoms parallel tet
to eNiAS;[MnP+
= I of MnP sex-
for spins perpend&lar to cNiAI
T(K)
[tr]
[MnPf]
[MnP-+]
283 395 404
91 80 70
2 3 5
7 17 26
408 410 412 415 432
59 49 24 0 0
4 8 12 11 0
36 43 64 89 loo
KNSE
results on heating and cooling are indistinguishable and thus dependent on measuring temperature only, whereas for a meteoritic sample [6] spin flips were detected in differing intervals on heating and cooling; thus, the relative concentrations were dependent both on measuring temperature and heating history. The ease of the spin flip here, being complete at 432 K in a series of successive heating, contrasts with a meteoritic sample [IO] where the spin flip was still uncompleted after 23 days at 429 K and associated with an activation energy E, of -2 kJ mol-‘. The rapid reaction and the considerably lower temperatures required in the synthetic sample indicates a much smaller Ea. The value of E, could, however, not be measured since the changes in spin configuration concentrations were too rapid to be detected by the method used. Moreover, the absence of hysteresis for the reverse spin flip indicates both a small E,, and that the concentrations of the two spin configurations are functions of temperature alone. Although difficult to detect due to the slow reaction rate, this might also be the case for meteoritic FeS at equilibrium. The present similarities on heating and cooling for the spin concentrations may affect also the [troiliteE [MnP] relations through the spin orientation-a transition coupling mentioned above. The absence of hysteresis in the a transition, the steepness of the d[MnP]/dT slope, and the lower [MnP] after hightemperature measurements in synthetic FeS would hence be coupled to the easy spin flip on cooling. Miissbauer studies on the influence of V, Co, and Cr substituting Fe in synthetic FeS have been reported [21-231. For Fe,,,975 Co,,,,,, S and Fe,,g,, V,,,, S a T, of 400 K was obtained [23]; for Fe,,,,Cr,,,,,S TX was given as 384K [21]. Thus, the a transition is affected by the presence of these elements. For Fe,,V,,, S a hysteresis-free transition at 390 K was reported [22], with [MnP] leveling off at 20% at room temperature. For Fe 0,975V0,025 S a hysteresis of 4 K was observed (400-396 K), but also a rapid variation in [MnP], being zero at 375 K. No corresponding results were given for Cr. However, since the mentioned elements do affect the transition it is likely that the hysteresis and sluggish transition reported for the meteoritic sample above are coupled to Cr and the lower concentrations of other elements, e.g. Co. An increase in the a-axis length and a corresponding decrease in c with increasing Cr concentration was reported by [21]; the cell volume was however approximately constant. This indicates a successive weakening of the cluster bond strength in the a-a plane. Thus, lower temperatures for the formation and break-up of the clusters would be required, which is consistent with the T, values reported for FeO.grrCr,,r, S.
.
Spin flip and troilite-MnP
2.9.5
n
. n
Ab Y
2.90
2.85
0
1
2
transition
in FeS
1597
likely that the effect is caused by the collective electron clouds associated with the clusters, resulting in repulsion along c.
.
z -0
structure
. .
3
P (GPa) Fig. 4. Interatomic distances d in synthetic troilite vs hydrostatic pressure. Squares: d between Fe atoms directly above/below one another. Triangles: d between Fe atoms obliquely above/below one another. Crosses: d between Fe atoms of the same cluster. Data from [2].
The nature of the troilite clusters and the mechanism of their formation have been discussed by Goodenough [4,24] who suggested cluster bonding to be due to delocalization of the sixth, minority spin electron of the involved Fe atoms. The literature data on the stability in (P, T, x) space for the troilite structure is reviewed above. Despite both experimental results and theoretical considerations, little effort has been spent on a general discussion of the transition to/from the troilite structure. A successive increase of the thermal expansion of a on heating in meteoritic troilite has been reported [lo], indicating a gradual weakening of the cluster bond strength. The critical Fe-Fe distance for the breakup of clusters was 2.93 A at normal pressure. Measurements of the Fe-Fe spacings in Fe, _,S vs x [20] showed the distances between atoms obliquely above or below one another (Fig. 1, right) to become smaller than even within the cluster as x increased. This would disturb and eventually destroy the cluster bonds, causing the a transition. Figure 4 shows Fe-Fe distances in synthetic troilite vs hydrostatic pressure at 294 K based on literature data [2]. Also here the distances between obliquely above/below Fe atoms become successively smaller, decreasing below the 2.93 A mentioned above. Thus, the cluster bonding is indicated to be disturbed prior to the final breakup, e.g. by a spilling over of electrons from the collective orbitals of the clusters to Fe d,2 orbitals interacting along cNiAs . Such a gradual weakening would explain the negative K+ discussed in [2]. The larger distances between Fe atoms straight above/below than between those obliquely above/below were suggested to be due to repulsion between Fe atoms. However, it is more
Clustering in compounds containing Fe and S is known also in biomolecules, where Fe-S clusters with mostly one, two, or four iron atoms are active cores in the electron transfer enzymes in most electron transport chains in metabolism [25]. The low frequency of 3-Fe cores may be connected to the possibility of forming clusters with stable electron configuration as exemplified in troilite. The strong bonding in such a configuration would require a relatively large E, for enzymatic reactions; this might have been an evolutionary drawback. The high stability connected to an Fe, cluster may be related to the arrangement of occupied and empty cluster energy levels associated with this particular surrounding.
Acknowledgemenfs--I wish to thank the following people for helpful guidance: Jean-Louis Calais for fruitful discussions and for reading through the manuscript; drjan Amcoff, Tore Ericsson and Hans Annersten for critical reviews; and Miss Kersti Gloersen for the language check.
REFERENCES 1. Evans H. T., Science 167, 621 (1970). 2. King H. E. and Prewitt C. T., Acta Crysr. B 38, 1877 (1982). 3. H&ggG.andSucksdorffI.,Z.phys.Chem.B22,444(1933). 4. Goodenough J. B., Mater. Res. Bull. 13, 1305 (1978). 5. Haraldsen H., Z. anorg. al/g. Chem. 246, 195 (1941). 6. Kruse 0. and Ericsson T., Phys. Chem. Minerals 15,509 (1988). 7. Kullerud G., Bell P. M. and England J. L., Carnegie Inst. Washington Yearb. 64, 197 (1965). 8. Fleet M. E.,-Amer. Mineral 53, 1‘846 (1968). 9. Tonel-Schadt J. and Muller W. F., Phvs. Chem. MAerals 8, 175 (1982). 10. Kruse O., Amer. Mineral 77, 391 (1992). 11. Horwood J. L., Townsend M. G. and Webster A. H., J. Solid State Chem. 17, 35 (1976). 12. Andresen A. F. and Torbo P., Acta Chem. Stand. 21, 2841 (1967). 13. Hafner S. and Kalvius M., Z. Krisrallogr. 123,443 (1966). 14. Goncharov G. N., Ostanevich Yu. M., Tomilov S. B. and Cser L., Phys. Star. Sol. 37, 141 (1970). 15. Gosselin J. R., Townsend M. G., Tremblay R. J. and Webster A. H., J. Solid Stare Chem. 17, 43 (1976). N. N.. Sidorov S. V. 16. Voitkovskii Yu. B., Razumovskava and Butyugin M. A.. Sov. Phyi. Crysrallogr. 23, 306 (1978). 17. Jernberg P. and Sundqvist T., Uppsala University Institute of Physics Reports 1090 (1983). R. and Trooster 18. van Dongen Torman J., Jagannathan J. M., Hyperfne Interactions 1, 135 (1975). 19. Yund R. A. and Hall H. T.. Econ. Geol. 64,420 (1969). ’ 20. Kruse O., Amer. Mineral 75, 755 (1990). 21. Ok H. N., Baek K. S. and Kim C. S.. Phvs. Rev. B 26. 5223 (1982). 22. Baek K. S., Park Y. S. and Ok H. N., Phys. Rev. B 30, 404 (1984). 23. Baek K. S., J. Magn. Magn. Mater 54, 925 (1986). 24. Goodenough J. B., J. appl. Phys. supplement to 33:3, I197 (1962). 25. Wachtershauser G., Microbial. Rev. 52, 452 (1988).
1. Phys. Chem. Solids Vol. 54, No. 1 I, pp. 1.593-l 597. 1993 Printed in Great Britain.
SPIN FLIP AND TROILITE-MnP TRANSITION IN FeS AS STUDIED SPECTROSCOPY
STRUCTURE BY Mi)SSBAUER
OLOF KRUSE~ Department of Mineralogy and Petrology, Institute of Geology, Uppsala University, Box 555, S-751 22 Uppsala, Sweden (Received 2 March 1992; accepted in revised form 7 June 1993)
Abstract-Synthetic FeS (troilite) was analyzed by Miissbauer spectroscopy in the temperature range 283 Q T d 460 K. Spectra were analyzed applying the full Hamiltonian. The a transition between the troilite and MnP-type structures was completed at 413 K on heating. Compared with literature data of a meteoritic troilite, the behavior differed as follows: the transition occurred without hysteresis, with a steeper slope in an MnP structure concentration ([MnP]) vs temperature plot, with a lower [MnP] at room temperature, and with heating and cooling trends coinciding. The spin flip in the MnP structure occurred rapidly and is suggested to be coupled to the ease of the a transition. The absence of trace elements, e.g. Cr, in the synthetic sample is suggested to be of importance to the rapid spin flip and a transition. A reported increase in compressibility of troilite with increased pressure is suggested to be due to a successive weakening of the Fe, cluster bond strength. The relatively scarce occurrence of electron transfer enzymes with reaction centres with three Fe atoms is suggested to be due to an evolutionary drawback caused by the possibility of forming energetically stable Fe, clusters unsuitable for reaction. Keywords:
MBssbauer, full Hamiltonian, FeS, a transition, spin flip.
1. INTRODUCTION Depending interrelated at ambient
on the temperature, FeS assumes different crystallographic and magnetic structures pressure. At room temperature, it occurs
as hexagonal troilite with space group P62c [l]. The troilite structure, which has been reported only for FeS [2], may be regarded as a NiAs structure derivative (Fig. 1) with the cell axis lengths a and c being ,/3 and 2 times those of the NiAs subcell axes, respectively [3]. In troilite, all Fe atoms are in triangular three-atom clusters forming planes perpendicular to the c axis. These clusters are suggested to be due to Fe-Fe bonds formed by 3d electrons if the distances between the involved atoms are below a critical value [4]. The clusters, and hence the troilite structure, cease to exist at the tl transition. The cluster breakup may be accomplished either by increased temperature, increased pressure, or by vacancies at Fe sites. For FeS at ambient pressure, the temperature for a completed transition into the orthorhombic MnP-type structure (Prima, [2]) has often been given as 413 K. However, the transition is gradual as shown by a weakening of the troilite supercell reflections of synthetic FeS after heating to and measurement at 375 K [5]. In meteoritic FeS, the situation is not fully reversible. The c( TPresentaddress: SecoTools AB, S-737 82 Fagersta, Sweden.
transition on heating is complete at 413 K, while the MnP to troilite transition starts at 410 K; a residual 20% of the Fe atoms may still be observed in the MnP structure after slow cooling to 280 K [6]. The transition temperature T. decreases if the pressure increases [7] and equals room temperature at 3.4 GPa [2]. The latter investigation indicated a negative value of the pressure derivative of the isothermal bulk modulus (K+ = -5(4)) for troilite, indicating this phase becomes more compressible with an increase in pressure. At ambient conditions, quenched samples of Fe,_,S compositions made by dry synthesis show troilite superstructure X-ray reflections for x < 0.047, but not for larger x values [3,8]. The latter investigation also showed sharp cell-parameter discontinuities at x x 0.047. On heating at ambient pressure, MnP-structured FeS transforms into the NiAs structure with space group P6,/mmc [9] at _ 508 K [lo]. Below the magnetic ordering temperature (TN z 600 K) the spins of the Fe atoms are antiferromagnetically coupled. Below the spin flip temperature T, the spins point along the NiAs-subcell c axis (cNiAs);above T, they are oriented in planes perpendicular to cNiAs[l 11. The spin flips occur only in the MnP phase [6]. Various values for Ts have been given, e.g. 445 K for susceptibility measurements on a synthetic Fe0,W6S single crystal [l 11, 458 K for neutron
1593
OLOF KRUSE
1594
at
g 2~
0-s
.=Fe
Fig. 1. FeS structures represented by atomic configurations. The c axis of the NiAs subcell is vertical. Left: the NiAs structure. The line connecting the Fe atoms is straight; the planes of S atoms are perpendicular to this line. Middle: the MnP structure. Compared to the NiAs structure the Fe-Fe lines and S-planes are tilted. Right: the troilite structure. All Fe atoms are members of triangular clusters forming planes perpendicular to the c axis. Every other Fe-Fe line is tilted with respect to E.
room-temperature
for
20
months
prior
to
measurements. Mdssbauer measurements were made in a vertical transmission set-up with a constant acceleration drive using a 57CoRh y source. The powder sample, containing 7 mg Fe cm-*, was kept on a horizontal Be disc in an evacuated furnace. The temperature could be measured within an estimated accuracy of f2 K. A total of 37 measurements were made in the range 283 < T < 460 K, where the temperature was both increased and decreased stepwise. The velocity scale was calibrated with an Fe foil at room temperature. The 512 channel mirror spectra were folded and then analyzed using a full-Hamiltonian program [ 171. Due to the thin absorber the spectral lines were assumed to be of Lorentz type; thus, no thickness correction was made. The quadrupole splitting Qs was defined as QS = 1/2V,eQ(l +rl*/3)“* [18]. The sample was analyzed before and after Miissbauer measurements by X-ray diffractometry at room-temperature using KC1 as internal standard. Peaks were measured at a goniometer speed of 20 = 0.001” s-l. Five KC1 peaks were used to calculate second-degree polynomials for corrections of d values. The composition of the sample was computed from the d value of the 102 peak by an expression giving a standard error of 0.002 in x [19]. The obtained values of x in Fe, _,S before and after heating runs were +O.OOl and -0.004, respectively. The computed values of cell parameters, using eight troilite peaks before and after heating runs, were a = 5.969(l), c = 11.760(4) and a = 5.968(l) A, c = 11.756(4) A, respectively.
diffraction on synthetic FeS [12], and the temperature ranges -410-470 K on heating and -450-360 K on cooling for a meteoritic sample [6]. The sluggishness of both the MnP to troilite structure transition and the spin flip in the meteoritic sample is in stark contrast to the corresponding situation in synthetic FeS where both transitions occur reversibly in narrow temperature intervals [l 11. The presence in meteoritic troilite of trace elements in solid solution has been suggested as being important to this difference [6]. The present investigation was undertaken in order to compare the behavior of synthetic FeS with that reported for meteoritic troilite. 3. RESULTS AND DISCUSSION It has been shown [13] that a good interpretation of Mijssbauer spectra of troilite requires considerA Mijssbauer spectrum with the sextets correation of the full Hamiltonian. However, also in later sponding to troilite and MnP structure is shown in investigations (e.g. [l&16]) spectra of synthetic FeS Fig. 2. Parameters for the sextets were obtained were interpreted according to a perturbation model. following [6]. Thus, spectra were fitted with a total of Thus, the magnetic hyperfine interaction has been three sextets representing troilite and the two spin assumed to be much stronger than the electric one, configurations of the MnP phase, i.e. MnP r and which is considered as a mere perturbation. The MnP -. The intensities I of these sextets were free to present investigation was also intended to yield full vary. The crystallographic positions of all Fe atoms Hamiltonian Miissbauer data on synthetic FeS at were assumed to be equally well defined and hence the elevated temperatures. half-widths w were constrained to be equal. The QS values for troilite were set negative. The Fe atoms in 2. EXPERIMENTAL the MnP phase were all assumed to have the same environment, except for the spin directions. The Samples were obtained by dry synthesis of Fe powder (99.999% purity, reduced at -900 K in H, centroid shift (CS ) and hyperfine magnetic field strength (&) values were hence constrained to the gas for 90 min) and S (99.9999% purity). Fusion was same values, respectively. The QS value was fixed performed at 823 K for 23 h in an evacuated glass ampoule. The reaction products were then milled same values, respectively. The Qs value was fixed at -0.25 mm ss’ and the 0 values (which here under toluene and again heated at 843 K for 15 days. equals the angle between the cNiAsaxis and B,,) to 0 The ampoule was then slowly cooled and kept
Spin Sip and troilite_MnP structure transition in FeS
I
1595
I
I
I
I
I
I
b ,
-6
-4
-2
0
2
4
6
Fig. 2. Mtissbauer spectrum of synthetic FeS at 410 K. The transmission is given in per cent vs y source
velocity in mm s-‘. Dots indicate data points, the solid line a fitted function. The troilite structure is represented by the broader sextet, the MnP structure by the narrower one. The relative intensities of the sextets are 49 and 51%, respectively.
and 90”. No value of the asymmetry parameter q = (V,, - V,)/V, can be deduced for the troilite and MnP structures. However, for symmetry reasons q equals zero in the high-temperature NiAs structure. Since no significant discontinuities occur in the parameters CS, QS, or Bbl at the NiAs-MnP structure transition, q may be assumed to have the value zero also in the MnP structure [6]. In fitting troilite spectra, variations in q have been shown to have only minor influence on the obtained parameters [6]. Therefore, q was fixed to the value zero also for troilite. The small differences in composition and cell parameters obtained before and after Miissbauer measurements were assumed to be negligible. The obtained values for the parameters CS, B,, and QS are given in Table 1. The corresponding results for natural FeS given in [6] are almost identical. Hence these parameters cannot be the base for a discussion of the differences in reaction kinetics between natural and synthetic FeS. The obtained concentration of MnP phase, [MnP], as a function of temperature in the range 283415 K, is given in Fig. 3. A comparison with the corresponding plot for a natural sample with a composition close to Fe,.,,Cr,,,,S [6] shows the following differences: (1) The heating and cooling curves are indistinguishable for the synthetic sample, whereas natural FeS when heated for the first time shows [MnP] increasing linearly from zero at 405 K to 100% at 413 K and decreasing slowly to an apparently stable value of 20% on cooling. In the present case both starting and end concentrations are close to 10%. (2) For the natural samples, the troilite to MnP transition was completed at 413 K, whereas the reverse transition started at 410 K. In the present case, an extrapolation gives a completed transition at 413 K, but no hys-
teresis. (3) The change in [MnP] on cooling is more accentuated in synthetic FeS, where a 50% value is reached at 409 K. In the natural samples, the corresponding temperature was 396 K. The assumption that the measured difference in chemical composition is negligible is supported since the obtained [MnP] has the same starting and end values. However, the Miissbauer parameters for Fe atoms in the MnP structure are similar to those of troilite-structure Fe atoms with an Fe vacancy as a nearest neighbor [20). The obtained sextet may thus be assigned to Fe atoms in two different environments. The relative intensity due to vacancies, I,, can Table 1. Values of Miissbauer parameters CS, B,,r,and f&S for synthetic FeS in the temperature range 283 5 T 5 460 K. A total of 37 spectra were measured; data for the excluded spectra do not deviate from the trends of the listed ones. The obtained parameter values were seemingly unaffected by heating history. “tr” and “MnP” indicate sextets representing troilite and MnP-type structure, respectively. The sign ” !” indicates that the value has been a fixed input in the computations T(K)
Sextet
283 313
%nP tr MnP
335 352 368 387
GnP tr MnP tr MnP tr MnP
406 432 449 460
%nP MnP MnP MnP
CS (mm s-‘)
E,,,(T)
0.76 0.67 0.74 0.66 0.73 0.70 0.71 0.67 0.70 0.69 0.69 0.66 0.68 0.69 0.68 0.68 0.66
31.2 27.7 30.8 26.7 30.5 26.5 30.3 26.3 29.9 26.1 29.5 25.4 28.9 24.9 23.5 22.5 21.8
QS (mm s-r) -0.88 - 0.25! -0.88 -0.25! -0.87 -0.25! -0.86 -0.25! -0.86 -0.25! -0.85 -0.25! -0.85 -0.25! -0.25! -0.25! -0.25!
OLOF
1596 . . . . . . i
E ..
.Z OI
280 300 320 340 360 380 400 420 T(K)
Fig. 3. Relative MnP sextet concentration in synthetic FeS vs temperature. be computed as Z, = 100 x 2x/( 1 - x)% [20]. For the present starting value x = 0.001, Z, = 0.2%. It can hence be concluded that the influence of vacancies is negligible in the MnP sextet. The presence of the high-temperature MnP phase in the initial run at room temperature indicates that the sample had not attained equilibrium. Since [MnP] reaches the same value after measurements it can be assumed that the intended equilibration at room temperature for 20 months had negligible effect. For natural FeS the a transition and its connection to the spin relations has been discussed [lo]. The sluggishness of the MnP to troilite structure transition was suggested to be due to structural incoherence between the phases below 413 K and the preferred Fe spin orientation in the MnP structure being perpendicular to cNiAsand thus to the orientation necessary for the transition. In Table 2 the present obtained relative intensities of the three fitted subspectra are given. These correspond to the relative concentrations of troilite phase and to the two spin configurations, -+ and t, of the MnP structure. The Table 2. Relative intensities I of subsnectra for synthetic FeS in the temperature range 283 5 T 5 432 K. ltrl = I of troilite sex&t in per cent; [M&f] = I of MnP sextet for magnetic spins of Fe atoms parallel tet
to eNiAS;[MnP+
= I of MnP sex-
for spins perpend&lar to cNiAI
T(K)
[tr]
[MnPf]
[MnP-+]
283 395 404
91 80 70
2 3 5
7 17 26
408 410 412 415 432
59 49 24 0 0
4 8 12 11 0
36 43 64 89 loo
KNSE
results on heating and cooling are indistinguishable and thus dependent on measuring temperature only, whereas for a meteoritic sample [6] spin flips were detected in differing intervals on heating and cooling; thus, the relative concentrations were dependent both on measuring temperature and heating history. The ease of the spin flip here, being complete at 432 K in a series of successive heating, contrasts with a meteoritic sample [IO] where the spin flip was still uncompleted after 23 days at 429 K and associated with an activation energy E, of -2 kJ mol-‘. The rapid reaction and the considerably lower temperatures required in the synthetic sample indicates a much smaller Ea. The value of E, could, however, not be measured since the changes in spin configuration concentrations were too rapid to be detected by the method used. Moreover, the absence of hysteresis for the reverse spin flip indicates both a small E,, and that the concentrations of the two spin configurations are functions of temperature alone. Although difficult to detect due to the slow reaction rate, this might also be the case for meteoritic FeS at equilibrium. The present similarities on heating and cooling for the spin concentrations may affect also the [troiliteE [MnP] relations through the spin orientation-a transition coupling mentioned above. The absence of hysteresis in the a transition, the steepness of the d[MnP]/dT slope, and the lower [MnP] after hightemperature measurements in synthetic FeS would hence be coupled to the easy spin flip on cooling. Miissbauer studies on the influence of V, Co, and Cr substituting Fe in synthetic FeS have been reported [21-231. For Fe,,,975 Co,,,,,, S and Fe,,g,, V,,,, S a T, of 400 K was obtained [23]; for Fe,,,,Cr,,,,,S TX was given as 384K [21]. Thus, the a transition is affected by the presence of these elements. For Fe,,V,,, S a hysteresis-free transition at 390 K was reported [22], with [MnP] leveling off at 20% at room temperature. For Fe 0,975V0,025 S a hysteresis of 4 K was observed (400-396 K), but also a rapid variation in [MnP], being zero at 375 K. No corresponding results were given for Cr. However, since the mentioned elements do affect the transition it is likely that the hysteresis and sluggish transition reported for the meteoritic sample above are coupled to Cr and the lower concentrations of other elements, e.g. Co. An increase in the a-axis length and a corresponding decrease in c with increasing Cr concentration was reported by [21]; the cell volume was however approximately constant. This indicates a successive weakening of the cluster bond strength in the a-a plane. Thus, lower temperatures for the formation and break-up of the clusters would be required, which is consistent with the T, values reported for FeO.grrCr,,r, S.
.
Spin flip and troilite-MnP
2.9.5
n
. n
Ab Y
2.90
2.85
0
1
2
transition
in FeS
1597
likely that the effect is caused by the collective electron clouds associated with the clusters, resulting in repulsion along c.
.
z -0
structure
. .
3
P (GPa) Fig. 4. Interatomic distances d in synthetic troilite vs hydrostatic pressure. Squares: d between Fe atoms directly above/below one another. Triangles: d between Fe atoms obliquely above/below one another. Crosses: d between Fe atoms of the same cluster. Data from [2].
The nature of the troilite clusters and the mechanism of their formation have been discussed by Goodenough [4,24] who suggested cluster bonding to be due to delocalization of the sixth, minority spin electron of the involved Fe atoms. The literature data on the stability in (P, T, x) space for the troilite structure is reviewed above. Despite both experimental results and theoretical considerations, little effort has been spent on a general discussion of the transition to/from the troilite structure. A successive increase of the thermal expansion of a on heating in meteoritic troilite has been reported [lo], indicating a gradual weakening of the cluster bond strength. The critical Fe-Fe distance for the breakup of clusters was 2.93 A at normal pressure. Measurements of the Fe-Fe spacings in Fe, _,S vs x [20] showed the distances between atoms obliquely above or below one another (Fig. 1, right) to become smaller than even within the cluster as x increased. This would disturb and eventually destroy the cluster bonds, causing the a transition. Figure 4 shows Fe-Fe distances in synthetic troilite vs hydrostatic pressure at 294 K based on literature data [2]. Also here the distances between obliquely above/below Fe atoms become successively smaller, decreasing below the 2.93 A mentioned above. Thus, the cluster bonding is indicated to be disturbed prior to the final breakup, e.g. by a spilling over of electrons from the collective orbitals of the clusters to Fe d,2 orbitals interacting along cNiAs . Such a gradual weakening would explain the negative K+ discussed in [2]. The larger distances between Fe atoms straight above/below than between those obliquely above/below were suggested to be due to repulsion between Fe atoms. However, it is more
Clustering in compounds containing Fe and S is known also in biomolecules, where Fe-S clusters with mostly one, two, or four iron atoms are active cores in the electron transfer enzymes in most electron transport chains in metabolism [25]. The low frequency of 3-Fe cores may be connected to the possibility of forming clusters with stable electron configuration as exemplified in troilite. The strong bonding in such a configuration would require a relatively large E, for enzymatic reactions; this might have been an evolutionary drawback. The high stability connected to an Fe, cluster may be related to the arrangement of occupied and empty cluster energy levels associated with this particular surrounding.
Acknowledgemenfs--I wish to thank the following people for helpful guidance: Jean-Louis Calais for fruitful discussions and for reading through the manuscript; drjan Amcoff, Tore Ericsson and Hans Annersten for critical reviews; and Miss Kersti Gloersen for the language check.
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