Morphological investigations on iron potassium sulfide KFeS2: grinding effect on thermal behavior

April 1995

EISEVIER

MaterialsLetters23 (1995) 133-138

Morphological investigations on iron potassium sulfide KFeS,: grinding effect on thermal behavior And.& Galembeck, Oswald0 Luiz Alves * Laborathio

de Quimico do Estado So’lido. Institute de Quimica - UNICAMP, Caixa Postal 6154, CEP 13081-970, Campinas, SP, Brazil

Received26 October 1994;in final form 27 December 1994; accepted 6 January 1995

Abstract Fe& (pyrite form), K2S04,K$e( SO,) 3 and cx-Fe*O,are the products of thermal decomposition of crystalline KFe&, from room temperature up to lOOO”C,in air. Grinding of iron potassium sulfide prior to heating leads to a modified reaction pattern; the decomposition products are the same, but their formation temperatures are considerably lower than in the case of the unground crystals. Morphological features of the thermal reaction products are also considerably modified by KFeSz grinding.

1. Introduction

Transition-metal compounds forming extended structures mainly in olne direction and containing paramagnetic ions are of considerable interest as they show unusual physical properties. Chain compounds such as AFe& (A= K, Rb, Cs) are used as model cases for demonstrating relationships between the crystal structure and magnetic properties [ 1,2]. KFeS2 linear chains are built by edge-sharing FeS, tetrahedra extended along the c axis, K atoms lying between these chains. As the intrachain separation of Fe atoms (2.70 A) is much shorter than interchain separation (6.50 A), there is an antiferromagnetic iron-iron interaction within the chain; this solid is a model of a quasi-nnidimensional antiferromagnet [3,41. These features turned this compound into an interesting subject of research in widely separated areas like biochemistry, magnetism, electrochemistry, molecular physics and topochemistry [ 1,2,5-l 31. Nevertheless, * Corresponding

author.

0167-577x/95/$09.50 0 1.995 Elsevier Science B.V. All rights reserved SSDI0167-577x(95)00020-8

little work was done on its chemical behavior, stability and reactivity. In this work, we report a study of thermal behavior of KFeS* when heated under air atmosphere and of the morphological changes caused by heating this solid. We also observed an important effect associated with crystal grinding on its thermal decomposition.

2. Experimental

procedure

2.1. Preparation of KFeS2

Polycrystalline KFe$ was prepared from analytical grade metallic iron (Sigma), sulfur (Riedel), K&O3 and Na2C03 (Merck), following the method of Deutsch and Jonassen [ 141, with a 80”C/h cooling rate. Needle-shaped (up to 2.0 cm length) crystals were obtained and identified by X-ray diffraction (XRD) . 2.2. Heat treatment The samples (0.25 g) were heated under room atmosphere at temperatures ranging from 100 to

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lOOO”C,within a horizontal furnace for 2 h and cooled to room temperature. Further heating up to 24 h did not lead to any additional change. This procedure was the same for both ground (in a glass mortar, 100 mesh) and unground crystals (0.1 to 2.0 cm long). They will be designated as (g) KFeS,/X and (u) KFeS,/X, for ground (g) and unground (u) samples respectively; X is the heating temperature ("C) .

Letters 23 (1995) 133438

r

2.3. Physical measurements XRD patterns were obtained using a Shimadzu, model XD-3A, diffractometer utilizing Ni filters and Cu Kcuradiation obtained using 30 kV and 20 mA, at 2”/min scan rate. The measurements were carried out on samples spread on a conventional glass sample holder, at room temperature. Infrared spectra were recorded in the 4000-400 cm-’ region using Nujol dispersions between alkali halide windows, in a Perkin-Elmer 1600 Fourier transform spectrometer. Scanning electron microscopy (SEM) micrographs were obtained in a JEOL, model JSM T-300, microscope, from gold-coated dispersed powder samples.

1

l( 1

I

20

30 28

3. Results and discussion As all products obtained by heating ground KFeS, showed a large number of more intense features on the XRD diagrams, which made easier their identification, these results will be presented first. 3. I. X-ray diffraction Grinding KFe& changed its XRD pattern by diminishing and broadening the peaks, which effects are commonly associated with lower crystallinity of the ground solid as compared to the unground crystals. Besides, no new reflections were observed, suggesting that no further structural changes of the solid were caused by the mechanical effect (grinding). X-ray diagrams of the heated ground KFe& samples are shown in Fig. 1. The pattern of the non heat-treated sample remains intact up to near 3OO”C,when the first new peak appears (Fig. la). At 320°C an almost complete breakdown of the structure takes place leading to at least two new prod-

I

I

40

50

6

t DEGREES)

Fig. 1. XRD patterns for sample (g) KFe$, heat-treated for 2 h at (a) 300, (b) 320, (c) 350, (d) 4OOand (e) 700°C. (0) Fe& (0) K,SO,, ( + ) K,Fe(SO,,), and (a) a-Fe20,.

Many peaks can be related to the arcanite form of K,SO,. A third group of lines, probably related to K,Fe(SO,), [ 151 appeared at 350°C as shown in Fig. lc. One set of peaks (marked with the (0) symbol), observed at 320-35O”C, could not be clearly identified. We tried to separate this portion by dissolving I&SO, with water, filtering and drying the insoluble portion of the (g) KFeS2/320 sample. A brown solid with all the XRD unidentified peaks on KFeS2/320 was obtained. Its pattern is very close to that reported in the literature for the pyrite form of FeS, [ 151, This was confirmed by the IR spectrum which showed an absorption near 420 cm-’ assigned to v( Fe-S), already described for pyrite [ 161. The unexpected detection of FeS, may be explained by the fact that KFe& possesses a very close Fe-S packing and then the chains [Fe&]:may suffer a partial oxidation as follows: [Fe&] E- + nFeSz + ne-, ucts.

A. Galembeck, O.L. Alvez/Materials Letters 23 (1995) 133438

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which cannot extend throughout the whole chain, due to the competing oxiclation to K2S04. Heating to 400°C causes Fe& to transform to (YFe203 as shown in Fig. Id. Further heating causes significant modification, observed only at 7OO”C,when K,Fe( SO,) 3 decomposes into K2S04 and a-Fe203.

Table 1 IR frequencies ( 13OCMOOcm-‘) for (g)KFeSr thermally at different temperatures

3.2. Infrared spectroscopy

1150sh 1109s

IR spectroscopy m’easurements are presented in Fig. 2 and summarized in Table 1 [ 18,191. KFeS2 does not show absorption between 4000 and 400 cm-’ but all compounds resulting from its thermal decomposition present important features in the 1300-400 cm-’ region. All spectra confirm our earlier statements about K2S04 and FeS2 identification at 320°C (Fig. 2a), transformation of pyrite near 400°C (Fig. 2c), formation of K,Fe( SO,):, at 350°C and its subsequent decomposition at 7OCPC(Fig. 2d). The most important structural information provided by IR spectroscopy refers to (g) KFeSJ350, where two active modes of the tetrahedral sulfate ion, v3 and v4, are split into three bands and the other two modes vi and y became active. This phenomenon is a typical case when a lowering of symmetry proceeds from Td to CZV[ 171, which seems to be the symmetry of the

Wavenumber (cm-‘) 320°C

983sh

617s

423m

samples treated

Assignment a

350°C

400°C

1238m 1158sh 1121s 1021w 985w 648~ 634~ 617s 592m

1237m 1146sh 1119s 1021w 986~

473w 417m

471w

634s 617s 592m 554s

1144sh 1113s

so:-

(VA

981sh

so:-

(v,)

617s I 560s 475s >

SOY(G)

v( Fe-O) So:- (vz) v(Fe-S)

a Y,: symmetric stretching mode; v2: out-of-phase deformation; g: asymmetric stretching mode; v.,: in-phase deformation. sh: shoulder; w: weak, m: medium; s: strong.

SOi- ion in K3Fe( SO,), and occurred concomitantly with the identification of this compound by XRD. 3.3. Unground KFeS, The products of thermal decomposition of unground KFe$ were identified by the same procedure as described in the previous sections. They are the same as detected for the ground solid, but the temperature ranges are quite different. The formation of the mineral yavapaiite KFe( SO,), and of K2S203, reported earlier in an oxygen atmosphere was not observed [ 201. Table 2 shows a comparison of the thermal behavior between (g) and (u)KFeS,. It is important to note the large difference between ground and unground samples at the start of decomposition, near 180°C. Another remarkable feature is Table 2 Temperature range (“C) of formation of the decomposition products of KFe!$ for ground (g) and unground (u) solid

1200

1000

I 600

WAVENUMBER

600

400

(cm-‘)

Fig. 2. IR spectra for the !sample (g)KFe$, (a) 320, (b) 350, (c) 400 and (d) 700°C.

heat-treated for 2 h at

Compound

(g)KFcS2

(u)KFcS2

Fe& &SO, &Fe(SO,)r o-FeO,

320- 350 320-1000 350- 600 400-1000

500 500-1000 600- 700 550-1000

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that FeS2 forms to a small extent in the unground ple, as denoted by the small peaks on its XRD.

sam-

3.4. Scanning electron microscopy Figs. 3 and 4 show scanning electron micrographs of unground and ground KFe&, at different stages of heating. Unground KFeS, is shown in Fig. 3A. The crystals show a highly ordered fibrous morphology consistent with the quasi-unidimensional character of this solid. The fibers are oriented along the c axis and are very thin (2.0 to 7.6 pm wide). The solid can be heated up to 400°C without losing its fibrous nature (Fig. 3B). At 550°C (Fig. 3C), cylindrical holes are seen on the surface of the crystal, probably associated with oxidation of the crystals. Fig. 3D is a 1 cm long crystal, heated at 700°C that retained its shape even

Fig. 3. SEM micrographs

for the sample (u)KFeS,

Letters 23 (1995) 133-138

though other morphological features are quite different from the pristine material. It is clear that at this temperature, oxidation of the material is extensive. Grinding causes a collapse of the crystal morphology (Fig. 4A) and the original stacking is destroyed, leading to a large increase in the surface area and crystal defects. Heat treatment of the ground sample under air atmosphere causes a further modification in the morphology. At 320°C the particles tend to aggregate and crystallites seem to sinter (Fig. 4B). At 4OO”C, further aggregation occurs and larger porous particles are observed as in Fig. 3C. Finally, at 700% two morphologically different domain types are clearly distinguished; one is compact with larger blocks and irregular surface ( K2S04) ; the other has regular geometric shapes, cubic or nearly hexagonal (0.6 to 2.2 pm across) corresponding to a-Fe203. This was identified

(a) without heat treatment; heat-treated

for 2 h at (b) 320, (c) 400 and (d) 700°C.

A. Galembeck, O.L. Alvez/ Materials Letters 23 (1995) 133438

by extracting KZS04 with water and isolating only the insoluble iron oxide. The final product of (u)KFe& obtained at 800°C shows the same morphology. These results show that grinding lowers the thermal stability of KFe& under air. It is well known that solids may be kinetically stalbilized due to a slow rate of nucleation of the crystalline reaction product; for this reason the larger the number of defects in the crystal, the greater its reactivity, other conditions being equal. Boldyrev [ 2 1] pro’posed that all thermal decomposition reactions may be placed into two groups. The first one includes chemical reactions in which rupture and formation of new bonds occur locally. The second group covers reactions in which charges are transferred within a path length much longer than the interatomic distances, the charges being carried by electrons or ions. The present case seems to belong to the first group,

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starting at the surface and being favored by the large increase in the surface/volumeratio provided by grinding. We propose that grinding KFeSZcrystals lowers their chemical stability towards thermal treatment for the following reasons: both crystals and surface defects are generated by mechanical stress. These are active centers, in which reactivity and mass transfer are enhanced at higher temperatures, as compared to the unground crystals. New phases (reaction products) may nucleate at these centers much easier than within the wellformed, stable pristine crystals. Further, many crystal planes other than the original ones, can be created by grinding, and probably match some planes of the reaction products and consequently play an important nucleating role for the whole process.

Fig. 4. SEM micrographs for the sample (g)KFeS* (a) without heat treatment; heat-treated 2 h at (b) 400, (c) 550 and (d) 700°C.

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Also, despite the formation of phase mixtures in all of the steps of degradation or decomposition (as evidenced by XRD), we could not observe different morphological domains in the SEM micrographs until the oxidation reaches completion, for both the ground and unground samples.

4. Conclusion Thermal decomposition of iron potassium sulfide under air atmosphere occurs in steps. Fe& (pyrite), K2S04, K3Fe( SOJ3 and cr-Fe203 were identified as the reaction products. Grinding the solid KFeS2 prior to heating decreases its thermal resistance, probably by increasing the surface area, by causing defects on the crystal structure and by creating new surface planes for nucleation. Besides, all morphological changes caused by oxidation are strongly dependent on the morphology of the starting KFeS2.

References [ 1] M. Nishi and Y. Ito, Solid State Commun. 30 ( 1979) 57 1. [2] W. Bronger, A. Kyas and P. Mllller, J. Solid State Chem. 70 (1987) 262.

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[ 31 J.W. Boon and C.H. MacGillavry, Reel.: Trav. Chim. Pays Bas 61 (1942) 910. [4] D.C. Johnston, S.C. Mraw and A.J. Jacobson, Solid State Commun. 44 ( 1982) 255. [5] D.O. Cowan, G. Pastemak and F. Kaufman, Proc. Natl. Acad. Sci. (USA) 66 (1970) 837. [6] W.V. Sweeney and R.E. Coofman, Biochim. Biophys. Acta 286 (1972) 26. [7] D. Raj and S.P. Puri, J. Chem. Phys. 50 (1969) 3184. [8] C.A. Taft, D. Raj and J. Danon, J. Phys. Chem. Solids 36 (1975) 283. [9] J. Zinkand K. Nagomy, J. Phys. Chem. Solids49 (1988) 1429. [lo] A.J. Jacobson, M.S. Whittingham and S.M. Rich, J. Electrochem. Sot. 126 (1979) 887. [ 111 A.K. Pant and E.D. Stevens, Phys. Rev. B 37 ( 1988) 1109. [ 121 J.W. Boon, Reel.: Trav. Chim. Pays Bas 63 (1944) 69. [ 131 H. Boiler, Monatsch. Chem. 109 (1978) 975. [ 141 J.S. Deutsch and H.B. Jonassen, Inorganic syntheses (McGraw-Hill, New York, 1960). [ 151 Powder DiffractionFile, Joint Commiteeof PowderDiffraction Standards. [ 161 H.D. Lutz, Cl. Kliche and H. Haeusler, Z. Naturforsch. 36a (1981) 184. [ 171 K. Nakamoto, Infrared spectra of coordination compounds, 3rd Ed. (Wiley Interscience, New York, 1978). [ 181 B. Gillot, P. Bouton, F. Chassangneux and A. Rousset, I. Solid State Chem. 33 (1980) 245. [ 191 El-Kababany, G. Said, Y. Badr and S. Taha, Phys. Stat. Sol. 67a (1981) 339. [ 201 NC. Furtado, C.A. Taft and J.O. Cassedane, J. Mater. Sci. 24 (1989) 2751. [21] V.V. Boldyrev, J. Therm. Anal. 40 (1993) 1091.