Raman spectra of MnPS3 intercalated with pyridine

0038-1098/88 $3.00 + .00 © 1988 Pergamon Journals Ltd.

Solid State Communications, Vol. 65, No. 5, pp. 419-423, 1988. Printed in Great Britain.

RAMAN SPECTRA OF MnPS3 INTERCALATED WITH PYRIDINE M. Hangyo, S. Nakashima and A. Mitsuishi Department of Applied Physics, Faculty of Engineering, Osaka University, Yamada-Oka 2-1, Suita 565, Japan and K. Kurosawa and S. Saito Department of Electronics, University of Osaka Prefecture, Mozu-Umemachi, Sakai, Osaka 591, Japan

(Received 28 August 1987 by J. Kanamori) Raman measurements of layered MnPS3 single crystals intercalated with pyridine have been made to understand the intercalation mechanism and the interaction between the host and guest. It is found that the spectrum in the frequency region above 200cm 1 is approximately the superposition of the lines corresponding to the host lattice and to the internal vibrations of the guest molecules. The splitting of some of the internal modes of pyridine indicates that pyridine between the host layers takes two different configurations; one corresponding to the physisorption and the other the chemisorption. INTRODUCTION METAL PHOSPHORUS trichalcogenides of the formula MPX3, where M represents metal ion and X represents S or Se, form a large and variegated class of compounds [1]. Most of them crystallize in layered structures similar to those of transition metal dichalcogenides. These compounds can be intercalated with ions or organic molecules between the layers (van der Waals gap) as well as the transition metal dichalcogenides [2]. The charge transfer (CT) mechanism from guests to hosts has been widely accepted as a driving force for the intercalation reaction in these materials [2]. Sch611horn et al. have proposed a model for the 2H-TaS2-pyridine system that 4,4'-bipyridine and pyridinium are formed as a result of the CT and the ionic bond binds the guest and host rather tightly [3, 4]. The same mechanism was suggested for the intercalation reaction of MPX 3 [1]. R'aman spectra will give much information on the intercalation mechanism and the interaction between the host lattice and guest molecules. Mathey and coworkers insist from the analysis of the Raman and infrared spectra that metallocenes intercalated into MnPS3, CdPS 3 and ZnPS3 take cationic forms [5-7]. In this Communication, Raman spectra of layered MnPS3 intercalated with pyridine, which is expressed by the formula MnPS3 (pyridine)4/3, have been measured and compared with the spectra of pure MnPS3 and pyridine. The high frequency portion ( > 200 cm- J) of the spectrum of MnPS3 (pyridine)4/3 is 419

interpreted as the superposition of the lines corresponding to the host lattice and to the internal vibratons of the guest molecules. On the other hand, the spectrum of MnPS3(pyridine)4/3 below 200cm -~ is considerably different from that of MnPS3. No traces of lines corresponding to bipyridine or pyridinium are found in the spectra. Some of the lines corresponding to the internal modes of pyridine split into doublets in MnPS3(pyridine)4/3. By comparing the frequency of these modes with those of pyridine in various states, it is concluded that there exist two different types of pyridine in the van der Waals gap; one corresponding to the physisorption and the other the chemisorption. EXPERIMENTAL Single crystals of MnPS3 were grown by the method described in [8]. The plate-like crystals with a color of transparent green were obtained. Typical size of the crystals was about 5 x 5 x 0.1 mm 3. The intercalation of pyridine was carried out by the following procedure: The single crystals of MnPS3 were sealed in an evacuated glass ampoule with liquid pyridine distilled in vacuum. Then, the ampoule was heated to 60°C and kept at this temperature for one or two weeks. The intercalation reaction occurs even at room temperature. No significant color change was observed after the reaction for both crystals and liquid. The intercalation compounds were characterized by the X-ray diffraction and thermogravimetric analy-

420

MnPS3 I N T E R C A L A T E D W I T H P Y R I D I N E TGA

Vol. 65, No. 5

i.. ,.~s~

-%"t

E4 I--r

MnPS3(pyridine)4/3

:

.~

237

: ',

274 37755

"',

o

I

X110

"

MnPS3(pyridine)t,/3

~

221

556 583 624

b

:,.,,,'.,,~,,,'~,'~.11\ ill\

~

A'~~ t

2 t-- 4

I

_Z

RT

3

o

6

,6o

26o

36o

/.880 A

~6o

~80 154

273

TEMPERATURE (°C)

Fig. 1. T G A curve for MnPSa(pyridine)4~3. 1

sis (TGA). The (00 1) diffraction series showed that the interlayer spacing is 12.6 A, being larger than that of the pure MnPS3 crystal 6.1 A. The T G A was obtained using a D u p o n t Model 990 Thermal Analyser at a heating rate of 5°C rain- J in a N2 atmosphere of the 65 ~ 90 ml min-~ flow rate. A typical weight vs temperature curve is shown in Fig. 1. The result shows a gradual decrease above room temperature (RT), which indicates that the deintercalation of pyridine occurs gradually in the temperature range of 20 ~ 140°C. The curve shows a sharp decrease at 140°C. It reaches a constant value above 150°C because of the complete deintercalation. The deintercalation was confirmed by the fact that the interlayer spacing goes back to the original one, 6.1 A, after the T G A measurements. The composition of the intercalation compound determined was MnPS3(pyridine)L35 from a loss of weight (36.9 wt %), which is very close to MnPS3 (pyridine)4/3 and we use this notation for pyridine-intercalated MnPS3. R a m a n spectra were excited by the 4880 A line of an Ar + ion laser. The spectra were measured in the quasi-backscattering configuration on the c'-face (basal plane). The scattered light was analyzed using a Spex 1403 double m o n o c h r o m a t o r in conjunction with a conventional photon counting system. The spectral resolution was c.a. 2 c m ~. RESULTS AND DISCUSSION Figure 2 shows the R a m a n spectra of MnPS3(pyridine)4/3 and MnPS3 in the wavenumber region below 700cm J measured at RT. The spectrum of MnPS3(pyridine)4/3 does not depend on the position of the sample. This fact indicates that pyridine is intercalated uniformly in the host crystal. As seen in the

0 --

200

400

600

RAMAN SHIFT (crff 1)

Fig. 2. R a m a n spectra of MnPS3(pyridine)4,3 and MnPS3 below 700 cm i measured at RT. The broken curve shows the spectrum corrected for (n + 1).

figure, the overall features in the region 200 ~ 600 cm ~of MnPS3 are retained after the intercalation, especially one-to-one correspondence being apparent in the region 200 ~ 400 cm 1. The shifts of the lines by the intercalation are less than 10cm except for the lines in the region 500 ~ 600 cm- ~. The lines at 567 and 580cm-~ of MnPS3 appear to split into three lines peaked at 556, 574 and 583 cm ~. The lines at 624 and 652 c m l are assigned to the modes corresponding to the internal vibrations of pyridine since there are no first order R a m a n lines above 600cm ~ for MnPS3. The structure below 200cm l is changed significantly by the intercalation. The intensity of the 154cm l line for MnPS3 becomes very weak after the intercalation. In the spectrum of MnPS3(pyridine)4.3, new unresolved bands appear below 150cm t. The broken line in Fig. 2 is the spectrum corrected for (n + 1), where n is the Bose factor. A detailed structure analysis of MnPS3 has been performed recently by Ouvrard et al. [9]. The crystal has a FePS~-type monoclinic structure (C2j,) that deviates slightly from the hexagonal symmetry [10]. Each layer in MnPS3 is regarded as consisting of the Mn -~+ cations and the hexatiohypodiphosphate anions P2S~ , which are combined with each other by the M n - S bonds. The M n - S bonds are thought to be weak compared with the bonds in the PzS~ units

Vol. 65, No. 5

MnPS3 I N T E R C A L A T E D W I T H P Y R I D I N E

421

¢l because in some cases the metal ions are expelled from MnPS3(pyridine)4/3 i the MPS 3 layer during the intercalation reaction (sub] 3 4880 A RT Xlo lx~ stitution-intercalation reaction) [11-14]. Accordingly, it is allowed to divide the vibrational modes of MnPS3 into the internal modes of the ethane-type P2S4- units > - 0 I-and the external modes corresponding to the transla- ~4 z tion and libration o f Mn 2+ and P2S4- [5]. Since the z deviation from the hexagonal symmetry is small, the 2 vibrational modes are classified based on the ap0 proximate layer symmetry of D3d. The primitive unit 600 800 i000 ~200 lz,00 1600 3000 3200 RAMAN SHIFT (cm-I) cell of MnPS3 contains two formular units and the vibrational modes at the F point are decomposed into Fig. 3. Raman spectrum of MnPS3(pyridine)4/3 in the the following irreducible representations: F = 5Alg region 500 ~ 3200cm -~ measured at RT. Raman -k- 2AEg k- 5Eg 'F A~u Jr 4AEu k- 5Eu (O3d). Among spectrum of liquid pyridine is also shown for comthese modes, three A~g and three Eg modes are the parison. Raman active internal modes and two Eg modes are the Raman active external modes. The polarization measurements show that the of solid pyridine ( 1.18 Mg m - 3) [ 15] and several broad strong lines of MnPS3 peaked at 245, 383 and lines corresponding to the lattice modes are observed 580 cm-~ have the AI~ character (polarized) and those below 160 c m - ~for pyridine solidified at high pressure at 223, 273 and 567 cm-~ have the Eg character (de- [16]. polarized). These modes are assigned to the internal Figure 3 shows the Raman spectra of MnPS3(pymodes of the P2 S~- unit which are expected from the ridine)4/3 and liquid pyridine in the wavenumber refactor group analysis. The low frequency lines peaked gion 500 ~ 3200 cm -t measured at RT. Since there at 114 and 154 cm ~ have the Eg character and are are no first order Raman lines above 600 cm l in the assigned to the external modes. The fact that the MnPS3 spectrum, the lines at above 600 cm-~ are asfrequencies of the lines in the region 200 ~ 600 cmcribed to the internal vibrations of the intercalant. As do not shift significantly by the intercalation indicates seen in the figure, two spectra are essentially the same that the bonds in the P284 unit are little affected by except for fine structures. The wavenumbers of the the guest molecules in the gap. On the other hand, the lines of MnPS3(pyridine)4/3 above 600cm i (RT and remarkable intensity change of the 154cm-~ line, LHeT) are summarized in Table 1 together with those which may be associated with the translational motion of liquid pyridine. of the Mn 2+ ions, indicates that the Mn-S bonds are According to Sch611horn et al. [3, 4], the reaction considerably modified by the intercalation. This is product of the 2H-TaS2-pyridine system is formulated reasonable if we take into account the fact that in a s some cases the metal ions are expelled from the MPS3 layers during the intercalation reaction owing to the (pyr)0.5-2x(pyr. H ÷ )x (pyr-pyr)x/2 (TaS2)X , weak M - S bonds. There is a possibility that Mn 2+ with x - 0.2, where pyr, pyr-pyr and pyr. H ÷ denote ions are expelled from the layers in MnPS3(py- pyridine, 4,4'-bipyridine and pyridinium, respectively. ridine)4/3. However, this is denied because the whole The value x represents the amount of the CT from the spectra in the region 20 ~ 700 cm l before the inter- guest to the host layer per one TaS2 unit. From the calation and after the complete deintercalation are the careful inspection of the spectrum, it is concluded that same, indicative of the reversibility of the reaction. there are no lines corresponding to pyridinium and There are possible explanations for the origin of 4,4'-bipyridine in the spectrum of MnPS3(pyridine)4/3 the broad bands below 150cm ~ in the MnPS3(py- [17, 18]. This result demonstrates that the intercalaridine)4/3 spectrum. First, they are the disorder ac- tion mechanism proposed for the 2H-TaS2-pyridine tivated acoustic phonon modes of the host layer, system by Sch611horn et al. cannot be applied to the namely, the modes which become Raman active MnPS3-pyridine system. owing to the relaxation o f the translational symmetry As seen in Fig. 3 and Table 1, YI, v2, Y3, Y5and Y6 caused by the disordered arrangement of the pyridine lines of liquid pyridine split into doublets in the specmolecules in the van der Waals gap. Second, they are trum of MnPS3(pyridine)4/3. The spectrum in the ring librational and/or translational modes of pyridine in stretching vibration region of pyridine is shown in Fig. the gap. This is probable because the calculated den- 4. The spectra of liquid pyridine and aqueous solution sity of pyridine in the gap (0.9 M g m -3) is close to that ofpyridine (2 x 10 -2 mol %) are also shown for com-

422

Vol. 65, No. 5

MnPS3 I N T E R C A L A T E D W I T H P Y R I D I N E

Table 1. Raman peaks of MnPS3(pyridine)4/3 in the region 600 ,,~ 3500 cm t. Raman peaks of liquid pyridine are also shown for comparison MnPS 3(pyridine)4/3 RT (cm ') 625 652 765 886 948 991 1008 1031 1041 1065 1217 1235 1570 1581 1601 3056 3077

LHeT (cm l) 601 630 651 767 887 948 ~ 990 [1009 ~1031 [1041 1068 1151 ~1218 ~1235 1570 1584 1600 ~3055 ~3078 3142

Liquid pyridine RT (cm ')

C-type r-1006 i

4880 A RT

Assignment

P-type

604

v3, Ai

654 748 885 942

vl2, B1 v23, //2 V25, O 2 v24, B2

992

vl, AI

1032

v6, AI

1069 1148

Vs, A] v16, BI

1219

vs, A]

1483 1574 1583 1599

v9, A~ v14, Bi v4, AI

3058

v2, Ai

MnPS3(pyridine)4t3.j I.Z p_

992

i

1032

lO65

~

03

z

pyridine(liq.)/~ --

o /

1069, -

1076

--"-----

pyridine (aq. s

900

i

Io7o i I I 940 980 1020 1060 RAMAN SHIFT (¢rff1)

1100

Fig. 4. Raman spectrum of MnPS3(pyridine)4/3 in the ring stretching vibration region of pyridine. Raman spectra of liquid pyridine and aqueous solution of pyridine are also shown for comparison.

3148

parison. In aqueous solution of pyridine, a hydrogenbonded molecular complex is formed. The lines peaked at 991 and 1031 cm -~ of MnPS3(pyridine)4/3 are very close in frequency to those of liquid pyridine peaked at 992 (v~) and 1032 (v6)cm t, respectively. It is well known that the v~ and v6 modes of pyridine chemisorbed on silver surfaces (adsorbed by the chemical bonds) shift upwards compared with those of liquid pyridine whereas the frequencies of the Vl and v6modes of physisorbed pyridine (adsorbed by the van der Waals force) are essentially the same with those of liquid pyridine [19]. The vl and v6 modes of pyridine also shift upwards when it is hydrogen-bonded or coordinated to metal ions [19]. From these facts, we assign the 1008 and 1041 cm ~ lines to the Vl and v6 modes, which shift upwards owing to the bonding to the MnPS3 layers. This type of pyridine is termed "C-type", which means chemisorbed pyridine by the analogy of adsorbed pyridine. On the other hand, we assign the 991 and 1031cm ] lines to the Vl and v6 modes of pyridine bonded to the MnPS3 layer by the van der Waals force and call it "P-type" pyridine, which means physisorbed pyridine.

Although the relation between the state of pyridine and the vibrational frequencies of the internal modes is not sufficiently established, C-type pyridine may be bonded to the host layers rather strongly by its lone pair electrons located at the nitrogen atom [19]. One possible model for C-type pyridine is that pyridine is coordinated to the Mn z+ ions since the translational mode of Mn 2+ is much affected by the intercalation. In summary, Raman spectra of MnPS3(pyridine)4/3 are measured. It is found that the internal vibrations of the P2S64 units in the host layer and pyridine are little affected by the intercalation whereas the translational mode of Mn 2+ is considerably affected. From the splitting of some of the pyridine modes, it is concluded that there exist two types of pyridine in MnPS3 (pyridine)4/3; one is weakly bonded to the host layer by the van der Waals force, and the other is bonded to the host layer by its lone pair electrons of the nitrogen atom.

Acknowledgements - - The authors thank the Instrument Center, Institute for Molecular Science, for the use of Dupon Model 990 Thermal Analyser.

Vol. 65, No. 5

MnPS3 INTERCALATED WITH PYRIDINE REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

J.W. Johnson, Intercalation Chemistry, p. 267, (Edited by S. Whittingham & J.A. Jacobson), Academic Press, New York (1982). See for example, Intercalated Layered Materials, (Edited by F.A. Ldvy), D. Reidel Publishing Company, Dordrecht, Boston, London (1979). R. Sch611horn, H.D. Zagefka, T. Butz & A. Lerf, Mat. Res. Bull. 14, 369 (1979). R. Sch611horn, Physica 99B, 89 (1980). Y. Mathey, R. Clement, C. Sourisseau & G. Lucazeau, Inorg. Chem. 19, 2773 (1980). C. Sourisseau, J.P. Forgerit & Y. Mathey, J. Phys. Chem. Solids 44, 119 (1983). C. Sourisseau, J.P. Forgerit & Y. Mathey, J. Solid State Chem. 49, 134 (1983). K. Kurosawa, S. Saito & Y. Yamaguchi, J. Phys. Soc. Japan 52, 3919 (1983). G. Ouvrard, R. Brec & J. Rouxel, Mat. Res. Bull. 20, 1181 (1985). W. Klingen, G. Eulenberger & H. Hahn, Z. Anorg. Alleg. Chem. 401, 97 (1973).

11. 12. 13. 14. 15. 16. 17. 18. 19.

423

R. Clement, J. Chem. Soc. Chem. Commun. 647 (1980). R. Clement, J. Am. Chem. Soc., 103, 6998 (1981). A. Michalowicz & R. Clement, Inorg. Chem. 21, 3872 (1982). Y. Mathey, R. Clement, J.P. Audiere, O. Poizat & C. Sourisseau, Solid State Ionics 9-10, 459 (1983). D. Mootz & H.-G. Wussow, J. Chem. Phys. 7, 1517 (1981). A.M. Heyns & M.W. Venter, J. Phys. Chem. 89, 4546 (1985). M. Kobayashi & M. Imai, Surf Sci. 158, 275 (1985). H. Kihara & Y. Gondo, J. Raman Spectrosc. 17, 263 (1986). I. Pockrand, Surface Enhanced Raman Vibrational Studies at Solid/Gas Interfaces, SpringerVerlag, Berlin, Heidelberg, New York, Tokyo (1984).