Microscopic studies of transition metal chalcogenides

Journal of Crystal Growth 34 (1976) 239—244 © North-Holland Publishing Company

MICROSCOPIC STUDIES OF TRANSITION METAL CHALCOGENIDES R.R. CHIANELLI Corporate Research Laboratories, Exxon Research and Engineering Company, Linden, New Jersey, USA Received 15 October 1975; revised manuscript received 31 December 1975

The reaction of single crystals of transition metal dichalcogenides (MX

2 layer structures) with n-butyllithium to form LiMX2 have been followed with the optical microscope. Small crystals are observed to intercalate Li with movement of a distinct front through the crystal. Larger crystals crack but remain intact. The reaction of single crystals of transition metal trichalcogenides (MX3-chain structures) to form Li3MX3 have also been observed and here large crystals fragment to smaller crystals. Both reactions are topochemical reactions in which structural integrity is retained. In the first case (MX2) slight structural rearrangement occurs and in the second case moderate structural rearrangement occurs. The reaction of uS2 with NH3 is also discussed.

1. Introduction Recently Thomas [1] has shown how topology plays a critical role in reactions occurring within solids which yield products difficult or impossible to obtain by other types of reactions. These reactions are termed topochemical reactions and are characterized by retention of crystallinity and lack of phase separation during completion. They are distinct from topotactic reaction where phase separation occurs during reaction (see for example Curtin and Paul [2]). Topochemical reactions are governed more by the precise atomic or molecular orientations and stacking faults within the crystal than by the intrinsic electronic properties of the compound and therefore unusual and perhaps metastable materials result. An example of such a reaction of recent interest is the solid state polymerization of (SN)~from (SN)2 [3]. Other examples are the reactions of lithium with the transition metal di- and trichalcogenides. The former is the special case of intercalation in which only a lattice expansion takes place upon reaction, the layers of MX2 sandwiches remain intact and only weak van der Waals bonds are broken (fig. 1) [4]. In the latter, however, covalent bonds are broken and the structural disruption is more severe [5]. The transition metal diand trichalcogenides are of interest because of their potential use as battery cathodes. It is the nature of the electrochemical reaction of the solid with lithium 239

that determines their reversibilities as cathodes [61. In particular it is the degree of disruption of the original structure during uptake of lithium which is of greatest effect [7,8]. As part of continuing effort to understand the chemistry and physics of the transition metal chalcogenides we have undertaken this optical study to gain insight into the mechanism of lithiation of these compounds. For this study we have chosen the reaction of n-butyllithium with di- and trichalcogenides which has recently been reported [4,5,8]. This reaction has been shown to be related to electrochemical lithiation and has been termed “electrodeless lithiation” [10]. The reaction of n-butyllithium with the transition metal dichalcogenides (MX2 where M Ti, Zr, Hf, Nb, Ta and X = S, Se) is an intercalation reaction (fig. 1): MX2 + CH3(CH2)3 Li LiMX2 + ~ CH3(CH2)6CH3 —

-~

The reaction of n-butyllithium with the transition metal trichalcogenides (MX3 where M = Ti, Zr, Hf, Nb, Ta and X = S, Se) proceeds in a similar manner: MX~X2)+ 3 CH3(CH2)3— Li Li3 MX3 -~

+ ~CH

c

3(CH2)6 H3 However, in this case polychalcogenide bonds which line a Van der Waals gap are broken (fig. 2) and three lithium atoms surround the infinite MX6 chains which remain 2

R. R. Chianelli / Microscopic studies of transition metal chalcogenides

240

: T

Ti

LA V E R S

5 2 L AVER

MOVING

\ I

FRONT

/

SINGLE

~

______

INTERCALATING CRYSTAL

I • ~

________________

a

INTERCALATING

Li

Fig. 1. Strucrure and intercalation of lithium in TiS

2.

intact causing a lattice expansion perpendicular to the chain axis (fig. 2). These reactions have been followed continuously with the optical microscope and have al-

so been studied with the scanning electron microscope (SEM), X-ray diffraction, infrared spectroscopy and other techniques.

A~A~ATA~ ATATATA~

~

VAN DER WAALS GAP OF POLYSULFIDE BONDS

I

~

b— AXIS PROJECTIONS

POLYSULFIDE BONDS

3Li

OF TRIGONAL PRISMS SHADING MEANS TRANSLATION OF

I

I

U UNIT CELLALONG b—AXIS

0

0

0

~:M3Ti~M5_________

Fig. 2. Structure of TiS 3 and reaction with n-butyllithium.

0

R.R. Chianelli / Microscopic studies of transition metal chalcogenides

2. Experimental Single crystals of the layered dichalcogenides were prepared by vapor transport reactions [11] and the crystals of the trichalcogenides were preparedby direct reaction of the elements [12]. A selection of crystals varying in size from about 0.01 to 1 mm were placed in Beckman spectrophotometer cells which were taped to glass slides making them suitable for mounting in the mechanical stage of the microscope. The cells were then loaded in a dry nitrogen atmosphere with 1.65 M (or further diluted) n-butyllithium in hexane which reacts vigorously with water (Foote Minerals Co., Exton, Penn.) and sealed with stoppers or vacuum grease and placed on the stage of a Leitz Ortholux-Pol microscope. The reaction was then followed with time. The cells

Tr176

241

varied with path length from 1 to 10mm depending upon the working distance of the desired objective. Single crystals were observed during reaction using a suitable combination of transmitted, reflected or oblique lighting and with polarizing, interference contrast or dark field devices. In the case of NH3 gas a 10 mm path length cell was connected to a rubber bladder Iii. led with dry NH3. SEM photographs were taken before and after intercalation in most cases. 3. Results and discussion 3 1. MS -dichalcogenides 2

During the intercalation of uS2 with Li from n-butyllithium (observedin reflected light falling parallel to

T~236 Fig. 3. Cracking and rifting of TiS2 crystals.

T

~296ulN

R.R. Chianelli / Microscopic studies of transition metal chalcogenides

242

g~T-1800 AX

AT

seC

-

AX

•

-

3000

sec

4~

Fig. 4. Intercalation of perfect crystal of TiS

2 with n-butylli-

thium (3 mm

1 nm).

the c-axis) two distinct effects were noted. Many crystals observed showed an increasing amount of cracking and riftingas seen in fig. 3. TiS2, normally a golden reflective yellow, would initially show spots which turned a non-reflective brown. At a later time these areas would begin to crack after which the coloration (attributed to thin layers at an angle to the incident light and thus non-reflective) disappeared. The cracking continued until deep rifts appeared in some places and high magnification (1000X or greater) showed that these rifts were V-shaped cuts in which steps appeared. This cracking 0 0 (which usually occurred at 120 or 602 angles) not and thedid crystal extend to below areas of about 10 pm as a whole retained its crystalline integrity as determined by X-rays. Most larger crystals and crystals with imperfections showed this effect. Smaller crystals with perfect hexagon morphology (less than 0.1 mm) showed the effect indicated in fig. 4. A front beginning at the edge of the crystal moved through the crystal, until it disappeared at the center (fig. 5) four to five hours later for an average crystal of about 50 pm. Crystals which showed this effect were not cracked after intercalation was complete. A lattice expansion of approximately 10% occurs upon intercalation with Li [4] and the front shown in figs. 4 and 5 is the boundary between the expanded and unexpanded lattice. The front is quite sharp initially but becomes broader as the center of the crystal is reached. Generally the boundary between the TiS2 and UTiS2 portions of the crystal was about 2 pm A schematic representation of this process is shown in fig. 1. X-ray studies of n-butyllithiation of TiS2 with time showed the presence of 0.0! reflections of inter-

.

AT

Ax

-

3 0 00 sec 4 Idly-

AT A

-

X

-

l2~

Fig. 5. Progression with time oflithium

3 9 00 a 5~.

front through TiS 2

single crystal.

calated and unintercalated material with no smearing between them in agreement with the optical study. We believe the cracking is due to “twinning” or stacking faults which have been reported in TiS2 [13,14]. This idea is supported by several observations. The type of “twinning” reported which can be considered mirroring across the 001 planes is only possible in trigonal space groups such as uS2 (P3m1) and not possible in hexagonal space groups. This cracking was not observed in 2H TaS2 which belongs to a hexagonal space (P63mmc). This difference in behavior between TiS2 and TaS2 during reaction with n-butyllithium cannot be explained by the difference in flexibility of the layers in TiS2 and TaS2 as has been suggested [15] because crystals were observed to crack which were much smaller and thinner than those that did not crack suggesting the presence

R.R. Chianelli / Microscopic studies of transition metal chalcogenides

243

of two types of crystals. In fact a recent X-ray study has confirmed the existence of “twinning” and “untwinned” crystals and shows that the “twinned” crystals cracked when reacted with n-butyllithium as described above [16]. ZrS2, which is initially a transparent red material, be. similar mann:r except that the crystals quirkindicating that the material was becoming more metallic with lithium uptake. The reaction of TiS2 single crystals with NH3 gas also proceeded in a similar manner to the n-butyllithium reaction except that the reaction time was much shorter, typically about 10—15 mm.

H

_______________________________________________

3.2. Trichalcogenides (MX3)

The trichalcogenides have either a needle-like or board-like (pinacoidal) morpology with the b-axis of the monoclinic cell along the needle axis. TiS3 and NbSe3 crystals are silvery-black reflective crystals, but when viewed in transmitted light thin crystals of TiS~ are partially transparent with a red color. Hafnium and zirconium trisulfide crystals are red-orange and are transparent in transmitted light but the corresponding selenides are silvery-black in bulk. All crystals extinguished cleanly in reflected or transmitted polarized light along their needle axes. When reacted with n-butyllithium, TiS3 appears to become fluffier in the bulk and microscopic examination after lithiation shows that the smaller individual crystals (> 5 pm in width) have retained

B AXIS

CRYSTAL

.

.

Fig. 7. SEM of Li3tltS3 single crystal(2400 showing brous naturephotograph after reaction with n-butyllithium X).fi-

their crystalline integrity as seen from the fact that they extinguish cleanly as before. However, larger crystals broke down into smaller crystals. When placed in n-butyllithium a TiS3 crystal almost immediately began to lose reflectivity and became black and striations appeared along the b axis. As this process continued small

STRIATED CRYSTAL EXTINGUISHED

FINAL CRYSTALLITES

ALONG b - AXIS IN POLARIZED LIGHT

CLEANLY EXTINGUISHED IN POLARIZED

/

Fig. 6. Schematic of breakdown of HfS3 crystal to smaller crystallites.

LIGHT

244

R.R. Chianelli / Microscopic studies of transition metal chalcogenides

crystals began to separate from the edges as shown schematically in fig. 6. When complete, the larger crystals were heavily striated but still extinguished in polarized light along the b axis and many small perfect single crystals had separated from larger crystals. The same behavior was observed with HIS3 in transmitted light, but the transparency of the crystals was retained after reaction. SEM photographs also show the heavily striated, fibrous nature of the product (fig. 7). Larger crystals break up along the needle-axis into smaller crystals because of expansion of the lattice perpendicular required to incorporate the three lithium atoms (see fig. 2) into the existing structure without severe disruption of the MX6 chains. A further demonstration of the flexibility of this structure was noted when a sample of Li3 HIS3 was placed in an optical cell and several drops of water were added. Gas bubbles were seen forming and individual crystallites were seen to change from their characteristic red-orange semitransparent state to completely transparent colorless crystals. Probably the Li3MX3 incorporates water into the lattice, again topochemically, iiberating H2S. These reactions give striking examples of macroscopic manifestations of microscopic processes and demonstrate the remarkable flexibility of the layered and chain structures of the transition metal chalcogenides.

and J.Chu for assistance. I would also like to thank A.H. Thompson and C.R. Symon who prepared the sulfides studied here.

References [1] J.M. Thomas, Phil. Trans. Roy. Soc. London 277 (1974)

251.

[2] IC. Paul and D.V. Curtin, Science 187 (1975) 19. [3] V.V. Walatka, Jr., M.M. Sabes and J.H. Perlstein, Phys. Rev. Letters 31(1973)1139. [41 MS. Whittingham and F.R. Gamble, Mater. Res. Bull. 10 (1975) 363. [5] R.R. Chianelli and M.B. Dines, Inorg. Chem. 14 (1975); R.R. Chianelli and M.B. Dines, Paper at Am. Chem. Soc. Meeting, Phlladelphia, Pa., April 6—11 (1975); M.B. Dines, Mater. Res. Bull. 10 (1975) 287. [6] MS. Whittingham, J. Electrochem. Soc., in press; MS. Whittingham, Paper 40, Electrochem. Soc. Meeting, Canada, [7] Toronto, R.R. Chianelli andMay M.B.11—16 Dines,(1975). Paper 443 RNP, Electrochem. Soc. Meeting, Toronto, Canada, May 11—16(1975). [8] R.R. Cliianelli,M.B. Dines and M.S. Whittingham, Paper 31, Electrochem. Soc. Meeting, Dallas, Texas, Oct. 5—9 (1975). 191 Meeting, D.W. Murphy and F.A. Trumbore, Paper 26, Electrochem. Toronto, Canada, May 11—16 (1975). [10] M.S. Whittingham, R.R. Chianelli and MB. Dines, Paper

[11] [12]

Acknowledgements

[13]

I wish to thank my colleagues, M.B. Dines, J.P. deNeufville, M.S. Wfhittingham, B.G. Silbernagel, and F.R. Gamble, for their useful discussions and J. Alonzo for SEM photographs, J. Scanlon for X-ray diffraction,

[14]

1151 [16]

35, Electrochem. Soc. Meeting, Dallas, Texas, Oct. 5—9 (1975). A.H. Thompson, to be published. E. Bjerkelund and A. Kjekshus, Acta. Chem. Scand. 19 (1965) 701. R.R. Chianelli, J.C. Scanlon, M.S. Whittingham and FR. Gamble, Inorganic Chem. 14 (1975) 1691. S.F. Bartram, Ph.D. Thesis, Rutgers Univ. (1958). A. Weiss, personal communication. R.R. Chianelli, J.C. Scanlon and A.H. Thompson, Mater. Res. Bull. 10 (1975) 1379.