Physical properties of intercalated solids

Solid State Ionics 9 & 10 (1983) 59-70 North-Holland Publishing Company

59

PHYSICAL PROPERTIES OF INTERCALATED SOLIDS

A.D. Yoffe

Physlcs and Chenistry of Solids Cavendish Laboratory Madingley Road Cambridge C B 3 0 H E UK This paper discusses the physical properties of the layer type transition metal dichalcogenides following intercalation with three classes of intercalate atoms or molecules. I) nitrogen containing molecules such as NH3, N2H 4 and organic amines. 27 Simple metals such as Li, Na,-K, Ag, Cu. 3) the 3d transition metals such as V, Cr, Mn, Fe, Co, Ni. All these act as electron donors. Changes in the transport, optical and magnetic properties, and the effect on Fermi surfaces will be considered. In addition to being good electronic conductors, some intercalate complexes are also good ion conductors and the system Li-TiS 2 will be dealt with in outline.

1.

INTRODUCTION

Many of the solids which can be classed as fast ion conductors have structures which contain channels, tunnels, or two dimensional layer structures along which ions can move with relative ease and with low activation energy. In this talk I will concentrate on the physical properties of some of the socalled low dimensional solids which have similar kinds of networks after intercalation with a variety of atoms or molecules. By low dimensional solids we refer here to solids composed of clusters, chains or layers. Solids with chain like structures such as polyacetylene or (SN)x polymer are often referred to as "Id" systems, while examples of layer structures such as graphite and the transition metal dinhalcogenides belong to the "2d" systems. These are highly anisotropic solids held together by relatively weak forces loosely called van der Waals forces but which embrace a number of different types of interactions. An important property is the ease with which the layers can be separated to allow guest species to diffuse in and out of the gap of the host lattice. Intercalation with a variety of foreign atoms or molecules can alter the physical properties and this is the aspect I wish to discuss since these solids have possible use as electrode materials in high energy density batteries. I will concentrate on intercalation of the layer structure transition metal dichalcogenides such as HfS2, TiS2; NbS2, TaS2; MoS2, WS 2 and many of the phenomena I will touch on are common to the other low dimensional solids mentioned above. Three classes of intercalate species will be considered. (I) alkali metals such as Li Na K but including metals such as Ag Cu, and also H. (2) The "3d" transition metals such as Ti, V, Cr, Mn, Fe, Co, Ni, and (3) Nitrogen containing molecules such as NH3, hydrazine N2H4, and organic molecules such as long chain amines, amides etc. These are all electron donors.

0 167-2738/83/0000-0000/$ 03.00 © 1983 North-Holland

There are two main effects in intercalation when we introduce foreign molecules between the layers. First of all we can separate the layers by quite large distances, and this in a way makes it possible to look at the properties of single layers, since the interaction between the layers can be weakened quite considerably. Here I refer mainly to electronic properties such as band structure, electrioal conductivity and superconductivity. The second effect illustrated in Figure 1 is the transfer of charge (electrons) from the intercalate atom or molecule (M) to the host lattice (TX2). The extend of this charge transfer varies with the particular system but for the alkali metal atoms such as Li complete charge transfer can take place, e.g. Li+Li ++(e). When this happens, in general we now replace the relatively weakish "van der Waals" forces with stronger Coulomb interactions and the lattice does not expand very much along the C direction

van der W a a l s

gap

X T X

~-

M

~,+

X

~-

T X

Figure 1 normal to the layers. The system can now be considered to be more three dimensional. 2.

INTERCALATION WITH ORGANIC MOLECULES

i.~en a crystal such as TaS 2 is heated up in liquid or vapour of say cyclopropylamine then

A.D. Yoffe / Physical properties of intercalated solids

60

Gamble, Geballe and their colleagues and others were able to show that molecules of cyclopropylamine enter between the layers. For long chain molecules such as stearamide or octadecylamine, bilayers may be formed in the "van der Waals" ga~, and the layers themselves of the order of 6 % thick can become separated by distances as large as 57 ~. The precise orientation of the molecules between the layers depends on the chain length, and some probable orientations deduced from X-ray work by various workers (7,8) are illustrated schematically in Figure 2.

For small molecules such as pyridine (9) and ammonia (10) it seems rather surprisingly that the orientations are as in figure 3.

b)

S

I~b

" --

S

,jLone pail" sp= orbit'a[.

N~ Nb

(.i) "S

NbSz'pyridin~ tl~'

T~

.-H

H

H

C ~N-.. H

C H3-N,~H

S

S

S

C ,)

S

Tel

TcI . . . . . . . . . . . . S

methyL~m~ne

H~

short chain.

~

/H

Ta

(ii) STa .... S

~

Nf

NJ

C4- C9

Figure 3 : Possible orientation of pyridine (i) and ammonia (ii) in the metallic layer compounds.

am,nes.

Although NH 3 is considered as possibly forming NH4 + by a redox reaction, there is no evidence that hydrazine N2H 4 forms a hydrazinium ion (N2H5)+. There has also been a suggestion by Dines (9) that back bonding to the intercalate molecules may also occur, for example, in isocyanides RNC, where R = CH3, C4H 9 etc. The argument is analogous to the adsorption of simple molecules such as CO on metal such as Ni, and is familiar to those interested in catalysis.

medium chain [engths.(7L

> E16

I

I

I

I

/N \ tN-.

TiTi

i!11 .

.

long

.

.

.

chains,

.n.octadecylamine~ ste~ramide~efc. Stage I. fully inferc~Qted. e.g.

ToS

2.213I.

Figure 2 : Suggested orientations of intercalate molecules for (i) short chain amines, (ii) medium chain length amines, (iii) long chain molecules.

) Figure 4 : Densities of states for Groups IV, V and VI transition metal dichalcogenides with octahedral and trigonal prismatic'coordination.

So much for structural effects. What happens to the electronic properties? Before we consider these, let us look at the simple electron energy schemes which have been developed for these materials and derived from band structure calculations, optical properties, photoemission, electron energy loss and other approaches (figure 4).

/ ...... EF

~---E F -

~--EF

N (E) ZrS 2 TaS2 octaheclra[,1T

TaS Mo$ trigonol prismatic.

2H

A.D. Yoffe / Physical properties of intercalated solids

61

For metals such as 2H-TaSe 2 and |T-TaS 2 the shape of the Fermi surfaces are illustrated in figure 5 (11).

2H

1T

hole

elec

pockets

Figure 5 :

Sketches of Fermi surfaces of 2H-TaSe 2 and IT-TaS 2

If we look at the optical absorption spectra of say a metal such as 2H-NbSe 2 before and after intercalation with cyclopropylamine, then we can identify a Drude edge due to the presence of free carriers in the dz 2 conduction band and also interband transitions (figure 6).

NbSe 2 + cyc|opropy~mine / .....

..

of the host lattice takes place. The dz 2 conduction band becomes more than half filled, and the number of carriers (holes in this case are the important carriers) falls and the Fermi surface shrinks (figure 7). From the position of the

,.o ~

after intercalation

, r I°

Figure 7 : Schematic illustration of "d" band filling and change in Fermi surface cross sectional area of a metallic layer compound after intercalation.

,

I I

i

I 2

,

I 3

i

I 4

(eV)

Figure 6 : Optical absorption versus photon energy in e.v. for 2H-NbS 2 intercalated with cyclopropylamine. After intercalation the Drude edge moves to longer wavelengths by about 0.2 to 0.3 eV. The simplest explanation is that partial charge transfer from the amine to the conduction band

plasma edge ~ and using ~p2 = 4~Ne2/m, we can give rough estimates of the extent of the charge transfer, q, expressed as electrons per intercalate molecules. These range from around 0.;5 to 0.5 electrons for pyridine. The assumption is that only a fraction of the molecules ionise, as for ammonia, giving up an electron to the host lattice. It is more likely, however, that some form of dative bonding does take place and that complete ionisation does not occur. Attempts have also been made to estimate this charge transfer on energetic grounds taking into account the energies involved.

A.D. Yoffe / Physicalproperties of intercalated Solids

62

-

/

Ionisation potential

q

\

electron affinity

dielectric

-

\

constant

transfer

Acrivos and her coworkers have also used the molecular orbital approach to obtain the relative position of the orbitals of the intercalate relative to the Fermi level of the host lattice and to decide on donor-aeceptor conditions for the charge transfer. Electrically the intercalate compounds are much more anisotropic, and for the Group V metals there is a conductivity ratio parallel and perpendicular to the layers o11/o ± ~ 3x10 2 before intercalation at room temperature, but >>10 5 after intercalation. What is more interesting is that Gamble, Geballe and their colleagues (4) have found that the metals such as 2H-TaS 2 remain superconductors even when separated by quite large distances, suggesting that the superconductivity may be associated with single layers when the separation is large and Josephson type coupling unlikely. Support for this concept comes from some very elegant experiments by Frindt (12) who looked at the thickness dependence of T for metallic 2H-NbSe 2. He found that even aCcrystal a unit cell thick (~ 12~) has a T c nearly 5 K which extrapolates to T c for a single layer of ~ 4 K (figure 8).

Co

2'C o

3'C e

interc=late I

I

20

I

charge

I

~,0

Figure 8 : Superconducting transition temperature (T c) for 2H-NbSe 2 as a function of crystal thickness. This is the value found with some intercalate compounds of 2H-TaS 2. Other evidence in favour of this idea can also be found, particularly those experiments involving chromocene intercalates discussed by Gamble and Thompson. The precise value for T c for the intercalates depends on a number of factors since we not only separate the layers, but with charge transfer

q

Madelung energy

di!ildipole

interaction, band energy etc.

altering the electron density of states at the Fermi level there will be a tendency for T to e . fall. On the other hand some of these materlals also exhibit the phenomenon of charge density waves and periodic lattice distortions which is a topic of intensive study at present. The presence of charge density waves can mean that energy gaps open up along sections of the Fermi surface thereby reducing the electron density of states. The intercalate molecules can destroy charge density waves tending to raise T c. There are thus two conflicting effects on intercalation which are difficult to separate. An interesting effect has been observed with IT-TaS 2. Here the CDW waves, for which a sinusoidal variation in electron charge density is coupled to a periodic lattice distortion, can lead to a very low electron density of states at the Fermi level. At low temperatures in the vicinity of I K, Di Salvo and his colleagues (13) find that resistivity follows at T -1/3 dependence reminiscent of two dimensional "Mott type" variable range hopping conduction at the Fermi level and there is an increase in susceptibility from diamagnetic behaviour to a situation where a Curie like tail develops. The explanation is that random potentials present in the lattice will induce "Anderson like" localisation of charge carriers. Various interpretations can be placed on this T-]/3 behaviour, such as two dimensional variable range hopping with a constant density of states at the Fermi level, or three dimensional hopping with a non constant density of states at the Fermi level. When hydrazine is intercalated into the JT-TaS 2 to give IT-TaS2.N2H 4 4/3 (14) a pronounced change in the nature of the phase transitions is seen, the resistivity increases due to the presence of a stronger charge density wave-periodic lattice distortion and an extension of the temperature ~!~ over which the resistivity obeys a near relation (figure 9). It is thought that the extra random potentials created by charged intercalated molecules (charge transfer is ~ 0.2 electrons per hydrazine molecule) increase the extent of loealisation and extends the T -|/3 regime. Good evidence for singly occupied localised states comes from the magnetic susceptibility which shows a Curie tail at low temperatures and the presence of an ear line in the same low temperature region. The extent of the charge transfer from the hydrazine appears to be 0.3 electrons per Ta atom.

A.D. Yoffe / Physical properties of intercalated solids

o

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,

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,

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% % o%

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i 200

,

,

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22

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,

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28

,

,

3/,

I

I

4+0

(KIT) I/3

Figure 9a : Plot of log resistivity p per unit thickness (d) as a function of temperature for ]T-TaS 2 before and after intercalation with hydrazine.

Figure 9b : Plot showing T -I/3 dependence in the low temperature regime.

Another baffling problem which has been looked into by Sarma, Beal and Friend is the nature of the anomaly seen in the transport properties of TiSe 2. Various interpretations have been put forward for this but intercalation of TiSe2, with hldrazine (figure ]0) suggests quite

strongly that the anomaly is not electronic in origin with coupling between electrons and holes but has to do mainly with the temperature dependence of the phonons. Before intercalation transport is by holes with a mobility of 20-25 cm2/V sec. and a carrier concentration of

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26

A.D. Yoffe / Physical properties of intercalated solids

64

r

Se p

TiSe2

Ti d

r

Ti d

L

Se p

L

TiSe2(N2H4)o.6

Figure l0 : Intercalation of TiSe 2 with N2H 4 (i) resistivity versus temperature, (ii) onset temperature of resistivity anomaly (Td) as a function of carrier concentration and conductivity (iii) schematic illustration of the change in band filling near the Fermi level before and after intercalation. 3xi020/cc. After intercalation R H becomes negative with a carrier mobility of m 7 cm2/V sec. corresponding to electrons in a relatively narrow d band and a concentration of 6x]021/cc. The stoichiometry of this complex is TiSe2.0.6N2H 4 givin~ an effective charge on the N2H 4 of N2H40"Y+ and on titanium of Ti-0"4~ The simplest explanation is that charge transfer (electrons) occurs to the host lattice. The alternative explanation of increased band overlap (p/d) due to the presence of intercalate molecules is less attractive. The figure also indicates that the onset temperature for Td for the anomaly appears to be independent of carrier concentration, leading to the conclusions mentioned above that this is not of electronic origin, as has been suggested by a number of workers but is due to an instability in one of the vibrational modes, as a soft phonon transition most likely a phonon driven antiferroelectric transition. I mention these points in order to illustrate the power of using intercalation as a tool to determine mechanisms of phase changes. As to the mechanisms of intercalation itself, this has been discussed by Acrivos, geal and their coworkers (15). Their experiments show quite beautifully how for low pressures of hydrazine in NbSe 2 the steps are probably adsorption on the surface of the crystal, formation of an activated complex on the surface and then rapid diffusion around the edge to enter between all the layers. This is a very different situation to graphite. 3.

cathode and using non aqueous solvents. In the case of Ag + intercalation Frindt and his colleagues (16) simply immerse the crystal in a silver nitrate solution with a silver rod in contact with the crystal. (iii) The use of Li butyl in the liquid state. This method used extensively by Dines is perhaps the cleanest technique, since only Li enters between the layers, and we have adopted this procedure. (iv) Immersion of the crystal in, for example, molten Na/alkali halide mixtures. (v) The use of metallocenes such as cobaltocene or chromocene in hexane solutions. The metallocene molecules act as electron donors. The alkali metal atoms between the layers ionise as K÷K + + e, and there can be profound changes in the electronic properties such as the optical and electrical properties. Structural work by Rouxel and his colleagues (17) has shown that just as for graphite it is possible to get different stages in crystals such as the titanium, zirconium and hafnium dichalcogenides. So for potassium compounds the following stages have been identified (figure II).

V///////////////////////A

000

V/////////////////////////I

000

F////////////////////////~

F////////////////////////~

0 0 0 F/////////////////////////~

V///////////////////////A

Stage 1

0 0 0 V////////////////////////A Stage 2

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V//////////////////////A Stage 4

INTERCALATION OF ALKALI METAL ATOMS

Turning to intercalation with alkali metal atoms such as Li, Na, K, Rb, Cs and also Ag, there are a number of experimental methods which are being used, and which have been developed in many laboratories. (i) The simplest is to immerse crystals in metal ammonia solutions, e.g. Na/NH 3 at -36°C. The disadvantage with this method is thaff NH 3 also goes in between the layers. (ii) Electrolysis, with the host acting as

Figure II : Different staging compounds possible for some Group IV dichalcogenides with potassium. Stage 3 absent. It is not clear why a stage C 3 is not observed. Some ideas concerning the staging mechanisms are currently being developed, (19,20). EXAFS experiments have been made on the NbSe2/Rb system and it has been found that there is an increase of Nb-Nb separation after intercalation by

A.D. Yoffe / Physical properties of intercalated solids

65

0.031~, with the Nb-Se distance remaining unchanged. The Se-Se bond decreases across a sandwich by 0.013~ so that the trigonal prism squashes down after intercalation. What about the electronic properties? One of the earliest experiments we carried out was to show that if you take a semiconductor such as MoS 2 and intercalate it with an alkali metal such as sodium, then you finish up with a metal, which was later shown by others to be a superconductor. Thus Group IV MoS 2

+

xK

÷÷

diamagnetic semiconductor.

HfS 2 ...... (D o o

'x X A

Kx Mo S 2

o.

paramagnetic metal T ~ 6.5 K c

.a

In addition the optical absorption spectrum changes, excitons are screened, and a Drude edge becomes apparent, see Figure 12:

LiHfS 2

.2

I

/ '/

2

3

4

5

6

eV

Figure 13 : Spectra of a crystal of HfS 2 before and after intercalation with Li. MoS2

It is interesting that T c both for pure metals such as NbSe 2 and NbS 2 and intercalates such as KxMoS 2 and KxHfS 2 are all in the region of 6 to 7 K. Now the density of electron states for these solids will be very different indicating that the electron-phonon interaction is as important as the density of states in determining T c .

o

t~ .Q

I

,

I

40

~

30

20

Wave

number

I0 (x [03cm 1)

Figure 12 : Optical absorption spectra at 77° K of a crystal of MoS 2 before (a) and after (b) and (c) intercalation with Na. Note the disappearance of excitons A, B due to screening by free carriers and appearance of a Drude edge C after intercalation.

for Group V compounds 2H - NbSe 2

+

xK

~

metal T ~ 7.2 K c

K x Nb Se 2 poor metal, for x = 1 expect a semiconductor.

for Group IV compounds HfS 2

+

semiconductor

xK

K x Hf S 2 metal and superconductor (see Figure

13).

Using the rigid band approach we visualise the electron from the alkali metal atom entering the conduction band of the host lattice (figure ]4) with the Fermi level moving from EFt to EF2. This is clearly a first approximation and there is sound experimental (optical), as well as theoretical (band structure calculations) evidence for mixing of the 's' orbitals of some of the alkali metal ions with the d conduction band orbitals in the case of the Group IV dichalcogenides, and the 's' orbitals and the d/p conduction band for the Group VI dichalcogenides as illustrated in figure 15. For LiNbSe 2 the s band may even drop to the dz 2 band, resulting in metallic behaviour even for the fully intercalated complex. This band picture works well for the metals enclosed

Til

v

Cr

Zr Hf

Nb Ta

~o

but there can be problems associated with the chromium and vanadium dichalcogenides. In fact crystals of CrS 2 cannot readily form but NaCrS 2 does exist and this is a semiconductor. On a simple band scheme this material might be expected to be a metal, but the Cr exists as Cr3+ ions, and the 'd 3' electrons are not delocalised to move in bands but instead are localised and there are local moments. The sulphurs are distributed about the chromium atoms in octahedral co-ordination with a slight trigonal distortion. The solid is antiferromagnetic at

A.D. Yoffe / Physical properties o f intercalated solids

66

sldap overl x~->',x8yr

r

s>p.7

d ~-t->xx~1%

s~p,L

d x z, yz. t

d2~(Z~-EFv_

Psc,(/+~ 12~~;~F .... 1. Group IV

dec'2~~ nsOz (4-"<~"~; V

d~ (2 . . . .

~--~- -~EF2.-~af[e r. --E FI'-'~ before. infercQlQfion.

POz

~2~ ----->"N(E) V LiTa52

> N(E) VI

e.g,13roup IV giHfS2

MX 2

Figure 14 : Sketches indicating position of Fermi level before (EFI) and after (EF2) intercalation of Group IV, V and VI layer compounds with alkali metals. low temperature (< 19 K), with a stacking of ferromagnetically ordered moments in the layers. Another important property of these alkali metal intercalate compounds such as LiTiS 2 and AgTiS 2 is that, in addition to being good electronic conductors, they are also very good "fast ion" conductors. This is also the case for AgCrS 2. They have a relatively low activation energy for the diffusion of the ions. For Li + it is of the order of 0. I eV, and it is this fact which has inspired a good deal of activity in many laboratories throughout the world in connection with the development of high energy density batteries. We have given a simplified account of the state of affairs for alkali metal intercalation, and many topics such as the effect of intercalation on charge density waves and phonons have not even been covered. For example LiHfS 2 is isoelectronic with ;T-TaS 2 but does not show very strong charge density wave-periodic lattice distortions. One possibility is that even though the band structures may be similar, with similar conduction bands, there may be substantial lattice stiffening in the alkali metal intercalated material so that any lattice distortion which involves an expansion in the unit cell perpendicular to the layers may be too costly in lattice strain energy. ~Another feature not discussed is the formation of ordered lattices of intercalate ions particularly where Ag + are involved as in Ag|/3TiS 2 and AgxNbSe 2. 4.

s/d p overlQp

d

INTERCALATION WITH '3d' TRANSITION METALS

Let us consider two effects and assume we are talking about crystals of say Crl/3NbSe 2. (i) The Cr sits between the layers and exists as Cr 3+ ions which is not now mobile. This means an electron has gone to the conduction band of the NbSe 2 per formula unit and on the rigid band

Figure 15 : Possible changes in the orbital character of the conduction bands after intercalation with alkali metals. model we expect the system to be less conducting and even approach being a semiconductor as for NaTaSe 2 or NaNbSe 2. Some band overlap of the s conduction band and the d/p band may however keep the system metallic. (ii) Since Cr 3+ with a d 3 configuration will have local moments we expect and indeed obtain interesting magnetic properties. The intercalates which have been prepared and studies are indicated in the table for M T X 2, where M is a 3d metal, x = I/4 or 1/3 an~ X = S,Se. Intercalate complexes formed with '3d' transition metals TX 2

IV

Ti

V

Cr

Mn

Fe

Co

Ni

TiX 2

/

/

/

ZrX 2

/

]

J

HfX 2

~

/

/

/

~S)

I

NbX 2

VX 2 £

¢

/

I

/ /

V

TaX 2

£

£

/

/

/

/

The intercalate compounds which have been investigated in greatest detail are derived from 2H-NbX 2 and IT and 2H-TaX 2. What happens is that in say Mn x NbS 2 or FexNbS 2 the 2H-NbS 2 retains the trigonal prism structure, while the Mn 2+ (d 5) ions, or the Fe 2+ (d 6) ions, sit in the van der Waals gap in octahedral sites (figure 16). The crystals are grown at high temperatures by heating the elements together. Since the Nb wants to sit in a trigonal prism environment while the Mn prefers the octahedral coordination, this means that the Mn does not

A.D, Yoffe / Physical properties of intercalated solids enter the Nb sandwiches substitutionally. There is thus a complete separation of the metallic layers of NbS 2 and the magnetic ions (Mn 2+) ordered between the layers. We have a beautifully simple system for looking at the kind of interactions that can lead to magnetic ordering. In Ta Fe by

the case of IT-TaX 2 intercalates where the is in an octahedral environment, doping with can, of course, lead to substitution of Ta Fe.

1.5

~

m

~\~.

/~.~" - ~ . . . . .

°\ e

1

~q

--

~\

i/

-2 n ,

×

~ /

c~0-5

o/-,

If we look at some of the compounds that can be formed such as:

V .

'13

.

.

z

-3

TCK,,.

.

120

-4

24O

X-~¢

d 7 :- Co2+1/3NbS2

Mnl/3TaS 2.

16 2

d 8 :- Ni2+I/3NbS 2

we see the valence on the '3d' ion is 2 + (note, however, than in Cr]/3NbSe 2 we have a d 3 Cr 3+ ion). The solids are all paramagnetic metals

<__

o

The Mn 2+, Fe 2+ etc. form ordered structures and super lattices between the layers.

d 5 :- Mn2+I/3NbS2 d 6 :- Fe 2 +]/3NbS2

67

..2

T~IO3

......

NlfC

E

21o* M T(K).

1 o-5 120'

.i.O.?~-Nn

~.

Figure 16 : Schematic diagram of intercalate compound composed of a metallic layer dichalcogenide (eg. TaS 2 or NbSp with trigonal prism coordination) and a '3d T metal such as Mn, giving Mn 2+, in octahedral coordination. at room temperature. The transport and magnetic properties however change quite markedly at low temperatures, and figure 17 for Mnl/3TaS 2 taken from the work of Parkin and Friend shows resistivity and Hall coefficient measurements for current in the layers and magnetic field perpendicular to them. Susceptibility data both parallel and perpendicular to the c-axis in a field of 330 G are also given. A simple interpretation is the following. There is a discontinuity in the resistivity curve just below 80 K. Above 80 K there is no long range magnetic ordering with strong spin disorder scattering of the carriers (holes). Here there is a strong interaction between the carriers in the conduction band based on Ta and the spins localised on Mn 2+ ions. That is to say above the magnetic ordering temperature electrons can lose momentum by flipping spins on Mn 2+ since they are not aligned in domains, and this results in an anomalous increase in resistivity.

~-

2/:0

Figure 17 : Resistivity p, and Hall coefficient R H (i) and susceptibility × (ii) data as a function of temperature for Mn]/3TaS 2. As we pass through the magnetic ordering temperature (Curie point) there is a change in slope similar to that found in rare earth metals such as Gd, Tb, Er and long range magnetic ordering occurs. In this regime it is difficult to flip the spins since we are now involved with magnetic domains. This suggests that the interaction of localised d electrons on the intercalate ions with the free carriers based on the Ta atoms in the layers is the most important one to consider, and the RKKY interaction is dominant resulting in a ferromagnetic solid. This is supported by polarised neutron diffraction experiments by Parkin, Marseglia and Brown (21) at ILL, Grenoble, where they were able to show that there is loss of magnetism density from the Mn 2+ to the Ta atoms in the host lattice and very little to the S atoms. Figure ]8(a) gives a spin density map of a unit cell and shows the difference in spin density between that observed and that calculated for spherically symmetrical Mn 2+ d 5 ions. Two features are clear. First there is a distortion of the Mn 2+ moment from spherical symmetry and secondly there is a polarisation around the Ta with two Ta sites, one with Mn immediately above and below and the other between these. It has been shown that the sulphur atoms do not participate in the spin density map, see figure 18(b). Overall there is a diminution of the moment from the Mn 2+ d 5 values by some 15% as a result of a net antiferromagnetic polarisation of the

A.D. Yoffe / Physical properties of intercalated solids

68

ving neighbouring chalcogen atoms and the localised electrons on the '3d' ions, is very large and more important than the RKKY interaction. The Fe intercalates form an intermediate case showing both ferromagnetic and antiferromagnetic orderings. In Fe complexes, therefore, the superexchange interaction is expected to be of intermediate strength between that for Mn and that for the Co and Ni intercalates. In summary, therefore, the magnetic ordering at low temperature is (i) Ferromagnetic below I00 K in the plane of the layer for Mn2+|/3 NbS2, Cr3+I/3NbS2 etc. These are soft magnets and total saturation occurs in a field as low as ~ I00 gauss. (ii) Co2+I/3NbS2, Ni2+I/3NbS2, Fe2+I/3NbS2 which are antiferromagnets. (iii) For Fel/3TaS2 on the other hand the system is ferromagnetic where the moment lies preferentially along the c-axis. Saturation only occurs in fields as high as ~ 6T with a high coercive force, and the system remains magnetic when the field is removed.

Figure 18(a) : Fourier difference map of the spin density Mgb s - M~aI~" for Mnl/4TaS 2 projected along 100,'where Mcalc is the calculated moment arising from the Mn 2+ at the Mn sites. Positive and negative contours are represented by full and dashed lines respectively at intervals of 0.02 DB/~

T

Parkin and Friend have developed a number of models to explain these effects, with the simple case of first of all the single electron crystal field splitting of the energy levels for the ions in an octahedral coordination carrying the slight trigonal distortion present in the structure. This configuration model assumes that crystal field splitting is smaller than the correlation energy coupling spins. For the many electron system, however, Parkin and Friend give a scheme for the ground and various excited states in what they term the intermediate crystal field model, and using a perturbation approach they have also worked out why some intercalates have an easy axis of magnetism along the layers while for others it is normal to the layers. The relation between the RKKY interactions and charge density waves involving Fermi surface nesting has also been discussed by them. 5.

Figure 18(b) : I00 projections of a Fourier synthesis map based on nuclear structure factors measured at II0 K showing the layers of Mn, S and Ta atoms along the c-axis. Comparison with the spin density map in Fig.18(a) reveals the absence of any spin density associated with the S layers. Ta atoms. In other systems such as the antiferromagnetic Co and Ni intercalates, the evidence is that superexchange interactions, invol-

THE Li-TiS 2 SYSTEM

We now look briefly at the Li-TiS 2 system since this has potential application as a high energy density system and this has been discussed by Whittingham and by Steele. Li intercalates into TiS 2 and to form finally LiTiS 2 with smoothly varying lattice and thermodynamic parameters. The Li + ion is small and fits into the octahedral interstices between the layers with only a small (~ 5%) "c" axis expansion. Band structure calculations suggest that the "s" conduction band of the Li + is far removed from the valence and conduction band edges of TiS 2 and supports the rigid band model in interpreting the transport properties. There does however appear to be somewhat less dispersion in the sulphur Pz bands for the complex suggesting the Li + may

A.D. Yoffe / Physical properties of intercalated solids weaken the covalent bonding within the layers. At the same time the "a" axis expands in the intercalate in contras~ to other less covalently bonded layered compounds which show a contraction.

REFERENCES (I) Levy, F.A., editor, Physics and Chemistry of Materials with Layered Structures, Reidel, Dodrecht, Volume 6, Intercalated Layer Materials (1979). (2) Yoffe, A.D., Chem. Soc. Rev. 5 (1976) 51, Ann. Chim. Fr. 7 (1982) 215. (3) Schollhorn, R., Physica 99B (1980) 89. (4) Gamble, F.R., Geballe, T.H., Treatise on Solid State Chemistry, Volume 3, Chapter 3, Inclusion Compounds (Plenum, New York, 1976). (5) Whittingham, M.S., Progr. Solid State Chem. 12 (1978) 41. (6) Wieting, T.J., Schluter, M., editors, Phonons in layered crystal structures, Reidel, Dodrecht, Volume 3 (1979). (7) Schollhorn, R., Sick, E., Weiss, A., Zeit fur Naturforsch. 28b (1973) 168. (8) Dines, M.B., Inorg. Chem. 17 (1978) 762, 763. (9) Riekel, C., Hohlwein, D., Schollhorn, R., Chem. Comm. (1976) 863. (I0) Schollhorn, R., Zagefka, H.D., Angew. Chem. Int. Ed. Engl. 16 (1977) 199. (II) Doran, N.J., Physica 99B (1980) 227. (12) Frindt, R.F., Phys. Rev. Lett. 28 (1972) 299. (13) Di Salvo, F.J., Graebner, J.E., Solid State Comm. 23 (1977) 825. (14) Sarma, M., Beal, A.R., Nulsen, S., Friend, R.H., J. Phys.C 15 (1982) 4367. (15) Beal, A.R., Acrivos, J.V., Phil. Mag. B37 (1978) 409. (16) Scholz, G., Joensen, P., Reyes, J.M., Frindt, R.F., Physica 105B (1981) 214. (17) Rouxel, J., Alkali metal intercalation compounds of transition metal chalcogenides. Physics and Chemistry of Materials with Layered Structures, Reidel, Dodrecht, Volume 6 (1979) 201. (18) Parkin, S.S.P., Friend, R.H., Phil. Mag. 41 (1980) 65, 95. (19) Safram, S.A., Phys. Rev. Lett. 44 (1980) 937. (20) Dahn, J.R., Dahn, D.C., Haering, R.R., Solid State Comm. 42 (1982) 179. (21) Parkin, S.S.P., Marseglia, E.A., Brown, P.J., J. Phys.C 16 (1983) 2769-2778. (22) Steele, B.C.H., Phil. Trans. Roy. Soc. Lond. A302 (1981) 361.

The Li + is highly mobile within the layers with a very high diffusion coefficient for a Li compound. The activation energy for diffusion is low (~ 0.1 eV) with an ionic conductivity of I0-3-10-4~-Icm-I. There does appear to be some ordering of the Li + at room temperature for certain values of Li + concentration. Since there is no structural change of the host lattice it could be a suitable material as an electrode in a high energy density battery and this has been discussed by others (5,22). For the system Li (electrode)

: Li + in non aqueous : or in polymer

TiS 2 electrode

the cell voltage is ~ 2 volts and the system is light. The simple reactions can be represented as Li÷Li + + e TiS 2 + e + TiS 2 Li + TiS2÷LiTiS 2 (intercalation by Li with no phase change) For recharging the system the voltage is reversed and LiTiS2÷Li + + on anode

TiS 2 (de intercalation)

In the stoichiometric pure state TiS 2 is a small band gap ~ 0.2 eV semiconductor which on intercalation becomes metallic (figure 19) with the resistivity decreasing by several orders of magnitude below the unintercalated TiS 2. Figure 19 : Density of state curves for stoichiometric TiS 2 showing position of Fermi level before (i) and after (ii) intercalation with Li.

E~i!

|I

~o-z;.~.~

- L~TiS 2

*Li

r (i)

69

(ii)