Electronic structure of intercalated metal disulfides (Ag13TiS2 and Fe13TiS2 ) studied by XPS and theoretical calculations

Journalof Alloys and Compounds 245 ( IYYh)30-39

Electronic structure of intercalated metal disuffides (Ag, ,,TiS2 and Fe, ,,TiS,) studied by XPS and theoretical calculations H. Martinez”. S.F. Matar’ hh,C. Auriel”, M. Loudef,

Rcccivcd

13 April

15)Ofx rcviscd

21 June

G. Pfister-Guillouzo”‘”

1YY6

Absfrnd The effect of metal intercalation (silver and iron) into lT-Cdl,-type TiS, layered crystals. expressed as M,TiS,. has been by X-ray photoelectron spectroscopy (XPS) and self-consistent electronic calculations (augmented sphere wave method). The spectra arc found to depend strongly on the guest metals, and we have shownhow the XPS valencebandsare modifiedby the ‘host-guest’interactions by using calculated densities of states. In the first approximation. the chemical shift of the core peaks arc correlated with Mulliken populationanalysis.

studied

Kt*yvorci~:

Intercalation:

C’alcutatud

dcnsitics

of states:

XPS: Valcncs hands: C’ow peaks

1. Introduction

authors have proposed the existence of a substitution

Transition metal dichalcogenides having the general formula TX? (T = Ti. Ta. MO,etc., X = S. Se, Te) have been extensively studied as a result of their twodimensional character and their highly anisotropic physical properties. New materials have been obtained by intercalating various atoms in these low-dimensional solids. The type and concentration of the interca-

lated species have a great effect on the physical properties of the host structure. resulting from charge transfer phenomena between the guest and the host [II* An X-ray diffraction study [2] has shown a considerable variation of parameter c (crystallographic axis of layer stacking) that depended on the type and con-

of titanium atoms by those of the intercalated metal 121.Other work, done with EXAFS [3] on M,TiS, compounds+ has shown that the metal-sulfur distance varied as a function of the concentration of the guest species. These authors concluded that M-S bonds were covalent. and that there was a non-rigidity of the host lattice structure (S-Ti-S). where inter-layer space became elongated as intercalation occurred.

This process was accompanied by the population of d antibonding levels of titanium atoms and thus an elongation of Ti-S bonds. In other words, they reported a guest-species-host-lattice structure interaction in M,TiS, compounds. Inoue et al. [ 11 reported magnetic properties, in particular for Fe,TiS,, where several phases appeared

centration of the intercalated species. When x varied from 0 to 1, lnoue et al. reported that parameter c changed from 5.68 to 5.78 A in the case of Fe,Ti&.

as a function of the Curie temperature. When the intercalation level was greater than 0.6. the authors observed a highly ferromagnetic state,

The ionic radius of the guest metal, ionicity or covalency of the bonds formed are responsible for this variation. In the plane (a,b), where a = i.. the lattice parameter is apparently unchanged, even if some

In the present work. we examine two intercalation compounds, Fe tIjTiSz and Ag, ,3TiSz. The electronic structure of iron and silver are sufficiently different to enable us to observe notable differences in behavior. The choice of stoichiometry (M,,,TiS,) was done in

‘Correspondingauthor. Tel: ( + 33) 59 92 30 IY; fax: ( + 33) 59 92 30 29: c-mail: [email protected]. ‘Tel: 56 84 26 90; fax: 56 84 27 61.

0925-X388/Y6/$15.00 PII

SO925-8388(

0 1996 Elscvier Science S.A. All rights reserved 96)02445-O

view of the known stability of these phases, Spectroscopies studies [4,5] and theoretical calculations [69] have been reported in the literature, and have led to

31

a better understanding of the electronic structure of these materials. However. little X-ray photoelectron spectroscopy (XI’S) precise studies have been done on Ag, ,.iT% The purpose of this paper is to realize a confrontation between XPS data (core peaks and valence hands) and self-consistent electronic calculations. We have chosen the augmented spherical wave (ASW) method [IO] which enables a combination of fast and accurate results: According to Mulliken population analysis. we interpret qualitatively and in a first approximation the charge transfer in relation to the chemical shift of the core peaks. Studies conducted by projected densities of states allow us to discuss the nature of the experimental valence bands and the interactions occurring in intercalated compounds. Furthermore. crystal overlap orbital population (COOP) CUNCS. which have been for the tirst time introduced in a selfconsistent method, enable us to study the hond order of Ti-S. Ag-S and Fe-S. With the help of these electronic characteristic results. we will interpret our next work. concerning scannrrq probe microscopy studies on intercalated compounds.

2. Crystallographic slrucbe of lhe compounds studied and computational details Titanium disulfide belongs to the transition metal dichatcogenides that crystallize in the CdI, system (space group P3rrrl). It can bc described as a simple juxtaposition of stacked layers along a crystallographic axis, generally c. In these layers. a succession of polyhedra share corners in two spatial dirc:t*tlns. leading to the strong anisotropy observed for I!CSC compounds. In the case we studied. such polyheura are octahedra for lT-TiS, and the structure can be considered as a compact stacking along the c axis, with cations occupying half of the octahedral sites. This cationic distribution leaves empty every other layer of the octahedral sites in the stacking direction. corresponding to the van der Waals gap. This conception enables us to understand that this compound is a good candidate For atomic intercalation, i.e. iron or silver in

our case, by the occupation of free octahedral sites. In the case of M,TiS, compounds, when x = I13 intercz lation m the van der Waals gap forms a lattice structure in the triangular (a.b) p!ane of parameter

U&X a&30”, where u is the distance separating two titanium atoms. The group forming the basic lattice structure is composed of three titanium atoms, six sulfur atoms and one atom of intercalated metal when the intercalation stage is first order. We can note that in the unit cell, one of the titanium atoms (Til)

plumb with metal (Fe or Ag) atoms and the two

(a) C&l Aalcms

w

0

a

S atoms

()

Matcms

0

Ti atoms

Fig. I Rcprwznl,rtic>n

of the laycrcd-like

unit cell. BiIsai pldnc projection

material TiS, and the Cd& phase.

of the intercalated

others (Ti2) arc below or above empty octahedral sites of the van der Waals gap. Fig. I (a) and Fig. l(b) show the basic three-dimensional TiS, lattice structure and a projection in the (001) plane of an M,,,TiS, compound. lattice parameters were determined by X-ray diffraction [11,12], and are summarized in Table 1. The ground state electronic structures of the sulfide systems were calculated using the ASW method in a scalar relativistic implementation ]13]. All valence electrons were treated as band states. In the minimal ASW basis set !lO]. we chose the outermost shells, i.e. Ti. Fe: 4s. 4p. 3d: Ag: 5s. 53. 42 and S: 3s. 3~. to represent the valence states. Energetically low-lying Ag (4d”‘) states were considered as core states. The ASW method is based on the density functional theory (DFT). in which the effects of exchange and correlation are treated within the local density approximation (LDA) using the parametrization scheme of

von Barth and Hedin [14] and Janak [15]. Moreover, within the atomic sphere approximation (ASA), the ASW method assumes overlapping spheres centered on the atomic sites where the potential has a spherical symmetry+entral potential. The volume of the spheres has to be equal to the cell volume because the Table

1

Lattice paramctcrs determined

by X-ray diffraction

[ll.l2l

32

wave equation is solved only in the spheres. Our non-unique choice of the atomic sphere radii was subjcctcd to the following ratios used throughout the calculations: r,,/rlutr_, = 1.52: rsIr,L.ITi, = 1.64: rllcli,,,, e*I:c/ blLl,,,and r,,,.,., are respectively introduced rwr.0 = 1.46) at the tetrahedral and octahedral sites and they serve as a unit for the different radii), These values were found to minimize the overlap between the atomic sphcrcs. For the host compound TiS,. we have included one sphere with zero nuclear charge on the octahedral site in the van der Waals gap and four empty spheres on the tetrahedral sites. For the intercalated compounds. two ‘octahedral’ and twelve ‘tetrahedral’ empty spheres were introduced. The Brillouin zone (BZ) integration was carried out for a sufficiently large number of k points (ten in each one of the three directions) in the irreducible wedge of the first zone of the hexagonal

Bravais

lattice.

Self-consistency

was

obtained when no variation of the chatgt tr;lnsfers (AQ < 10 “) and of the total variational ec:rgy E,.,, (AE < 10 ’ Rydberg) could be observed upon additional cycles. In this work, further addressing of the chemical bonding is proposed on the basis of the so-called COOP. of which a comprehensive account was given by Hoffmann from the quantum chemistry standpoint (extended Hiickel calculations) fl6]. This allows for the density of states (DOS) features to be discussed on the basis of chemical bonding criteria by weighting them with the sign and magnitude of the overlap integral between the relevant orbitals. One of the authors of this paper recently implemented the COOP in the ASW method with the objective to extract further information on the chemical bonding from first principles. For each system examined, TiS?, Ag,,jTiS, and Fe, ,,TiS,. we plotted the COOP and total DOS-in an energy window covering the valence zone and the lowest conduction bands. We chose to visualize the associated partial state densities thus: at the level of titanium: to each of the five 3d orbitals; at the level of sulfur: to the group of 3p orbitals and to the 3s orbital; at the level of iron: to the group of 3d; at the level of silver: to the group of 4d. In the considered energy window. the weak participa-

tion of Ag5p. /ig%, Fe 4p and Fc 4s orbitals is not reported on the DOS curves. Each energy mentioned for theoretical curves is related to E,.

3. Experimental The titanium dichalcogenides and the intercalated compounds were prepared by direct synthesis from the elements. Ti, S and Fe or Ag elements, mixed in the

stoichiometric proportions, were placed into a silica tube sealed under vacuum. The mixture was gradually heated to 640°C and then kept at this temperature for 1 week. The product of this reactian was ground and sealed again in a silica tube with a small amount of iodine (less than S mg cm -‘) to promote crystallization. The same heating procedure was applied to this silica tube. Single crystals, mostly platelet-shaped, were obtained in the cold end of the tube. Compounds were studied by XPS experiments. XPS analyses were performed with a Surface Science Instrument spectrometer (model 301) using a focused (diameter of the irradiated area: 300 pm) monochromatized Al Kol radiation (1486.6 eV) and interfaced to a Hewlett Packard CM00microcomputer. The residual pressure inside the analysis chamber was in the 5 X 10~’Pa range. Chemical composiGon and phase purity were examined by Castaing electronic microprobe (for the bulk) and quantitative XPS analysis (for the surface).

4. Results and discussion 4,l. Valence bands 4.1.1. Experimentul

results

The results obtained for the valence zones of TiS,, Ag , ,>TiS, and Fe, ,,TiS, are reported in Table 2 and the corresponding spectra are presented in Fig. 2. TiS?

For TiS, (Fig. 2(a)). two principal massifs are observed, with maxima 9.2 eV apart. That on the high energy side corresponds to a single band centered at 12.6eV and with a full width at half maximum (FWHM) of 2.2 eV. The broad band at lower binding

Table 2 XPS data valunce --

bands

Binding energy (cV)

TiS,

12.6

Ag,., TiSl

13.0

Fe,., TiS,

13.2

FWHM

(ev)

4.9

3.3

2.0

2.2

1.6

1.3

6.4

5.3

3.6

2.2

2.4

1.5

1.4

L.6

I.6

1.7

5.8

3.6

2.5

0.7

2.4

2.2

2.0

2.0

1.4

-

b

19

I5

11

7

Fig. 2. XPS valcncc hands of TiS. (a). Ag, ,TiS, (h) ml

energies is mt:re complex: it is about 7 eV wide and could be decomposed into three components which involve prmcipally S 3p orhitals with a weak contribution from the Ti 3d orhitals.

The experimental valence band of Ag, ,,TiS, (Fig. 2(b)) shows well-differentiated shapes resulting from different types of interaction. The signature of the intercalate is seen primarily in the broad band appearing towards high binding energies. decomposed into four components for the two substrates.

For the experimental valence band of Fe,,,TiS,. the shape of the triplet characteristic of the titanium

3 Billdktg amgr & Fc, ,TiS, (c).

disultides is conserved. but we note the appearance of a new component towards low energies attributed to the trace (Fig. 2(c)) of the intercalated metal. In addition. the Fermi level appears within this new component, reflecting the electron density characteristics of a metallic compound. The S 3s band centered at E = 13.2 eV and the width of the Ti 3d-S3p-Fe 3d. about IOeV. are in agreement with the valence band reported by Fujimori et al. [5] for Fe,,,TiSz. However, the line shape near E, isdifferent: If we compare with TiS,, the appearance of a new component towards low binding energies, attributed to the trace of the intercalated metal. is more pronounced than in Ref. [5]. In the case of Ag,,,TiS?. the weak intensity of the S 3s band in the experimental spectra (Fig. 2(b)), compared with TiSz or Fe,,jTiSz, can be associated

with the high value

of the photoionization cross-section of Ag4d and the number of valence electrons with regard to other atomic orhitals which participate to the valence band. (Photoionization cross-sections reported by Scoficld [ 171 are: 4d Ag = 1S5, 3d Fe = 0.17. 3s s = 0.14. 3p s = 0.07). WG note that the second component of the valence band. situated towards low binding energies, is distributed over a wide energy range (1OeV) for the two

intercalated species, compared with the host compound (7 eV). Fujimori et al. [5] have subtracted the spcclrum of TiS, from Fe ,,,TiS,. in order to highlight the chang: induced by inlercalation. The authors show that the difference spectrum obtained is also distributed over a *wide energy range. extended from E, to

E = 10 eV. and not only in a low binding energy (E E, = 3 eV). In this way, they suggest that this result

4-b

z-

C

32-

could be explair.rd by multiplet and satellite structure. We think that it is very difficult to affirm that the broadening of the S 3p-Ti 3d-Fe 3d compared with the S 3p-Ti 3d interaction in TiS, is uniquely due to the satellite structure because: the M 3d or M 4d modify the interaction between Ti 3d-S 3p states:

0.15

,,k.,d,

O,o;:A

,

almost, we can make difference valence band spectra if the two compounds have exactly the same work function (the same position of the Fermi energy and the same surface crystal state). which is very difficult to obtain with experimental compounds synthesized by the chemical vapor-transport technique. Of course, satellite structures could be clearly reported in the core peak energy range. For example, it

is well known that the 2p core-level XPS spectra of Ni or Co exhibit satellitc neaks, while iron shows less pronounced phenomena !5].

TiS, Based on the corresponding projected DOS curve

(Fig. 3), the theoretical valence band could be associated with S 3s orbital ionization with strong localization between - 11.3 and - 13.4 eV, and with the mixing of S3p and Ti 36 states between 0 and -5.2 eV. Beyond the Fermi level of the TiS, calculated densities of states, two sets appear, characteristic of the t,,/eg splitting of the d block of titanium, Between 0 and 4.1 eV, we observe two bands localized primarily on Ti 3d f2%and eg orbitals and where the contribution of S 3p orbitals is clearly visible and uniform throughout the entire zone.

The presence of silver leads to a considerable perturbation of the characteristic triplet of TiS,. The sum of the projected DOS per type of atom (Ag, Ti, S)

I;; -15

-5

-10

0

5

Energy (E-En eV Fig. 3. TiS~: total (a) and partial DOS fur S 3s (h). S 3p (c). Ti 4s and Ti 4p (in dolkd

lines) (d)

and Ti

3d (c) orbitals.

(Fig. 4) enables the most important attributions to be carried out: - between - 14 and - I2 eV, a band characteristic of S3s orbitals, with a very slight participation from Ti 3s orbitals states: - between -6.2 and 0 eV, a more highly structured band than the above involving primarily S 3p orbitals and Ag4d otbitals. with weak participation from the 3d orbitals of titanium atoms.

The first conduction band is localized primarily on Ti 3d orbitals of titanium, and further, the patticipation of S 3p orbitals is visible and uniform throughout the zone. The detailed analysis of these results shows that the intercalation of silver modifies the valence band of the host. There is a mixing between Ag4d and S 3p

orbitals. It should be noted that this interaction is ncft accompanied by a gain in energy since the ahovementioned states are occupied. A number of authors have stressed that this fact is responsible for the limit

-

-

c

B40

-

Fy?. 5. Fe, :TiS.:

total (a) and partial

DOS

for S 3s (hb.

S3p (c).

Fc .%I (d). Til 3d (c) and Ti2 3d (I) orbitals. Energy (E-E~J eV Fig. 4. Ag, Ag4d

,TiS,:

(d). Til

total (a) and partial

DOS

for S3s (h). S3p (c).

3d (c) and Ti2 3d (f) orbitok.

of interca!dtion (x < 0.42) of silver in Ag,TiS? [X]. In addition. the Ti 3d-S 3p interaction (in comparison with TiS,) is weaker, so the covalency of the titaniumsulfur bond is weaker in the intercalated compound. The calculated position of silver 4d orbitals on the DOS curve corresponds to the experimental spectrum,

where the main modification with respect to the host compound occurred towards high binding energies. The appearance of a weak electronic density at the Fermi level is clearly seen on the DOS curves. This density, localized mainly on Ti 3d orbitals. leads to the loss of the semiconductor nature of the host lattice, TiSI. These results are in agreement with calculations of electronic structure by Motizuki and Suzuki [8] in LAPW methodology.

Fe,,.,TiS, The sum of projected DOS by types of orbital (Fig. 5) enables us to attribute the ;y,incipal valence band and the first conduction bands. Between - 14 and - 12 eV there is a band characteristic of S 3s orbit& with a slight participation of titanium atoms, Between -5.9 and 0 eV, a band separated into two distinct components constitutes the last occupied states. The first component (-5.9 and - 1 eV) is characteristic of S 3p orbitals with a non-negligible participation from Ti 3d orbitals. The second component, from - 1 to

ON is associated with Fe 3d orbitals with a slight contribution from Ti 3d orbitals. The tirst conduction band (from 0 to 6eV) is a mixture of 3p orbilals of sulfur, 3d of iron and 3d of titanium. the latter predominating. The electronic structure of the parent compound TiS, is modified by iron intercalation. in particular by an Fe 3d-Ti 3d-.S 3p interaction, occurring partially in occupied levels. The XPS analysis of Fe,,>Ti& of this zone. decomposed into four components, shows the participation of the intercalant species towards high binding energies. This result agrees with the corresponding DOS curve that contains two massifs: the first results from an S 3p-Ti 3d interaction, similar to TiS,. and the second is primarily iron localization. The Fermi level situated in the second component leads to a high electronic density, associated with Fe 3d orbitals (40%) Ti3d orbitals (45%) and S3p orbitals (15%). This result agrees with transport properties, which reflect a metallic character, characterized by an increase in electron density of the order of lo*‘IO” e cm -J for intercalated compounds, compared with that of TiS, which is about 10” e- cmF3 [18]. The width of the calculated spectra in the S3pTi 3d and M 3d valence band region (0-6eV) is inferior to the corresponding experimental Sands. This disagreement could be related to: - the theoretical method used; - the multiplet and satellite structure, as Fujimori et ai. [S] propose. In this way, one electron band

structure

account

catcutation could not hc applied to take of this phenomenon. The authors have done

(CI) calculations on the configuration-interaction clustc:r model. For example. their study on the NiS:,” cluster model show that WC can understand the broadening of the Ni! .iTiS, valence band by taking into account fhc intra-atomic correlation energy. A cluster CI calculation un ES:,” was not reported because of the high theoretical difficulty of treating a high-spin qcn shell with six electrons. Thus, we cannot exclude the possibility of a satellite structure. A gcnet-al remark may be made, according to the chemical shift observed of the S 3s state (in the band) between TiS, and expcrirncntal valence Ag, , ,TiS? (0.4 eV) and Fc , ,,TiS, (0.6 eV). It could be explained by theoretical calculaiions: the DOS curve of TiS, exhibit a band characteristic of S 3s orhitals between - 11.7and - 13.3 cV with a very slight participation from Ti 3d orhitals (Ti 4s. Ti 4~). The same band for the intercalated compounds is situated bctwecn -12.1

- 12.1 and - 14eV (Ag!,>TiS,) and between and - 13.5, cV [Fc,,,TiS,). which is in agree-

ment with the chemical shift observed in the experimental spectra. The M 3d orhitals modify weakly the mixing between 3s S (occupied states) and 3d Ti (unoccupied states) in the energy range (- 14 to in - 12eV). and vary the order This Fe, ,>TiS, -Ag,,,TiS,--+TiS,. modification stabilizes the hand m&y as&&ted with S 3s orbitals. In the DOS curves. by integrating the participation of S 3s and Ti3d states in the energy range - I4 to - 12 eV. we find that: 3s S-3d Ti (TiS, ) = 36. I 3s S-3d Ti (Ag, ,jTiS1) = 16.4 3sS-3dTi (Fc,,,TiS,) = 11.2

do not lead to a satisf;uztory interpretation of each of the experimental XPS valence bands and core-peaks: for Fe, ,,TiS, and Ag, ,,TiS,, the Mulliken population analysis is misinterpreted with the extcndcd Hiickcl tight binding approximation, namely for the different charge transfer from Til and Ti2. Essentially. the iron high contribution to the DOS at E, in the theoretical curve seems overestimated. if WC compare with the XPS spectrum, where the Fermi level is within a low intensity component. Furthermore. the calculatrd position of silver 4d orbitals on the DOS curve. in the middle of the massif. does not correspond to th.x experimental spectrum, where the main modification with regard

to the host compound

occurred

towards

high binding energies. In this sense, ASW calculations presented in this paper are better. If we compared DOS curves and the energetic position of atomic orhitals, LAPW [i] and APW [6) calculations are in agreement with our results. Furthermore, p(E,) and the interaction between M 3d or M 4d and Ti 3d states decreases as the atomic number of M increases. as Suzuki et al. [IU] have reported in a paper which reports the electronic band structures and bond orders of M, ,,TiS, (M = Mn. Fe, Co, Ni). In order to specify the different interactions occurring with the guest metal (Fe 3d or Ag Jd states) and TiS,. we have studied bond orders for intercalated compounds. Considering two atoms that delimit a bond in the basic lattice in a given energy range. we can account for the slates by the bonding or antibond-

ing nature provided by the overlap population. The resulting plot (COOP) is an additional view of DOS curves at the level of electronic distribution. Band intensity in this case is. in addition to the number of states involved, related to the degree of overlap and the value of the weights attributed to the orbitals in question.

These results correspond to a higher mixing between 3s S and 3d Ti for Fe,, ,TiS,. compared with Agl,,TiS, and TiS,. Our theoretical calculations are in agreement with the experimental gap between the two main components of the valence band of TiS,. Fe, ,,TiS, and Ag, ,.lTiS,. (For the three compounds. the experimental and theoretical gap are calculated between the maximum of the S 3s band and Ihe maximum of the middle of the characteristic triplet of TiS,): Exp.

TiS, (gap S 3s-S 3plTi 3d) 9.3 eV Ag, ,,TiS, (gap S 3s-S 3piAg 4dl 9.4 eV Ti 3d) Fe, ,,TiS, (gap S 3s-S 3p/Ti 3d/ 9.6 eV Fe 3d)

Theory 9.2 eV 9.5 eV 10.5 eV

Previous TB-EHT calculations done by our group

The analysis of the Ti-S hond in TiS, (Fig. 6(a)) shows an essentially bonding character before the Fermi level (corresponding to the mixing between Ti 3d eglt,, and S 3p states) and antibonding after (corresponding to the mixing between Ti 36 tzl!leg and S 3p states). The COOP study of the same bond in intercalated compounds (Fig. 6(b) and Fig. 6(c)) reveals weaker intensity bands than in the case of TiS,. The integration of the three curves up to E, provides information on the overlap population of the bond. It is smaller for Fe I ,JTiSz and Ag, ,,TiS, ( I .81 and I.88 respectively) than for the host system ( 1.94). In other words, and as we have mentioned above, the covalency of the Ti-S bond is weakened by the intercalation process. Our results for Fe, ,,TiS,, very similar to TiS,, are in agreement with bond orders reported by Suzuki and coworkers [20,19]. The COOP curve for the Ti-S bond in Ag, ,3TiS, presents some differences compared with

37

-2+---i----l -5

0

5 E=rgy

Fig. 7. I‘OOP

curve’s for the Fe-S

(a) and Ag-S

(E-E0 (b) bonds.

the energy range -0.5 to 4 eV corresponds to the lack of interaction between silver and sulfur atoms. Energy (E-Ef) eV

42.1.

Fe, ,,TiS,: the interaction between Ti 3d (cg) and S 3p states leads to weaker bonding bands (in the energy range -6 to -4 eV), while the mixing between Ti 3d (t?,) and S 3p states corresponds to higher bonding bands (in the energy range - I to I) eV). The COOP curves of the Fe-S and Ag-S bonds are reported in Fig. 7. For Fe,,,TiS,: (i) the weak interaction between Fe 3d (eg and tze) and S 3p states leads to bonding bands in the energy range -5.6 to - 1 eV; (ii) the interaction between Fe Jd (t,,) and S 3p states leads to antibonding bands in the energy range -1 toOeV; (iii) after the Fermi level. the interaction between Fe 36 (eg) and S 3p states leads to antibonding bands. For Ag, ,,TiS,: the antibonding bands before the Fermi level in the Ag-S bond (Fig, 7(b)), between - 3.8 and - 0.5 eV, are characteristic of an interaction between occupied levels (Ag 4d’O-S 3p states) and correspond respectively to the mixing between Ag 4d (t,,) and (eg) with S3p states. In other words, the Ag-S band is weaker than the Fe-S bond. The very weak intensity band in

E.ywintental

results

Stringent handling conditions were taken to avoid contamination by oxygen and carbon. We proceeded to a precise determination of the binding energies associated with the peaks S 2p,,,_ ,,?. Ti 2~~,~_,,~, Ti 3s. Ti 3p. Ag 3ds,z_.t,z and Fe 2p3,?_,,?. characteristic of these compounds (see Table 3). The S2p,,,-,,, doublets are perfectly resolved and present energies (associated with the two components 2P,,, and 2p,,,) characteristic of sulfur in a metaltic environment and similar for the three compounds. The full widths at half maximum are comparable between the host lattice structure and the intercalated samples. The Ti 2p, ,? _i ,? peak at 456.1-462.2 eV (Fig. 2) for TiS, is well defined and no trace of contamination by TiO, was observed. The same core peak for Ag,,,TiS, and Fe,, ,TiS, is clearly broadened with a pronounced shoulder towards low binding energies. The 312 and l/2 components can be dec\?mposed into two bands. This phenomenon can be interpreted by an initial and/or final state effect which will be discussed below. It should be noted that in the case of the Ti 3s and Ti 3p peaks, there was only a broadening (less pronounced than for Ti 2p) for the two intercalated compounds, compared with TiS,, with no change in tile chemical shift. (707.4-720.3 eV) for The Fe 2~~,~_~,~ peak

Fe, ,TiS_ appears at B higher binding energy than for metallic iron (7M.6 [ 1.I j-710.5 [ 1.61eV) probably re-

flecting the appearance of a positive charge on the atom (FWHM in brackets). The shift of lhe 3d5,2_J,2peak of intercalated silver (367.8-373.X eV) compared with metallic silver (368.1 [ 11-374.2 il] cV) towards low energies is identical to that observed for a number of silver-based compounds [2lj. One example of this is the binding energy of the Agjd,,, J,I peak for Ag,S (367.8 [I]373.1)[l] eV). very similar to that showtl in the table for intercalated silver.

4.X

Mullikw

popdurim

ud_vsi.s ad

di.wmim

a first approximation. the interpretation of the chemical shift of the core peaks can be correlated with the calculated population analysis. which is characteristic of the initial state of the samples. A Mulliken population analysis (sumnlariled in Table 4) was carried out on the basis of the charge distribution in the ground state. As empty spheres are coordinated by sulfur atoms. we can redistribute their net charges on these atoms. fn this way. sulfur atoms have a total charge of 6.70, 6.65 and 6.70 for TiSI. Ag,,,TiS, and Fe, ,,TiS, respectively, For intercalated compounds, titanium atoms occupy different cryslallographic sites and are differentiated by their respective net charges. It is to be remembered that in the basic lattice of Ag,,,TiS, or Fe,,, TiS?. one of the titanium atoms (Til ) is in the vertical axis of a silver or iron atom, As

Tahlc 4 Total charge calculated (Mullikcn id

Ld

NC

sites and non-uquivaknt

Til Ti2 S

population

analysis): FL_,. Ii,_,

with octahedral tclrahudral sites in the structures

rcspcctivclyempty sphucs aswciatcd TiS,

Fc,, ,TiS,

b,,Ti%

2.9 2.5, S.K

3.1 2.9 fl.1

3.0 2.Y h.O 9.7

AK FC

6.9

E 111 I

0.4

3.3

0.35

E ,Lll.,I

0*2

0.2

0.20

E Q.#,.,?

0.3

0.3

0.30

while the other two (Ti2) are below (or above) the two empty octahedral sites of the van der Waals gap. Charge transfer from the intercalated metal towards the host lattice is shown by the loss of 1.3 electrons from silver and 1,I electrons from iron that is recovered partially by titanium atoms that become charged (compared with the same atoms in TiS,) and hy empty spheres. The position of the S 2p,,? l ,? peak, equivalent for the three compounds, agrees with calculated charges on the sulfur atoms, and is of the same order of magnitude for both the host lattice structure and the intercalated samples. The broadening of the Ti 2p, Ti 3s and Ti 3p peaks, associated with a slight shift towards low binding energies (in particular for the Ti 2p peak) accounts for the charge transfer from the intercalant to Ti atoms. The differentiation of these two Ti atoms according to charge analysis (Table 3) results from rhe fact that they are at non-equivalent sites in terms of the intercalated metal. The interacting orbitals involved are thus different depending on the atom considered, leading to changes in electronic density (see DOS curves on Figs. 4 and 5). This supports the hypothesis of an initial state effect responsible for the modifications observed in XPS at the level of the Ti2p, Ti 3s and Ti 3p peaks. However, when we consider relaxation effects of ionized systems in the final state, this can also explain the observed phenomenon. The creation of a photohoie produces a rearrangement of the electronic structure which tries to screen the positive charge. This dynamic polarization phenomenon reduces the binding energy of the electron involved in the experimentally observed chemical shift by stabilizing the final state. It should be noted that in the case of the Ti 3s and Ti 3p peaks, we noted only a broadening with the two intercalated compounds, compared with TiS,, with no change in the binding energy. Since relaxation effects are greater in proportion as we consider states nearest the nucleus, we attribute peak broadening primarily to an initial state effect. The chemical shift observed for the Ti 2~~,~_,,~ peak involves not only the initial state but a final state effect as well. Prior work by Weitering and Hibma [4] on silver intercalation in TiS, explains the broadening

39

and the dissymmetric shape of the Ti 2p peak. The authors rejected the hypothesis of an initial state effect for two reasons: (i) the partial population of d lcvcls of the host lattice structure during intercalation would creak

two different

degrees nf oxidation of titanium

(Ti” and Ti”) and thus lead to two componcnts for the peak in question. The authors nevertheless considered that 3d electrons in TiS,-intercalates are delocalized and that their cffccts would not atoms

lead to significant changes of electronic repulsion; (ii) the lack of knowledge on the distribution of the intercalated metal in the van der Waals gag may give

rise to non-equivalent titanium sites. This point is undoubtedly due to the synthesis process employed: silver growth on TiS, (001) leading to the diffusion of metal atoms in the gap. In agreement with the conclusion of Fujimori et al. [Sl in work on the intercalation of transition metal atoms in TiS,. the authors concluded on a final state effect. In our point of view. as we have proposed. it is more rational to look at final

and initial states effects. It is seen that Fe atoms are positively charged with reference to their ground state, increasing to a respective net charge of + 1.I e ‘. The classical view admitting a charge transfer from the intercalated species towards the host lattice structure is consistent with the observed chemical shift towards high binding energies

for the Fe 2p,,, ,,? peak. This view. in agreement with the frozen orbitals concept, is insufficient to explain the shift of the 3d5,, .l,, peak of intercalated silver compared with metafiic silver. An initial state effect characteristic of Ag’ is superimposed in a linal state effect, capable in reversing the direction of the chemical shift seen in terms of polarity. The creation of a photohole in 2p of intercalated silver in oxidation state I would change bielectronic repulsion. In this case, the phenomenon is more pronounced than for metallic silver, since there is a greater contraction of d orbitals of a silver atom in oxidation state I. The final state is thus stabilized to a greater extent than the stabilization of the initial state and reverses the direction of the

chemical shift. in addition, electronic repulsion IS greater when the number of electrons arising from d levels is higher. This can explain the phenomenon observed with silver (d”‘) and to a lesser extent with iron (d’), as noted Gaarenstroom and Winograd [21] in a work on the initial and final states effects of silver and cadmium oxides,

5. Conclusion

We have studied the electronic structutc ~1 intercalation compounds M,KTiS2(M = Fe, Ag) L; using %-S and calculated densities of states. The valence ban& are shown to be strongly r jtified by the intercalant species. The XPS analysis 01 ~g, ,3TiS, and Fe, ,,TiS2 exhibit two kinds of bebior, interpreted as Siffertnt ‘host-guest’ interactions. 1 he &en&l shifts of the core peaks have been cor;@atei wit:? Initial a~4 final states. the former based on the charge transfer ~.&ich occurred from the jntercata.b* to !k hose tit.tGm atoms.

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