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Electronic structures and charge transfer in lithium and mercury intercalated titanium disulfides
0022~3697(95)00&g-4
Pergamon
J. Phvs Chem Solids Vol 57. Nos 6-g. pp. I Il7- 1122. I996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reset-& 0022-3697/96 $15.00 + 0.00
ELECTRONIC STRUCTURES AND CHARGE TRANSFER IN LITHIUM AND MERCURY INTERCALATED TITANIUM DISULFIDES P. MOREAU, Institut
des Matkriaux
G. OUVRARD,?
P. GRESSIER,
P. GANAL and J. ROUXEL
de Nantes, UMR CNRS I IO,2 rue de la Houssiniere, 44072 Nantes Cedex 03, France (Received 28 May 1995; accepted in revisedform 31 May 1995)
Abstract-Mercury can be intercalated into TiSz by a reaction between elemental mercury and TiSr at room temperature. The structure of the obtained compound Hgl.z4TiS2can be described as two non-commensurate monoclinic sublattices. The mercury atoms form metal chains inserted into trigonal prismatic channels created by the expanded TiS2 host lattice. The structural arrangement and interatomic distances for this compound indicate the presence of primarily neutral mercury, with very low charge transfer, and relatively weak Hg-S interactions. In order to understand this peculiar behaviour, electron band structure calculations have been made using the extended Hiickel method and compared with experimental data from different spectroscopies: XAFS, EELS and XPS. Pristine TiSz and its lithium and mercury intercalated compounds have been investigated. The experimental data are in good agreement with the calculated electronic structures. The main conclusion is that 0.24 electrons are transferred by lithium atom to the TiSr host structure for the composition Li,Ti$. This transfer is almost equivalent on titanium (0.09 electrons) and sulphur (0.075 electrons per atom). For the mercury intercalated TiSz phase, both calculations and experimental data show an electronic transfer from mercury to TiSz very close to zero. Keywords: A. chalcogenides, C. electron energy loss spectroscopy, C. photoelectron spectroscopy, C. XAFS,
D. electronic structure.
1. INTRODUCTION It has been shown that mercury can be intercalated into TiS2 by a reaction between elemental mercury and TiS2 at room temperature [l]. A stage 1 compound is obtained with a composition Hg1,24TiS2. Its structure can be described as two monoclinic sublattices which are incommensurate along their b axes, one corresponding to the c expanded and slightly distorted TiS2 host sublattice, and the other to the intercalated mercury sublattice [2]. The mercury atoms form metal chains located in trigonal prismatic channels created by the expanded TiS2 host lattice. Their structural arrangement and interatomic distances favour primarily neutral mercury, with very low charge transfer, and relatively weak Hg-S interactions. Room temperature magnetic susceptibility measurements [3] and TDPAC experiments on analogous mercury intercalated TaS2 compounds [4] suggest, by comparison with lithium intercalated compounds, that the electronic transfer could be about 0.3 electron per mercury atom. A iggHg NMR study for mercury intercalated TaSZ phase supports weak charge transfer and weak mercury bonding [5]. Such studies address the general problem of the nature and amplitude of guest-host electronic transfer in this unusual intercalation process. t Author to whom correspondence should be addressed. ICI
57:6,8-P
Accurate determination of the charge transfer requires good knowledge of the electronic band structures of both the pristine and intercalated compounds. Although the electronic structure of lamellar transition metal disulphides has been thoroughly investigated over the past 25 years, using different theoretical approaches, similar studies of intercalated phases are relatively uncommon. A review in 1987 [6] concluded that the problem of electronic transfer had been solved for lithium intercalated compounds, with lithium being fully ionized with its 2s electron being transferred to the transition metal d orbitals. Such a conclusion is mainly based on a rigid band model and bulk techniques. From a band structure calculation point of view, the review [6] cites the paper of Umrigar et al. [7] in which the calculations used the LAPW method for both pristine and lithium intercalated TiS.2. However, the results were not directly compared due to the unknown relative position of the Fermi level, which was taken as zero in both cases. Nothing is said in this paper about the nature and the amplitude of the charge transfer. A more recent work [8] on various 2H-TaS2 intercalation compounds concludes, from LMTO band structure calculations, that lithium only transfers 0.45 electrons per intercalated lithium. These electrons are shared by tantalum (0.07 electrons per atom) and sulphur (0.19 electrons per atom). Thus the question of lithium charge transfer in these hosts in
1117
P. MOREAU et al.
1118
unresolved. In order to contribute new insight into lithium charge transfer and the novel Hg1.24TiSz charge transfer problem, we performed band structure calculations together with various spectroscopic analyses, which are characteristic of occupied states, empty states and Fermi level, namely X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS) and electron energy loss spectroscopy (EELS). This study has been made on TiS2 and its lithium and mercury intercalates.
-Q.=O” . . . . . . .._
-a
2464
2466
2. EXPERIMENTAL TiSz powders have been obtained following a classical procedure [9]. Lithium intercalation has been performed by the n-butyl lithium technique at room temperature [lo] with the lithium content determined by flame spectroscopy. The mercury intercalate was prepared by a room temperature reaction between TiS2 powder and elemental mercury. In order to prevent sample degradation, they were handled in glove boxes and transferred to the different systems for analysis via appropriate air-tight containers. XPS experiments have been made using an SSI spectrometer and in situ surface cleaned samples. The incident monochromatized AIK_L radiation (1486.6eV) was focused on an area of 600pm diameter. The EELS experiments (titanium L2_s edge) were carried out on a CM30 Philips TEM, operated at 200 kV and equipped with a Gatan 666 multichannel spectrometer. The experimental conditions led to an energy resolution of 0.9eV FWHM for the zero loss peak. XAFS spectra were recorded using the French synchrotron radiation source at LURE (Orsay). The titanium K edge was studied using the DC1 storage ring, with a two Si3 11 crystal monochromator, and 0.2eV resolution. The same resolution was attained for the sulphur K edge recorded on the Super AC0 storage ring. Band structure calculations have been performed using the Extended Hiickel approach. It is known that this method, mainly due to its semi-empirical character, is not very accurate in describing the empty levels. Nevertheless, it is very useful for our purpose in its precise knowledge of the nature of electronic levels and the easy way to study, in a rigid band model, the evolution of an electronic structure in a topotactic reaction. In order to take into account the modifications of the atomic charges resulting from the studied charge transfer, an iterative procedure on the charge has been introduced for the lithium intercalated compound. For the mercury intercalate, due to the incommensurability of the structure, a supercell has been used for the calculations with a = 5.923A, b = 13.75A z 4 x 3.44A x 5 x 2.75A, c = 8.86A and
a
=
30”
-____a=5y
= 80”
2468 2410 Energy (eV)
2472
2414
Fig. 1. X-ray absorption spectra recorded at the sulphur K edge for a T& single crystal for different values of the a angle between incident radiation and the normal to the crystal ab plane.
/I = 102.3”. The higher complexity of these calculations kept us from applying charge iteration to these studies. 3. RESULTS
3.1. Pristine materials Our first step in the above approach was to verify the experimental spectroscopic data can be easily explained by comparison with calculated band structures of TiS*, lT- and 2H-TaS2. The shape of the valence band of TiSz was found to be directly comparable with the calculated density of states (DOS). The evolution of the edge position and peak intensities of the absorption edge for dichalcogenide single crystals versus the o angle between the radiation and the normal to the ub place of the crystal was also in good agreement (Fig. 1). We may then compare, for example, the experimental influence of the px, py and pz orbitals of sulphur and their relative contributions to the calculated DOS (Fig. 2). When cy = O”,only the px and py orbitals are involved in the electronic transition. Increasing Q leads to a gradual increase
-I
-12
-10
-8 Energy (eV)
-6
-4
Fig. 2. Density of states of TiS2, projected on the sulphur px (thin line), py (dotted line) and pz (broad line) orbitals.
1119
Li and Hg intercalated TiSz
-14
-12
-10
-8
-6
-4
-2
0
Energy (eV) Fig. 3. Total density of states calculated for TiSr (full line) and LiiTiSr (dotted line) after an iterative procedure taking into account the charge variation. The vertical bars indicate the Fermi level positions.
in the contribution of thepz orbitals to the transition. Due to the slightly lower energy of pz, the DOS explains why the X-ray absorption edge is shifted toward lower energy when o increases. The relative shapes and positions of the pz, px and py orbitals in the DOS are also in very good agreement with the increasing asymmetry of the first peak in the edge. The decreasing intensity of the second peak is also explained by the very low contribution of the pz orbitals in the DOS around -5eV, even if a more important effect would be expected. In the same way, it has already been shown that the suiphur edge shape can be used to characterize the tantalum coordination, i.e. octahedral and trigonal prismatic for the 1T and 2H forms of TaSz, respectively, when taken together with the electronic structures of these materials [ 111. 3.2. Lithium intercalated TiS2 Figure 3 shows total DOS calculated for TiSz and LilTiS using an iterative procedure. The main differences primarily concern the positions of their features (Table 1). Upon intercalation, the valence band is shifted by about 0.8eV to higher energies. At the same time, due to the displacement of the first accepting levels and their partial filling, the Fermi level (Er) is shifted in the same direction by 1.2 eV. The lower empty levels, which mainly consist of titanium T2g
orbitals, are also found at a higher energy by about 0.5 eV, while those corresponding mainly to the titanium Eg levels have not moved. Just above these levels are the titanium 4p levels whose energy is lowered by about 0.6eV. Many of these features, such as the decrease of the separation between the T2g and Eg levels and the lowering of the valence band relative to EF, are in agreement with the conclusions of Ref. [7], where values of 0.3 and 0.7 eV have been calculated for these values, respectively, compared with of 0.5 and 0.4 eV from our calculations. The EF can be related to the open circuit voltage (OCV) of a lithium battery, which, to a first approximation measures the potential of the first accepting levels of a positive material (Er) versus a constant value for the electrochemical couple Li/Li+. Therefore, the EF displacement must be close to the variation of the OCV. Experimentally this can be estimated around 0.8 eV [ 121,in agreement with the 1.2 eV we have calculated. XPS experiments measure the transitions between occupied electronic levels and EF. Table 2 gathers the energies of various peaks corresponding to core levels or valence states, before and after lithium intercalation. All the transitions move to higher energies by values ranging from 0.1 to 0.6 eV, with a mean value of 0.35 eV. The positions of the core levels have not been calculated. However, we may estimate that their shift with charge transfer is similar to that for the valence state, the position of both being largely modified by the same screening effect. The above mean value is in a very good agreement with the raising of these starting levels by 0.8 eV and EF by 1.2 eV (Table 1). In Table 1 the XAFS and EELS experimental data are also compared with the difference between the calculated valence band/core-level energy shift and the shifts of the other electronic levels. Only selected orbitals in the projected DOS for a given atom were considered. In effect, these spectroscopies are very selective for both the type of atom and the electronic levels involved in the dipolar transition. The L2.3 EELS titanium spectra shown in Fig. 4 correspond to the transition of 2pi12 and 2ps,z electrons to unoccupied titanium d states, consistent with the selection rule Al = 1. They consist of two double peaks corresponding to L2 and L3 edges around 462 and 455 eV,
Table 1. A comparison of calculated energy shifts (in eV) for lithium intercalation of TiSa (LilTiS*) with XPS. XAFS and EELS observations
Calculations Calculated energy shifts XPS Titanium K edge (XAFS) Titanium L2,3 edge (EELS) Sulphur K edge (XAFS)
Valence band
Fermi level
‘T2g’ group
‘Eg’ group
Titanium 4p
+o.s
+1.2 +0.4 +0.35
f0.5 -0.3
0.0 -0.8
-0.6 -1.4
-0.3
-0.8
-1.6 +0.4
1120
P. MOREAU et al.
Table 2. Energies of various XPS peaks for TiSr and Li,Ti$, corresponding to valence band and core electronic levels Peak nature
Valence band
TiS2 LilTiS
2.2 eV 2.6eV
respectively.
3SeV 3.9eV
By comparison
with the DOS projected
d orbitals,
on the titanium
5.leV 5.6eV
the lower energy part of
each double peak, which appears more as a shoulder, can be attributed corresponding
to T2g orbitals, with the main peak to Eg orbitals.
that, upon lithiation, the main
feature
respectively.
Our calculations
the energies of the shoulder are lowered
These
with the estimated
show
values
are in good
increase
0.8eV and the calculated
by 0.3 and
agreement
of the core
displacements
and
08eV,
levels by of the T2g
and Eg orbitals by +0.5 eV and 0.0 eV, respectively. The same comparison results.
Figure
pristine
and lithium
Only a slight displacement
intercalated for LilTi&.
with the 1s + 3p transition,
DOS projected
Ti!$.
of the edge toward higher
energy of about 0.4eV is observed in agreement
K edges of
the sulphur
and mercury
on the sulphur
3p orbitals
This is
based the (Fig. 6).
Indeed, a noticeable fraction of 3p sulphur empty states is found above the Fermi level. They have roughly
the same shape
and, as previously
as the titanium
S 2P1/2
Ti 2P3/2
Ti 2~112
162.1 eV 162.2eV
456. I eV 456.4eV
462.2 eV 462.8 eV
decrease
of the first peak with lithium intercalation,
which corresponds
d orbitals
discussed for Ti&, the double peak
to partial filling of the lower empty
3p sulphur states, referred as ‘T2g’ due to their overlapping with the T2g titanium strates
that
transfer
sulphur
orbitals. part
This demon-
in the electronic
with intercalation.
The intensity to unoccupied
states,
associated
takes
of the second peak, which corresponds remains
unchanged
with
intercalation,
as
expected. Finally we will consider corresponds
can be made using XAFS
5 compares
S 2P3jz 160.9eV 161.1 eV
K edge which
the titanium
to the titanium
1s + 4p electronic
transi-
tion (Fig. 7). A small pre-edge peak is observed, whose intensity decreases with lithium intercalation. The origin of this peak is not precisely known. It may be due to either a transition
to d levels, allowed
partial quadrupolarcharacter
of the transition,
small fraction
of p orbitals,
which were found at this
energy by band structure calculations, ever
its origin,
explained
by a or to a
its amplitude
or both. What-
reduction
is easily
by the partial filling of these lower energy
empty levels in TiS2. More important is the main edge shift towards lower energy. The 1.6eV displacement
observed at the edge can be related to overlapping T2g and Eg orbitals. We observe a smaller separation
increase in the core level energy and the lowering
between
the titanium
these peaks (around
2eV) than that calcu-
lated for the DOS (3 eV), corresponding to the known tendency of the Extended Hiickel method to overestimate upon
the empty
lithium
level separation.
intercalation,
Nevertheless,
an increase
in &
for LilTi&
is in good
agreement
3.3. Hg1.24TiS2 lithium intercalated
edges for pristine,
and mercury intercalated
about I .2 eV is calculated,
which is partially counter-
compared.
balanced
in the core level energies,
Hg1,24TiSZ are very close to those
by an increase
which is estimated to be 0.8 eV. These two values are in good agreement with the observed shift of the sulphur K edge. Another
important
feature is the significant
4%
460
465
Energy W) Fig. 4. EELS Lz.3 titanium edges for TiSz (thin line), LilTiS (dotted line) and Hg1,24TiS2 (broad line).
of
4p levels by about 0.6 eV.
In Figs 4, 5 and 7 the various
of
with the 08eV
pristine
Generally host,
for LilTiS unchanged
the features
while the principle
TiS2 are
associated observed changes
with in the
observed
are not seen. This is highlighted by the position of the titanium K edge observed
2470
2475
Energy (eV) Fig. 5. XAFS sulphur K edges for TiS2 (thin line), LilTiS (dotted line) and Hgl 24TiS, (broad line).
Li and Hg intercalated TiSr
mergy (eV)
Fig. 6. Density of states projected on the sulphur 3p orbitals in the cases of TiSz (thin line) and LirTiSr (dotted line). for Hgl,zJTiSz in Fig. 7. However, slight changes are
observed in the intensities of the prepeak for the titanium K edge and in the first peak for the sulphur K edge, and a slight displacement of the L2,3 titanium edge is observed indicating that some electronic exchange occurs upon mercury intercalation, in agreement with previous physical measurements [3]. Almost no change in the peak positions, upon mercuration, has been observed in XPS experiments.
4.
DlSCUSSION
The spectroscopy results are in good agreement with the band structure calculations and the proposed electronic modifications which appear in the position and filling of electronic levels for lithium and mercury intercalation. The results are especially accurate for the Li-TiSz system in which an iterative procedure on the charge has been used. Therefore, we may deduce from the calculated band structures the values of the charges on each atom via a Mulliken population calculation. For TiSz, a charge of +0.29 is calculated on titanium, counterbalanced by a charge of -0.145 on each sulphur atom. These rather low values are in
4970 Energy
4975 (eV)
Fig. 7. XAFS titanium K edges for TiSr (thin line), LirTiSr (dotted line) and Hgr,z.,TiSr (broad line).
1121
agreement with the highly covalent character of this compound. The same calculations for LilTiS show that 0.24 electrons per lithium are given to the host structure. This charge is shared almost equally between the titanium and sulphur atoms, whose charges become f0.20 (-0.09) and -0.22 (-0.075) per atom, respectively. These results are in good agreement with those of Guo et al. [8] and disagree with the simplistic schematic view of a fully ionized lithium, whose electron has been donated to titanium. The charge transfer of 0.09 electrons to the titanium d orbitals raises the energy level of the occupied electronic states by increasing the d electron screening effect. A simple calculation of this effect can be done by using Slater’s screening coefficient. The screening effect of Id electron on a 1s electron in a titanium is 8.2eV. The same calculation for a 2d electron effect in manganese is 18.1 eV. By comparison the above calculated charge transfer of 0.09 electrons on titanium d orbitals will raise the core levels by 0.7-0.8 eV in excellent agreement with the value of 0.8eV estimated from the spectroscopic data. Even if the band structure calculations are less precise for Hg1.24TiS2, a similar calculation estimates the charge on intercalated mercury to be close to 0.02 electrons. Such a low value explains the almost elemental behaviour of intercalated mercury and the physical measurements which suggest similar behaviour between mercury and dilute (around 0.25) lithium intercalated compounds. This is apparent from an interpretation of the magnetic susceptibility values observed for these intercalation compounds using the calculated band structures. We know that for metallic compounds, the Pauli paramagnetic susceptibility is proportional to the DOS at EF, N(&). For TiS2, & is found in the gap and N(Es) is zero. For LilTiS and Hg1,z4TiS2, N(&) is calculated at 3.5 and 1.5 states per eV, respectively. This corresponds to a ratio of 43% between the densities of states of mercury and lithium intercalated compounds, in perfect agreement with the 43% ratio between the experimental Pauli susceptibilities of both compounds. This study on TiSz intercalated compounds has shown that spectroscopy is essential in confirming the validity of band structure calculations. From such a comparison, an accurate determination of the charge transfer upon intercalation can be made. Here we have been able to determine precisely its nature in the lithium intercalates and to confirm that intercalated mercury is very close to an elemental state. Acknowledgements-We thank Dr Mike McKelvy for valuable discussions and corrections of the paper, Professor J. L. Mansot for his help in collecting EELS data and Dr D. Gonbeau for the XPS experiments. This work was supported by a Human Capital and Mobility EC grant (PG).
P. MOREAU et al.
1122 REFERENCES
1. Ong E. W., McKelvy M. J., Ouvrard Cl. and Glaunsinger W. S., Chem. Mater. 4, 14 (1992). 2. Ganal P., Moreau P., Ouvrard Cl., Sidorov M., McKelvy M. and Glaunsinger W., Chem. Mater. 7, II32 (1995).
7.
3. Moreau P., Ganal P. and Ouvrard G., Mol. Crvs?. Li9. Cryst. 244,325 (1994). 4. G&al P., Olberding W., Butz T. and Ouvrard G., Chemical Phvsics oflntercalation II (Edited bv P. Bemier. J. E. Fischer: S. Roth and S. Solin), Nato ASI Series, Vol: B305, p. 383. Plenum Press, New York (1993). 5. Gossard A. C., Di Salvo F. J. and Yasuoka H., Phys.
9.
Rev. B 9, 3965 (1974). 6. McKinnon W. R., Chemical Physics of Intercalation
(Edited by A. P. Legrand and S. Flandrois), Nato ASI
8.
IO. I 1.
12.
Series, Vol. Bl72, p. 181. Plenum Press, New York (1987). Umrigar C., Ellis D. E., Wang D. S., Krakauer H. and Postemak M., Phys. Rev. E 26,4935 (1982). Guo G. Y. and Liang W. Y., J. Phys. C: SolidState Phys. 20,4315 (1987). McKelvy M. J. and Glaunsinger W. S., J. Solid State Chem. 66, I81 (1987). Dines M. B., Mater. Res. Bull. 10,287 (1975). Moreau P. and Ouvrard G., Chemical Physics of Intercalation II (Edited by P. Bernier, J. E. Fischer, S. Roth and S. Solin), Nato AS1 Series, Vol. 305, p. 351. Plenum Press, New York (1993). Thomoson A. H.. Fast Ion Transoort in Solids (Edited bv P. Vaihishta, G.‘N. Mundy and G. K. She&y), p. 47. Elsevier, North Holland (1979).
Pergamon
J. Phvs Chem Solids Vol 57. Nos 6-g. pp. I Il7- 1122. I996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reset-& 0022-3697/96 $15.00 + 0.00
ELECTRONIC STRUCTURES AND CHARGE TRANSFER IN LITHIUM AND MERCURY INTERCALATED TITANIUM DISULFIDES P. MOREAU, Institut
des Matkriaux
G. OUVRARD,?
P. GRESSIER,
P. GANAL and J. ROUXEL
de Nantes, UMR CNRS I IO,2 rue de la Houssiniere, 44072 Nantes Cedex 03, France (Received 28 May 1995; accepted in revisedform 31 May 1995)
Abstract-Mercury can be intercalated into TiSz by a reaction between elemental mercury and TiSr at room temperature. The structure of the obtained compound Hgl.z4TiS2can be described as two non-commensurate monoclinic sublattices. The mercury atoms form metal chains inserted into trigonal prismatic channels created by the expanded TiS2 host lattice. The structural arrangement and interatomic distances for this compound indicate the presence of primarily neutral mercury, with very low charge transfer, and relatively weak Hg-S interactions. In order to understand this peculiar behaviour, electron band structure calculations have been made using the extended Hiickel method and compared with experimental data from different spectroscopies: XAFS, EELS and XPS. Pristine TiSz and its lithium and mercury intercalated compounds have been investigated. The experimental data are in good agreement with the calculated electronic structures. The main conclusion is that 0.24 electrons are transferred by lithium atom to the TiSr host structure for the composition Li,Ti$. This transfer is almost equivalent on titanium (0.09 electrons) and sulphur (0.075 electrons per atom). For the mercury intercalated TiSz phase, both calculations and experimental data show an electronic transfer from mercury to TiSz very close to zero. Keywords: A. chalcogenides, C. electron energy loss spectroscopy, C. photoelectron spectroscopy, C. XAFS,
D. electronic structure.
1. INTRODUCTION It has been shown that mercury can be intercalated into TiS2 by a reaction between elemental mercury and TiS2 at room temperature [l]. A stage 1 compound is obtained with a composition Hg1,24TiS2. Its structure can be described as two monoclinic sublattices which are incommensurate along their b axes, one corresponding to the c expanded and slightly distorted TiS2 host sublattice, and the other to the intercalated mercury sublattice [2]. The mercury atoms form metal chains located in trigonal prismatic channels created by the expanded TiS2 host lattice. Their structural arrangement and interatomic distances favour primarily neutral mercury, with very low charge transfer, and relatively weak Hg-S interactions. Room temperature magnetic susceptibility measurements [3] and TDPAC experiments on analogous mercury intercalated TaS2 compounds [4] suggest, by comparison with lithium intercalated compounds, that the electronic transfer could be about 0.3 electron per mercury atom. A iggHg NMR study for mercury intercalated TaSZ phase supports weak charge transfer and weak mercury bonding [5]. Such studies address the general problem of the nature and amplitude of guest-host electronic transfer in this unusual intercalation process. t Author to whom correspondence should be addressed. ICI
57:6,8-P
Accurate determination of the charge transfer requires good knowledge of the electronic band structures of both the pristine and intercalated compounds. Although the electronic structure of lamellar transition metal disulphides has been thoroughly investigated over the past 25 years, using different theoretical approaches, similar studies of intercalated phases are relatively uncommon. A review in 1987 [6] concluded that the problem of electronic transfer had been solved for lithium intercalated compounds, with lithium being fully ionized with its 2s electron being transferred to the transition metal d orbitals. Such a conclusion is mainly based on a rigid band model and bulk techniques. From a band structure calculation point of view, the review [6] cites the paper of Umrigar et al. [7] in which the calculations used the LAPW method for both pristine and lithium intercalated TiS.2. However, the results were not directly compared due to the unknown relative position of the Fermi level, which was taken as zero in both cases. Nothing is said in this paper about the nature and the amplitude of the charge transfer. A more recent work [8] on various 2H-TaS2 intercalation compounds concludes, from LMTO band structure calculations, that lithium only transfers 0.45 electrons per intercalated lithium. These electrons are shared by tantalum (0.07 electrons per atom) and sulphur (0.19 electrons per atom). Thus the question of lithium charge transfer in these hosts in
1117
P. MOREAU et al.
1118
unresolved. In order to contribute new insight into lithium charge transfer and the novel Hg1.24TiSz charge transfer problem, we performed band structure calculations together with various spectroscopic analyses, which are characteristic of occupied states, empty states and Fermi level, namely X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS) and electron energy loss spectroscopy (EELS). This study has been made on TiS2 and its lithium and mercury intercalates.
-Q.=O” . . . . . . .._
-a
2464
2466
2. EXPERIMENTAL TiSz powders have been obtained following a classical procedure [9]. Lithium intercalation has been performed by the n-butyl lithium technique at room temperature [lo] with the lithium content determined by flame spectroscopy. The mercury intercalate was prepared by a room temperature reaction between TiS2 powder and elemental mercury. In order to prevent sample degradation, they were handled in glove boxes and transferred to the different systems for analysis via appropriate air-tight containers. XPS experiments have been made using an SSI spectrometer and in situ surface cleaned samples. The incident monochromatized AIK_L radiation (1486.6eV) was focused on an area of 600pm diameter. The EELS experiments (titanium L2_s edge) were carried out on a CM30 Philips TEM, operated at 200 kV and equipped with a Gatan 666 multichannel spectrometer. The experimental conditions led to an energy resolution of 0.9eV FWHM for the zero loss peak. XAFS spectra were recorded using the French synchrotron radiation source at LURE (Orsay). The titanium K edge was studied using the DC1 storage ring, with a two Si3 11 crystal monochromator, and 0.2eV resolution. The same resolution was attained for the sulphur K edge recorded on the Super AC0 storage ring. Band structure calculations have been performed using the Extended Hiickel approach. It is known that this method, mainly due to its semi-empirical character, is not very accurate in describing the empty levels. Nevertheless, it is very useful for our purpose in its precise knowledge of the nature of electronic levels and the easy way to study, in a rigid band model, the evolution of an electronic structure in a topotactic reaction. In order to take into account the modifications of the atomic charges resulting from the studied charge transfer, an iterative procedure on the charge has been introduced for the lithium intercalated compound. For the mercury intercalate, due to the incommensurability of the structure, a supercell has been used for the calculations with a = 5.923A, b = 13.75A z 4 x 3.44A x 5 x 2.75A, c = 8.86A and
a
=
30”
-____a=5y
= 80”
2468 2410 Energy (eV)
2472
2414
Fig. 1. X-ray absorption spectra recorded at the sulphur K edge for a T& single crystal for different values of the a angle between incident radiation and the normal to the crystal ab plane.
/I = 102.3”. The higher complexity of these calculations kept us from applying charge iteration to these studies. 3. RESULTS
3.1. Pristine materials Our first step in the above approach was to verify the experimental spectroscopic data can be easily explained by comparison with calculated band structures of TiS*, lT- and 2H-TaS2. The shape of the valence band of TiSz was found to be directly comparable with the calculated density of states (DOS). The evolution of the edge position and peak intensities of the absorption edge for dichalcogenide single crystals versus the o angle between the radiation and the normal to the ub place of the crystal was also in good agreement (Fig. 1). We may then compare, for example, the experimental influence of the px, py and pz orbitals of sulphur and their relative contributions to the calculated DOS (Fig. 2). When cy = O”,only the px and py orbitals are involved in the electronic transition. Increasing Q leads to a gradual increase
-I
-12
-10
-8 Energy (eV)
-6
-4
Fig. 2. Density of states of TiS2, projected on the sulphur px (thin line), py (dotted line) and pz (broad line) orbitals.
1119
Li and Hg intercalated TiSz
-14
-12
-10
-8
-6
-4
-2
0
Energy (eV) Fig. 3. Total density of states calculated for TiSr (full line) and LiiTiSr (dotted line) after an iterative procedure taking into account the charge variation. The vertical bars indicate the Fermi level positions.
in the contribution of thepz orbitals to the transition. Due to the slightly lower energy of pz, the DOS explains why the X-ray absorption edge is shifted toward lower energy when o increases. The relative shapes and positions of the pz, px and py orbitals in the DOS are also in very good agreement with the increasing asymmetry of the first peak in the edge. The decreasing intensity of the second peak is also explained by the very low contribution of the pz orbitals in the DOS around -5eV, even if a more important effect would be expected. In the same way, it has already been shown that the suiphur edge shape can be used to characterize the tantalum coordination, i.e. octahedral and trigonal prismatic for the 1T and 2H forms of TaSz, respectively, when taken together with the electronic structures of these materials [ 111. 3.2. Lithium intercalated TiS2 Figure 3 shows total DOS calculated for TiSz and LilTiS using an iterative procedure. The main differences primarily concern the positions of their features (Table 1). Upon intercalation, the valence band is shifted by about 0.8eV to higher energies. At the same time, due to the displacement of the first accepting levels and their partial filling, the Fermi level (Er) is shifted in the same direction by 1.2 eV. The lower empty levels, which mainly consist of titanium T2g
orbitals, are also found at a higher energy by about 0.5 eV, while those corresponding mainly to the titanium Eg levels have not moved. Just above these levels are the titanium 4p levels whose energy is lowered by about 0.6eV. Many of these features, such as the decrease of the separation between the T2g and Eg levels and the lowering of the valence band relative to EF, are in agreement with the conclusions of Ref. [7], where values of 0.3 and 0.7 eV have been calculated for these values, respectively, compared with of 0.5 and 0.4 eV from our calculations. The EF can be related to the open circuit voltage (OCV) of a lithium battery, which, to a first approximation measures the potential of the first accepting levels of a positive material (Er) versus a constant value for the electrochemical couple Li/Li+. Therefore, the EF displacement must be close to the variation of the OCV. Experimentally this can be estimated around 0.8 eV [ 121,in agreement with the 1.2 eV we have calculated. XPS experiments measure the transitions between occupied electronic levels and EF. Table 2 gathers the energies of various peaks corresponding to core levels or valence states, before and after lithium intercalation. All the transitions move to higher energies by values ranging from 0.1 to 0.6 eV, with a mean value of 0.35 eV. The positions of the core levels have not been calculated. However, we may estimate that their shift with charge transfer is similar to that for the valence state, the position of both being largely modified by the same screening effect. The above mean value is in a very good agreement with the raising of these starting levels by 0.8 eV and EF by 1.2 eV (Table 1). In Table 1 the XAFS and EELS experimental data are also compared with the difference between the calculated valence band/core-level energy shift and the shifts of the other electronic levels. Only selected orbitals in the projected DOS for a given atom were considered. In effect, these spectroscopies are very selective for both the type of atom and the electronic levels involved in the dipolar transition. The L2.3 EELS titanium spectra shown in Fig. 4 correspond to the transition of 2pi12 and 2ps,z electrons to unoccupied titanium d states, consistent with the selection rule Al = 1. They consist of two double peaks corresponding to L2 and L3 edges around 462 and 455 eV,
Table 1. A comparison of calculated energy shifts (in eV) for lithium intercalation of TiSa (LilTiS*) with XPS. XAFS and EELS observations
Calculations Calculated energy shifts XPS Titanium K edge (XAFS) Titanium L2,3 edge (EELS) Sulphur K edge (XAFS)
Valence band
Fermi level
‘T2g’ group
‘Eg’ group
Titanium 4p
+o.s
+1.2 +0.4 +0.35
f0.5 -0.3
0.0 -0.8
-0.6 -1.4
-0.3
-0.8
-1.6 +0.4
1120
P. MOREAU et al.
Table 2. Energies of various XPS peaks for TiSr and Li,Ti$, corresponding to valence band and core electronic levels Peak nature
Valence band
TiS2 LilTiS
2.2 eV 2.6eV
respectively.
3SeV 3.9eV
By comparison
with the DOS projected
d orbitals,
on the titanium
5.leV 5.6eV
the lower energy part of
each double peak, which appears more as a shoulder, can be attributed corresponding
to T2g orbitals, with the main peak to Eg orbitals.
that, upon lithiation, the main
feature
respectively.
Our calculations
the energies of the shoulder are lowered
These
with the estimated
show
values
are in good
increase
0.8eV and the calculated
by 0.3 and
agreement
of the core
displacements
and
08eV,
levels by of the T2g
and Eg orbitals by +0.5 eV and 0.0 eV, respectively. The same comparison results.
Figure
pristine
and lithium
Only a slight displacement
intercalated for LilTi&.
with the 1s + 3p transition,
DOS projected
Ti!$.
of the edge toward higher
energy of about 0.4eV is observed in agreement
K edges of
the sulphur
and mercury
on the sulphur
3p orbitals
This is
based the (Fig. 6).
Indeed, a noticeable fraction of 3p sulphur empty states is found above the Fermi level. They have roughly
the same shape
and, as previously
as the titanium
S 2P1/2
Ti 2P3/2
Ti 2~112
162.1 eV 162.2eV
456. I eV 456.4eV
462.2 eV 462.8 eV
decrease
of the first peak with lithium intercalation,
which corresponds
d orbitals
discussed for Ti&, the double peak
to partial filling of the lower empty
3p sulphur states, referred as ‘T2g’ due to their overlapping with the T2g titanium strates
that
transfer
sulphur
orbitals. part
This demon-
in the electronic
with intercalation.
The intensity to unoccupied
states,
associated
takes
of the second peak, which corresponds remains
unchanged
with
intercalation,
as
expected. Finally we will consider corresponds
can be made using XAFS
5 compares
S 2P3jz 160.9eV 161.1 eV
K edge which
the titanium
to the titanium
1s + 4p electronic
transi-
tion (Fig. 7). A small pre-edge peak is observed, whose intensity decreases with lithium intercalation. The origin of this peak is not precisely known. It may be due to either a transition
to d levels, allowed
partial quadrupolarcharacter
of the transition,
small fraction
of p orbitals,
which were found at this
energy by band structure calculations, ever
its origin,
explained
by a or to a
its amplitude
or both. What-
reduction
is easily
by the partial filling of these lower energy
empty levels in TiS2. More important is the main edge shift towards lower energy. The 1.6eV displacement
observed at the edge can be related to overlapping T2g and Eg orbitals. We observe a smaller separation
increase in the core level energy and the lowering
between
the titanium
these peaks (around
2eV) than that calcu-
lated for the DOS (3 eV), corresponding to the known tendency of the Extended Hiickel method to overestimate upon
the empty
lithium
level separation.
intercalation,
Nevertheless,
an increase
in &
for LilTi&
is in good
agreement
3.3. Hg1.24TiS2 lithium intercalated
edges for pristine,
and mercury intercalated
about I .2 eV is calculated,
which is partially counter-
compared.
balanced
in the core level energies,
Hg1,24TiSZ are very close to those
by an increase
which is estimated to be 0.8 eV. These two values are in good agreement with the observed shift of the sulphur K edge. Another
important
feature is the significant
4%
460
465
Energy W) Fig. 4. EELS Lz.3 titanium edges for TiSz (thin line), LilTiS (dotted line) and Hg1,24TiS2 (broad line).
of
4p levels by about 0.6 eV.
In Figs 4, 5 and 7 the various
of
with the 08eV
pristine
Generally host,
for LilTiS unchanged
the features
while the principle
TiS2 are
associated observed changes
with in the
observed
are not seen. This is highlighted by the position of the titanium K edge observed
2470
2475
Energy (eV) Fig. 5. XAFS sulphur K edges for TiS2 (thin line), LilTiS (dotted line) and Hgl 24TiS, (broad line).
Li and Hg intercalated TiSr
mergy (eV)
Fig. 6. Density of states projected on the sulphur 3p orbitals in the cases of TiSz (thin line) and LirTiSr (dotted line). for Hgl,zJTiSz in Fig. 7. However, slight changes are
observed in the intensities of the prepeak for the titanium K edge and in the first peak for the sulphur K edge, and a slight displacement of the L2,3 titanium edge is observed indicating that some electronic exchange occurs upon mercury intercalation, in agreement with previous physical measurements [3]. Almost no change in the peak positions, upon mercuration, has been observed in XPS experiments.
4.
DlSCUSSION
The spectroscopy results are in good agreement with the band structure calculations and the proposed electronic modifications which appear in the position and filling of electronic levels for lithium and mercury intercalation. The results are especially accurate for the Li-TiSz system in which an iterative procedure on the charge has been used. Therefore, we may deduce from the calculated band structures the values of the charges on each atom via a Mulliken population calculation. For TiSz, a charge of +0.29 is calculated on titanium, counterbalanced by a charge of -0.145 on each sulphur atom. These rather low values are in
4970 Energy
4975 (eV)
Fig. 7. XAFS titanium K edges for TiSr (thin line), LirTiSr (dotted line) and Hgr,z.,TiSr (broad line).
1121
agreement with the highly covalent character of this compound. The same calculations for LilTiS show that 0.24 electrons per lithium are given to the host structure. This charge is shared almost equally between the titanium and sulphur atoms, whose charges become f0.20 (-0.09) and -0.22 (-0.075) per atom, respectively. These results are in good agreement with those of Guo et al. [8] and disagree with the simplistic schematic view of a fully ionized lithium, whose electron has been donated to titanium. The charge transfer of 0.09 electrons to the titanium d orbitals raises the energy level of the occupied electronic states by increasing the d electron screening effect. A simple calculation of this effect can be done by using Slater’s screening coefficient. The screening effect of Id electron on a 1s electron in a titanium is 8.2eV. The same calculation for a 2d electron effect in manganese is 18.1 eV. By comparison the above calculated charge transfer of 0.09 electrons on titanium d orbitals will raise the core levels by 0.7-0.8 eV in excellent agreement with the value of 0.8eV estimated from the spectroscopic data. Even if the band structure calculations are less precise for Hg1.24TiS2, a similar calculation estimates the charge on intercalated mercury to be close to 0.02 electrons. Such a low value explains the almost elemental behaviour of intercalated mercury and the physical measurements which suggest similar behaviour between mercury and dilute (around 0.25) lithium intercalated compounds. This is apparent from an interpretation of the magnetic susceptibility values observed for these intercalation compounds using the calculated band structures. We know that for metallic compounds, the Pauli paramagnetic susceptibility is proportional to the DOS at EF, N(&). For TiS2, & is found in the gap and N(Es) is zero. For LilTiS and Hg1,z4TiS2, N(&) is calculated at 3.5 and 1.5 states per eV, respectively. This corresponds to a ratio of 43% between the densities of states of mercury and lithium intercalated compounds, in perfect agreement with the 43% ratio between the experimental Pauli susceptibilities of both compounds. This study on TiSz intercalated compounds has shown that spectroscopy is essential in confirming the validity of band structure calculations. From such a comparison, an accurate determination of the charge transfer upon intercalation can be made. Here we have been able to determine precisely its nature in the lithium intercalates and to confirm that intercalated mercury is very close to an elemental state. Acknowledgements-We thank Dr Mike McKelvy for valuable discussions and corrections of the paper, Professor J. L. Mansot for his help in collecting EELS data and Dr D. Gonbeau for the XPS experiments. This work was supported by a Human Capital and Mobility EC grant (PG).
P. MOREAU et al.
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