An X-ray spectrscopy study of the electronic structure of TiS3

Article abstract of DOI:10.1134/S0022476614060067

X-ray photoemission and absorption spectra of TiS₃ are studied. The temperature dependence of the magnetic susceptibility is measured and the electron density of states is calculated. Titanium ions in different oxidation states are found to coe

Full text of DOI:10.1134/S0022476614060067

Journal of Structural Chemistry. Vol. 55, No. 6, pp. 1039-1043, 2014.
Original Russian Text © 2014 A. S. Shkvarin, Yu. M. Yarmoshenko, M. V. Yablonskikh, A. I. Merentsov, A. N. Titov.
AN X-RAY SPECTRSCOPY STUDY OF THE
ELECTRONIC STRUCTURE OF TiS3
1 1
A. S. Shkvarin , Yu. M. Yarmoshenko ,
UDC 544.17
2 1,3
M. V. Yablonskikh , A. I. Merentsov ,
1,3
and A. N. Titov
X-ray photoemission and absorption spectra of TiS3 are studied. The temperature dependence of the
magnetic susceptibility is measured and the electron density of states is calculated. Titanium ions in
different oxidation states are found to coexist.
DOI: 10.1134/S0022476614060067
Keywords: resonant photoemission, photoelectron and absorption spectroscopy, titanium trisulfide.
INTRODUCTION
Interest in the investigation of the electronic structure and the features of the chemical bond in TiS3 is due to the
anomalous temperature dependence of the conductivity found in this compound, which suggests the occurrence of the
collective electronic state [1]. It turned out that the properties of TiS3 in this state were similar to the properties of quasi-1D
conductors with the moving charge density wave (CDW) [2-4]. Nonlinear conductivity was detected to occur along and
across metal chains [2, 5], which has not so far been observed in conductors with CDW. Up to this point, materials with
moving CDW have not been found among chalcogenides of group IV transition metals. In [1, 6] the photoelectron spectra of
TiS3 were measured. Data on the studies of the electronic structure of this compound by other methods are unknown.
EXPERIMENT AND CALCULATIONS
Single crystal samples of TiS3 were grown in quartz ampoules by gas transport reactions using TiS2 as the initial
batch and sulfur excess. Transfer was performed to the cold end on which a temperature of 500 °C was maintained whereas
the TiS2 weighed portion was kept at a temperature of 700 °C. The crystal growth process took from a few days to a week.
The samples were faceted wiskers with the dimensions b×a×c (500-3000)×(10-200)×(1-20) µm.
Studies with the use of synchrotron radiation were carried out on an Elettra synchrotron, CIPO line[7]. Spectra of the
core levels and the valence band spectra were calibrated with respect to the Au4f line. The TiL2,3 absorption spectrum was
calibrated using the spectrum of pure titanium. All spectra were obtained under the conditions of high vacuum at room
temperature. Carbon and oxygen lines are absent in the survey spectra.
For the theoretical description of the density of states of the materials under study the ab initio calculations were
1
Institute of Physics of Metals, Ural Branch, Russian Academy of Sciences, Ekaterinburg, Russia;
2
3
shkvarin@ifmlrs.uran.ru. Sincritrone Trieste SCp A, Basovizza I-34012, Italy. Ural Federal University, Ekaterinburg,
Russia. Translated from Zhurnal Strukturnoi Khimii, Vol. 55, No. 6, pp. 1095-1099, November-December, 2014. Original
article submitted January 15, 2014.
0022-4766/14/5506-1039© 2014 by Pleiades Publishing, Ltd.
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Fig. 1. Fragment of the crystal structure of
TiS3, ZrSe3 structure type.
performed. The calculations were carried out within the full-potential linearised augmented-plane wave (FP-LAPW) method
implemented in the ELK program package [8]. In the calculations the experimentally found crystal structure parameters were
applied (structure type P21/m:b [9], lattice parameters a = 4.9664 Å, b = 3.4048 Å, c = 8.801 Å, β = 97.38°). A fragment of
the crystal structure is shown in Fig. 1.
The multiplet calculation of the absorption spectrum of titanium was performed using the program [10]. The calculation
parameters are listed in Table 1.
RESULTS AND DISCUSSION
Fig. 2 depicts the temperature dependence of the magnetic susceptibility of the ceramic sample of TiS3, which was
obtained from the field dependences of the magnetization of the ceramic TiS3 sample by linear approximation.
As seen from Fig. 2 the susceptibility reaches almost 9 emu/(mol⋅Oe), which is close to the value of the diamagnetic
susceptibility of the ideal superconductor of 4π ≈ 12.56 emu/(mol⋅Oe). For TiS3 whose conductivity has a semiconductor
type, such a high value of the magnetic susceptibility is anomalous.
The photoelectron spectrum of the S2p level of TiS3 shown in Fig. 3 has an additive character. As a result of its
decomposition with regard to the experimental resolution and the width of the respective core levels it is found that this
spectrum is a sum of contributions from sulfur atoms in two crystallographically non-equivalent positions. The corresponding
spectra are shifted relative to each other by 1.2 eV with the binding energy of 162 eV for S(1)2p3/2 and 160.8 eV for S(2)2p3/2.
The ratio of contributions S(1)/S(2) = 2. This is consistent with the structural data. The distances dS(1)Ti = 2.36.Å and
dS(2)Ti = 2.42 Å. Both these facts (the difference in the binding energy and interatomic distances) indicate strong polarization
of the chemical bond in the S(1)–Ti–S(2) chain.
In Fig. 4 for comparison the spectra of the Ti2p level of TiS3 and TiS2 are given. The spectra of the Ti2p levels have
a relative shift of 1 eV. In both cases, titanium is four-valent, however, TiS3 is characterized by lower ionicity.
TABLE 1. Multiplet Calculation Parameters for Two States of the Titanium Atom
Calculation
Slater
10Dq
e
t2g
Upd
Udd
Δ
g
1
2
0.9
0.4
0
0
1
1
1
0.8
3.2
0
0
0
0
1
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Fig. 2. Temperature dependence of the
magnetic susceptibility of TiS3.
Fig. 5 depicts the valence band spectra in a resonant excitation mode (eV) (452.4 (1), 455.8 (2), 457.95 (3), 460.7
(4), 597.4 (5) and far beyond its limits 1486.6 (6)).
It is seen that the spectra obtained in the resonant excitation mode beyond its limits (6) in Fig. 5, have a small peak
directly on the Fermi level (excitation energies of 456 eV and 458 eV). Moreover, it is possible to distinguish crossover:
a transition from the resonant Auger Raman spectrum (RARS) to the normal Auger spectrum. It is seen that the form of the
spectrum with a high (6 in Fig. 5) excitation energy differs from the spectra with a lower excitation energy. This is caused by
a change in the photoionization cross-section at a significant change in the excitation energy [11]. In order to distinguish the
partial contributions from the states of sulfur and titanium atoms to the electronic structure the calculation of the density of
states was performed for TiS3.
Fig. 6 depicts the partial densities of states for TiS3. From the partial densities of states of sulfur it is seen that there
are two sulfur positions: two atoms (S2 and S3 in Fig. 6) of one type of sulfur and one position of another type of sulfur (S1
in Fig. 6). The situation is similar to the spectrum of the S2p level. It should be noted that the densities of states of sulfur of
the S1 type are similar in structure to the density of states of sulfur in TiS2. The structure of the density of states for sulfur of
the S2 and S3 types differs for both p and s states. The density of states of titanium occupies the energy range up to 6 eV.
Above 6 eV only the states of the sulfur atoms are located. In the experimental spectrum obtained at an energy of 1486.6 eV,
the region of 10-15 eV contains two spectral contributions of S3s states at different binding energies (left panel in Fig. 6)
corresponding to two types of sulfur atoms, as is well seen from the calculation.
According to the calculation, near the Fermi energy the contribution of Ti3d states is insignificant unlike that in the
Fig. 3. Spectrum of the S2p level for TiS3. Darkened
peaks show the components of the spectrum. The
ratio of the sum of the areas of peaks 1 and 2 to the
sum of the areas of peaks 3 and 4 is 2.
Fig. 4. Spectrum of the Ti2p level for TiS3 and TiS2.
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Fig. 5. Valence band spectra obtained in the resonance Ti2p–3d
excitation mode (1-5) and far beyond its limits (6). The respective
excitation energies are given in the text. The RRAS line marks the
crossover of the pseudo-Raman peak into the normal Auger peak.
The dashed line R denotes the resonance peak.
experimental spectra.
Fig. 7 shows the total densities of states with regard to the photoionization cross-section value at the corresponding
excitation energy and concentration.
It is seen that with a change in the energy the intensity ratio of the main peak (0-7 eV) and peaks with the energies
ranging from 8 eV to 17 eV is considerably altered.
Fig. 8 depicts the experimental X-ray absorption (XAS) spectrum of TiL2,3 along with the results of model
calculations. In order to model the real state of the chemical bond we assumed the titanium atom to exhibit two oxidation
states. These oxidation states are characterized by some distinction in the iconicity and the interaction force of the Ti2p
vacancy and Ti3d electrons. These arguments follow from the above data on strong polarization in the environment of the
titanium atom and are taken into account in the selection of the calculation parameters (Table 1). Therefore the model
absorption spectrum is a total result of two calculations with different parameters. A combined analysis of the experimental
data on the binding energy of the Ti2p level, the spectral features in the valence band near the Fermi energy, and the first
peak in the absorption spectrum indicates the presence of the narrow Ti3d band localized in the vicinity of the Fermi level,
which is not reproduced by the band calculation of the electron density of states. This feature of the electronic structure of
Fig. 6. Partial densities of states for TiS3: s sulfur states (a), p sulfur states (b), titanium states (c).
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Fig. 7. Total densities of states at photoionization
cross-sections of 1486.6 eV and 440 eV.
Fig. 8. Absorption spectrum of titanium in TiS3 and the
4+
result of the multiplet calculation for Ti . Calculation
parameters are listed in Table 1.
TiS3 can reflect special states appearing as a result of strong polarization of the chemical bond of titanium atoms in the lattice
of the compound.
CONCLUSIONS
The experimental X-ray photoemission spectra and TiL2,3 absorption spectra of TiS3 are studied. A narrow band
located near the Fermi level is found. The occurrence of this band is due to the polarization of the chemical bond of titanium
atoms with the lattice. The measurement results of the temperature dependence of the magnetic susceptibility give evidence
of strong diamagnetism of the compound. The model calculation of the electron density of states does not completely
describe the experimental data. The analysis of the experimental data makes it possible to assume the coexistence of titanium
ions in different oxidation states.
The work was supported by RFBR (projects Nos. 13-03-96032-ural and 13-02-01228).
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