X-ray spectroscopy studies of the electronic structure and band-structure calculations of cubic TaCxN1-x carbonitrides
The electronic structure of almost stoichiometric cubic (NaCl structure) tantalum carbonitrides TaCxN₁₋x synthesized under high pressure-high temperature conditions (7-10 GPa and 2100-2400°C) was studied employing X-ray photoelectron spectroscopy (XPS), Xray emission spectroscopy (XES) and X-ray abs...
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irk-123456789-702682014-11-02T03:01:56Z X-ray spectroscopy studies of the electronic structure and band-structure calculations of cubic TaCxN1-x carbonitrides Lavrentyev, A.A. Gabrelian, B.V. Vorzhev, V.B. Nikiforov, I.Ya. Khyzhun, O.Yu. Rehr, J.J. The electronic structure of almost stoichiometric cubic (NaCl structure) tantalum carbonitrides TaCxN₁₋x synthesized under high pressure-high temperature conditions (7-10 GPa and 2100-2400°C) was studied employing X-ray photoelectron spectroscopy (XPS), Xray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS). The XPS valence-band and core-level spectra, the XES Ta Lβ ₅, C Kα and N Kα bands (reflecting energy distributions of mainly the Ta 5d-, C 2p- and N 2p-like states, respectively), as well as the XAS Ta LIII edges (unoccupied Ta d-like states) were derived and compared on a common energy scale for the compounds TaC₀.₉₈, TaC₀.₅₂N₀.₄₉ and TaN₀.₉₇ obtained under the mentioned high pressure-high temperature conditions. To investigate the influence of substitution of carbon atoms by nitrogen in the cubic TaCxN₁₋x system, the cluster self-consistent calculations of the electron density of states for cubic TaC, TaC₀.₅N₀.₅ and TaN compounds were carried out with the FEFF8 code. In the present work a rather good agreement of the experimental and theoretical results for the electronic structure of the TaCxN₁₋x system under consideration was obtained. 2006 Article X-ray spectroscopy studies of the electronic structure and band-structure calculations of cubic TaCxN₁₋x carbonitrides / A.A. Lavrentyev, B.V. Gabrelian, V.B. Vorzhev, I.Ya. Nikiforov, O.Yu. Khyzhun, J.J. Rehr // Физика и техника высоких давлений. — 2006. — Т. 16, № 4. — С. 135-143. — Бібліогр.: 22 назв. — англ. 0868-5924 PACS: 78.70.Dm, 78.70.En, 79.60.−i, 82.80.Pv http://dspace.nbuv.gov.ua/handle/123456789/70268 en Физика и техника высоких давлений Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України |
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The electronic structure of almost stoichiometric cubic (NaCl structure) tantalum carbonitrides TaCxN₁₋x synthesized under high pressure-high temperature conditions (7-10 GPa and 2100-2400°C) was studied employing X-ray photoelectron spectroscopy (XPS), Xray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS). The XPS valence-band and core-level spectra, the XES Ta Lβ ₅, C Kα and N Kα bands (reflecting energy distributions of mainly the Ta 5d-, C 2p- and N 2p-like states, respectively), as well as the XAS Ta LIII edges (unoccupied Ta d-like states) were derived and compared on a common energy scale for the compounds TaC₀.₉₈, TaC₀.₅₂N₀.₄₉ and TaN₀.₉₇ obtained under the mentioned high pressure-high temperature conditions. To investigate the influence of substitution of carbon atoms by nitrogen in the cubic TaCxN₁₋x system, the cluster self-consistent calculations of the electron density of states for cubic TaC, TaC₀.₅N₀.₅ and TaN compounds were carried out with the FEFF8 code. In the present work a rather good agreement of the experimental and theoretical results for the electronic structure of the TaCxN₁₋x system under consideration was obtained. |
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Lavrentyev, A.A. Gabrelian, B.V. Vorzhev, V.B. Nikiforov, I.Ya. Khyzhun, O.Yu. Rehr, J.J. |
| spellingShingle |
Lavrentyev, A.A. Gabrelian, B.V. Vorzhev, V.B. Nikiforov, I.Ya. Khyzhun, O.Yu. Rehr, J.J. X-ray spectroscopy studies of the electronic structure and band-structure calculations of cubic TaCxN1-x carbonitrides Физика и техника высоких давлений |
| author_facet |
Lavrentyev, A.A. Gabrelian, B.V. Vorzhev, V.B. Nikiforov, I.Ya. Khyzhun, O.Yu. Rehr, J.J. |
| author_sort |
Lavrentyev, A.A. |
| title |
X-ray spectroscopy studies of the electronic structure and band-structure calculations of cubic TaCxN1-x carbonitrides |
| title_short |
X-ray spectroscopy studies of the electronic structure and band-structure calculations of cubic TaCxN1-x carbonitrides |
| title_full |
X-ray spectroscopy studies of the electronic structure and band-structure calculations of cubic TaCxN1-x carbonitrides |
| title_fullStr |
X-ray spectroscopy studies of the electronic structure and band-structure calculations of cubic TaCxN1-x carbonitrides |
| title_full_unstemmed |
X-ray spectroscopy studies of the electronic structure and band-structure calculations of cubic TaCxN1-x carbonitrides |
| title_sort |
x-ray spectroscopy studies of the electronic structure and band-structure calculations of cubic tacxn1-x carbonitrides |
| publisher |
Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України |
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2006 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/70268 |
| citation_txt |
X-ray spectroscopy studies of the electronic structure and band-structure calculations of cubic TaCxN₁₋x carbonitrides / A.A. Lavrentyev, B.V. Gabrelian, V.B. Vorzhev, I.Ya. Nikiforov, O.Yu. Khyzhun, J.J. Rehr // Физика и техника высоких давлений. — 2006. — Т. 16, № 4. — С. 135-143. — Бібліогр.: 22 назв. — англ. |
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Физика и техника высоких давлений |
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Физика и техника высоких давлений 2006, том 16, № 4
135
PACS: 78.70.Dm, 78.70.En, 79.60.−i, 82.80.Pv
A.A. Lavrentyev1, B.V. Gabrelian1, V.B. Vorzhev1, I.Ya. Nikiforov1,
O.Yu. Khyzhun2, J.J. Rehr3
X-RAY SPECTROSCOPY STUDIES OF THE ELECTRONIC
STRUCTURE AND BAND-STRUCTURE CALCULATIONS OF CUBIC
TaCxN1−x CARBONITRIDES
1Department of Physics, Don State Technical University
Gagarin Sq. 1, Rostov-on-Don, Russian Federation
E-mail: alavrentyev@dstu.edu.ru
2Frantsevych Institute for Problems of Materials Science,
National Academy of Sciences of Ukraine
3 Krzhyzhanivsky str., 03142 Kyiv, Ukraine
E-mail: khyzhun@ipms.kiev.ua
3Department of Physics, University of Washington
Seattle, WA 98195-1560, USA
The electronic structure of almost stoichiometric cubic (NaCl structure) tantalum carboni-
trides TaCxN1−x synthesized under high pressure�high temperature conditions (7�10 GPa
and 2100�2400oC) was studied employing X-ray photoelectron spectroscopy (XPS), X-
ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS). The XPS va-
lence-band and core-level spectra, the XES Ta Lβ5, C Kα and N Kα bands (reflecting
energy distributions of mainly the Ta 5d-, C 2p- and N 2p-like states, respectively), as
well as the XAS Ta LIII edges (unoccupied Ta d-like states) were derived and compared
on a common energy scale for the compounds TaC0.98, TaC0.52N0.49 and TaN0.97 obtained
under the mentioned high pressure�high temperature conditions. To investigate the influ-
ence of substitution of carbon atoms by nitrogen in the cubic TaCxN1−x system, the cluster
self-consistent calculations of the electron density of states for cubic TaC, TaC0.5N0.5 and
TaN compounds were carried out with the FEFF8 code. In the present work a rather
good agreement of the experimental and theoretical results for the electronic structure of
the TaCxN1−x system under consideration was obtained.
Introduction
It is well known that almost all transition metal (TM) carbides and nitrides
form unlimited solid solutions with each other [1,2]. Some physical and chemical
properties of such TM carbonitrides are non-monotonous as a function of the
Физика и техника высоких давлений 2006, том 16, № 4
136
MC/MN ratio (M denotes a transition metal) and in many cases they depend
strongly on a ternary compound composition. The properties of TM carbonitrides
can be understood by considering their electronic structure. Additionally, the un-
derstanding of the electronic structure of TM carbonitrides is also of great techni-
cal interest because alloying the sublattices of the compounds with atoms of a dif-
ferent type is one of the most promising methods for modification of service char-
acteristics of materials [2,3].
Concerning the TaC�TaN system, the synthesis of a continuous cubic (NaCl
structure) solid solution by conventional powder metallurgy methods is not possi-
ble using as precursors usual forms of tantalum mononitride (CoSn structure) and
monocarbide (NaCl structure) [1,2]. As Weber [4] has revealed, a phase transition
from the fcc to the hcp structure occurs in the TaCxN1-x carbonitrides near x = 0.4.
A continuos nonstoichiometric cubic TaCxNy (x + y ≠ 1) solid solution was first
derived by Avakyan et al. [5] using the method of self-spreading high temperature
(SSHT) synthesis. However, the SSHT method does not allow to obtain stoichi-
ometric TaCxNy system (with x + y ≈ 1). The above problem was solved by
Gololobov et al. [6,7] employing high temperature−high pressure conditions for
obtaining the almost stoichiomertric TaCxN∼ 1−x system.
However, it is interesting to study the influence of substitution of carbon at-
oms for nitrogen on the electronic structure of the TaC−TaN system. The aim of
this work was to employ both experimental and theoretical methods for solution
of the above task. Series of theoretical band-structure calculations were devoted
either to TaC [8−12] or to TaN [13−16] compounds, however, to the best of our
knowledge, previously there were no attempts for theoretical investigation of the
electronic structure of a ternary TaCxN1−x system. For the above system, in the
present paper we have used also three experimental methods providing informa-
tion about the electronic structure of solids, namely, X-ray photoelectron spec-
troscopy (XPS), X-ray emission spectroscopy (XES) and X-ray absorption spec-
troscopy (XAS).
2. Experimental
Cubic tantalum carbonitride TaC0.52N0.49 and mononitride TaN0.97 investi-
gated in the present work were synthesized under high pressure�high temperature
conditions (7�10 GPa and 2100�2400oC) using the following powdered materials
as precursors: hexagonal mononitride TaN0.99 (CoSn structure), cubic monocar-
bide TaC0.98 (NaCl structure), and metallic tantalum. According to the data of a
conventional chemical analysis and XPS measurements [17], the tantalum
mononitride under consideration contains some admixtures of carbon and the
analytical formula TaC0.04N0.97 can be ascribed to the compound. The synthesis
method, the calculation of the burden�s chemical composition and of content of
the nitrogen-containing heterocyclic compounds in the burdens (melamine
(C3H6N6), 5,6-dimethylbenzimidazole (C9H10N2), γ, γ′−dipyridyl (C10N2H8))
Физика и техника высоких давлений 2006, том 16, № 4
137
were discussed previously [17]. By the data of the chemical analysis and tests
with a Microscan-5 X-ray microanalyser, an ES-2401 electron spectrometer, a
JAMP-10 Auger scanning electron microscope («JEOL», Japan), and a CS-244
carbon analyser («Leco», Germany) oxygen content in the studied tantalum car-
bonitrides was lower than 0.3 wt%, unbonded carbon was either absent or its
content was < 0.1 wt% [17].
Measurements of the XPS valence-band and core-level spectra were carried
out in an ion-pumped chamber of an ES-2401 spectrometer having a base pressure
less than 5·10−8 Pa. In the mentioned spectrometer, the Mg Kα1,2 radiation (E =
1253.6 eV) was used as a source of spectra excitation. The binding energy (BE) of
84.00 ± 0.05 eV of the XPS Au 4f7/2 core-level spectrum was used as a reference.
The surfaces of the studied specimens were prepared by Ar+ bombardment (1700
V, 30 µA/cm2, 10 min) with subsequent mechanical cleaning in the spectrometer
chamber, as reported in Refs. [17,18].
The X-ray C Kα and N Kα emission bands (K → LII,III transition) reflecting
the energy distribution of the C 2p- and N 2p-like states, respectively, in the tan-
talum carbonitrides under consideration were derived with the energy resolution
∆Emin of about 0.3 eV using an RSM-500 spectrometer. A diffraction grating with
600 lines/mm and a radius of curvature R = 6026 mm was used as a dispersing
element in the RSM-500 spectrometer. The detector was a secondary electron
multiplier VEU-6 with a CsI photocathode. The X-ray chamber was evacuated to
1·10−6 Pa. The C Kα and N Kα spectra were recorded under conditions of oil-free
evacuation practically eliminating impact of hydrocarbon vapours on the target. In
addition, the surfaces of the tantalum carbonitrides under consideration were pre-
pared by Ar+ bombardment in the spectrometer chamber (2100 V, 50 µA/cm2, 10
min). The fluorescent X-ray Ta Lβ5 emission bands (LIII → OIV,V transition) re-
flecting primarily the Ta 5d-like states in the above materials were obtained using
a DRS-2M spectrograph (the resolution was estimated to be ∆Emin ≈ 0.4 eV) in
the second order of reflection from the (0001) plane of a quartz crystal prepared
according to Johann [19]. An X-ray BKhV-7 tube with gold anode was used as a
source of spectra excitation.
The X-ray Ta LIII absorption spectra (unoccupied Ta d-like states) were obtained
using a KRUS-1 spectrometer. The method of «a variable field of absorption» was
employed for recording the spectra and a quartz crystal with the (13 4 0) reflecting
plane and a radius of curvature R = 872 mm was used as a dispersing element. The
absorber covering half of the rotating sample cell was made in the form of a thin
film of the investigated substance with a wax binder. Our attempts for investigating
quantum yields of the X-ray photoeffect in the area of the C K and N K absorption
edges in the tantalum carbonitrides were unsuccessful, probably, as a result of
strong screening of the carbon and nitrogen atoms by the heavier tantalum atoms.
An X-ray diffraction analysis employing Cu Kα radiation has revealed that the tan-
talum carbonitrides studied in the present work were single-phase materials [17].
Физика и техника высоких давлений 2006, том 16, № 4
138
Calculation procedure
Cluster self-consistent calculations of the total density of states (DOS) and
the partial Ta 5d-, N 2p- and C 2p-like DOS were carried out with the FEFF8
program [20] for TaC, TaC0.5N0.5 and TaN compounds possessing the structure
of NaCl-type. The ab initio calculations employing the FEFF8 program are car-
ried out in the approach of multiple scattering theory for clusters in real space,
i.e. without any restrictions on symmetry and periodicity of a crystal lattice.
Relativistic atomic charge densities are calculated at the very beginning and the
phase shifts derived with relativistic corrections using the above procedure are
used on the next steps of calculations of the self-consistent muffin-tin potential
in the approach of full multiple scattering for clusters consisted of approxi-
mately 45 atoms.
During the self-consistency procedure, a new electron density is calculated;
the number of 10 to 20 iterations is necessary to reach a coincidence of the Fermi
energy within 0.003 eV. Clusters consisted of approximately 200 atoms are used
for calculations of the total and partial DOS. Energy resolution (accuracy) of the
DOS calculations is restricted by (i) final dimensions of the clusters, (ii) life-time
of a core-level hole, (iii) a distinction of the muffin-tin potential from the total
potential. However, such integral characteristics as the total DOS, the Fermi en-
ergy and charge transfer values are almost insensitive to the above-mentioned pe-
culiarities of the FEFF8 program employed in the present work for calculations of
the electronic structure of cubic tantalum carbonitrides [20].
4. Results and discussion
Figure 1 represents a comparison of the Ta Lβ5, C Ka and N Ka emission
bands and the XPS valence-band spectra for the cubic TaCxN∼ 1−x carbonitrides
under consideration provided that a common energy scale is used. It is obvious
that, instead of the main maximum B, the presence of the very prominent high-
energy near-Fermi feature C and the low-energy feature A is characteristic of all
the spectra presented in Fig. 1. In accordance with results of Refs. [21,22], the
main maximum B of the Ta Lβ5 band in tantalum carbides and nitrides is created
by the Ta 5d-like states taking part in the formation of the covalent dTa−pC(N)
bonds due to Ta 5d−C(N) 2p-like hybridization, while the near-Fermi feature C of
the band is created by the Ta 5d-like states taking part in forming the metallic
component of the chemical bonding. Therefore, an increase of the relative inten-
sity of the near-Fermi feature C of the Ta Lβ5 band, when going from tantalum
monocarbide to tantalum mononitride through the intermediate tantalum carbonitride,
indicates that the metallic component of the chemical bonding increases when going
from TaC0.98 to TaN0.97. As Fig. 1 reveals, increasing the relative intensity of the
near-Fermi feature C of the Ta Lβ5 band in the sequence TaC0.98 → TaN0.97 leads to
broadening the band (i.e., to increasing the full width at half-maximum of the
band) in the mentioned sequence of compounds.
Физика и техника высоких давлений 2006, том 16, № 4
139
Since changes of shapes of the XPS
valence-band spectra of substoichi-
ometric cubic and hexagonal tantalum
carbides resemble changes of those ob-
served for the Ta Lβ5 band [21], we can
expect similarity of changes of the XPS
valence-band and XES Ta Lβ5 spectra
when carbon atoms are substituted for
nitrogen atoms in the cubic TaCxN∼ 1−x
system. This is observed in fact in the
present experiments. As one can see
from Fig. 1, similar to that observed for
the Ta Lβ5 band, the relative intensity of
the near-Fermi feature C of the XPS va-
lence-band spectra and their half-widths
increase somewhat when going from
TaC0.98 to TaN0.97. Additionally, energy
positions of the main peaks B of the Ta
Lβ5 band and the XPS valence-band
spectra, coinciding with the position of
the main peak B of the C Ka band in
tantalum monocarbide within accuracy
of the experiments, shift by about (0.7�
0.8) ± 0.2 eV away from EF when going
from TaC0.98 to TaN0.97, coinciding with
the main peak B of the N Ka band in
tantalum mononitride. Therefore, the strong Ta 5d−C(N) 2p-like hybridization is
characteristic of the TaCxN∼ 1−x carbonitrides under consideration.
The energy position of the main maximum B of the C Ka band remains constant
(within accuracy of the experiment) with respect to the Fermi level when going
from TaC0.98 to TaC0.52N0.49, as well as that of the N Ka band in the sequence
TaN0.97 → TaC0.52N0.49. Additionally, the shape of the C Ka (N Ka) band does not
change in the sequence TaC0.98 → TaC0.52N0.49 (TaN0.97 → TaC0.52N0.49). The
above facts indicate the absence of the C 2p−N 2p-hybridization in the cubic
TaCxN∼ 1−x system. As seen from Fig. 1, half-widths of the C Ka and N Ka bands
and relative intensities of their near-Fermi features C do not change within ex-
perimental errors when carbon atoms are substituted for nitrogen atoms in the cu-
bic TaCxN∼ 1−x system under study.
The above-mentioned experimental results on studies of the electronic struc-
ture of cubic tantalum carbonitrides are in a rather good agreement with the theo-
retical calculations presented in Fig. 2. In Fig. 2,a, the main occupied part of the
valence band of cubic tantalum monocarbide is formed mainly by contributions of
0
C
B
A
TaC0.98
TaC0.52N0.49
TaN0.97
In
te
ns
ity
, a
rb
. u
ni
ts
Energy, eV
�20 �10 0 10
Fig. 1. Comparison on a common energy
scale of the X-ray emission Ta Lβ5 (− − −),
N Kα (− · − · −), C Kα (· · · ·) bands and
the XPS valence-band spectra (�) for
the cubic tantalum carbonidrides under
investigation (zero of energy corresponds
to the position of EF of an ES-2401 spec-
trometer energy analyzer)
Физика и техника высоких давлений 2006, том 16, № 4
140
a b
the partial Ta d and C p DOS. The
theoretical calculations reveal that the
Ta 5d- and C 2p-like states are highly
hybridized in TaC, being in excellent
agreement with the experimental XES
and XPS data of this compound (cf.
Figs. 1 and 2,a). This fact indicates
that the covalent component of chemi-
cal bonding is extremely high in TaC.
The bottom of the valence band of
TaC is formed mainly by the C 2s-like
states (labelled as peaks E). In the en-
ergy region corresponding to the posi-
tion of the C 2s-like subband, small
contributions of the valence Ta s-, Ta
p- and Ta d-like states are also de-
tected (Fig. 2,a). Energy positions of
the main maxima C and of the near-Fermi shoulders A of the theoretical partial C
p and Ta d DOS (Fig. 2,a) well correspond to the positions of the fine-structure
features B and C of the XES C Kα and Ta Lβ5 bands, respectively (see Fig. 1).
The energy positions of the maxima B (C p-like DOS) and C (N p-like DOS) of
the TaC0.5N0.5 carbonitride coincide with those of the main maxima B of the ex-
perimental C Ka and N Ka bands of the TaC0.52N0.49 compound (cf. Figs. 1 and 2,b).
Fig. 2. Calculated total and partial density of
states of cubic TaC (a), TaC0.5N0.5 (b) and
TaN (c) (zero of energy corresponds to the
Fermi energy)
c
Физика и техника высоких давлений 2006, том 16, № 4
141
Furthermore, from the theoretical
curves presented in Fig. 2,c, it is ob-
vious that the energy positions of the
maxima B of the N p- and Ta d-like
DOS of TaN correspond to those of
the maxima B of the experimental N
Ka and Ta Lβ5 bands of the TaN0.97
compound (see Fig. 1). Additionally,
similar to the experimental observa-
tions (Fig. 1), the main maximum of
the valence band formed mainly by
contributions of the C(N) 2p- and Ta
5d-like states shifts in the direction
opposite to the position of the Fermi
energy when going from TaC to TaN.
It should be noted that the theoretical curves of the partial C p- and N p-like DOS
reveal an extremely weak C 2p�N 2p-hybridization within the main part of the
valence band of TaC0.5N0.5, being in excellent agreement with the experimental
results for the TaC0.52N0.49 compound (see Fig. 1). As Fig. 2,b shows, the C 2s-
and N 2s-like states form F and G subbands on the curves representing the above
partial DOS of TaC0.5N0.5. The mentioned subbands are well separated from each
other as well as from C 2p- and N 2p-like bands of the TaC0.5N0.5 carbonitride
(Fig. 2,b).
Dependences of the Ta 4f and N 1s
core-level BEs on content of carbon and
nitrogen atoms in the TaCxN1−x system
are presented in Fig. 3. An increase of
the XPS Ta 4f core-level BE in the
TaC0.98 → TaC0.52N0.49 → TaN0.97 se-
quence indicates that the substitution of
carbon atoms by nitrogen atoms leads to
increasing the positive effective charge
of tantalum atoms. Therefore, in the
TaC0.82 → TaC0.05N0.78 sequence the
ionic component of chemical bonding
increases. It was difficult to evaluate
changes of charge states of carbon and
nitrogen atoms when going from TaC0.98
to TaN0.97. Fig. 3 shows that the XPS N
1s core-level BE decreases slightly when
going from TaC0.97 to TaC0.52N0.49.
Nevertheless, as discussed in Refs.
[17,18], the XPS N 1s core-level spectra
Fig. 4. The X-ray Ta LIII absorption
edges of the cubic TaCxN1−x carboni-
trides under consideration (zero of energy
corresponds to the position of EF of an
ES-2401 spectrometer energy analyzer)
0.0 0.2 0.4 0.6 0.8 1.0
23.4
23.6
23.8
24.0
397.5
398.0
N 1s
Ta 4f7/2
B
in
di
ng
e
ne
rg
y,
e
V
y/(x + y)
Fig. 3. Dependence of the XPS Ta 4f7/2 and
N 1s core-level binding energies upon con-
tent of carbon and nitrogen atoms in the cu-
bic TaCxN1−x carbonitrides studied
Физика и техника высоких давлений 2006, том 16, № 4
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superimpose on the Та 4р3/2 spectra in tantalum carbonitrides. Additionally, the
XPS C 1s core level spectrum in the tantalum carbide and carbonitride under
study (ЕBE ∼ 283 еV) superimposes on the C 1s line of carbon- and hydrogen-
containing admixtures (ЕBE ∼ 285 еV). The above difficulties do not allow to
make precise measurements of the XPS N 1s and C 1s core-level BEs in the cubic
TaCxN1-x carbonitrides.
The X-ray Ta LIII absorption edges of the cubic tantalum carbonitrides studied
are presented in Fig. 4. From the above figure, it is obvious that the substitution of
carbon atoms by nitrogen atoms in the TaCxN1−x system does not lead to any
changes of shapes of the Ta LIII absorption spectra measured in the near-edge re-
gion. However, in the sequence TaC0.98 → TaC0.52N0.49 → TaN0.97 the inflection
point of the edges (marked by the arrows in Fig. 4) shifts by about 0.7 eV towards
higher energies. The above fact indicates the growth of the positive effective
charge of tantalum atoms in the mentioned sequence of compounds. This result
confirms the results of XPS Ta 4f core-level BE measurements presented in Fig. 3.
5. Conclusions
A rather good agreement of the experimental (XPS, XES and XAS measure-
ments) and theoretical (using the FEFF8 program) results for the electronic struc-
ture of the TaCxN1−x system has been obtained in the present paper. The results
indicate that a strong hybridization of the Ta 5d- and C(N) 2p-like states is char-
acteristic of the TaCxN1−x compounds studied, while the hybridization of the C
2p-like and N 2p-like states is absent.
It has been established that the substitution of carbon atoms by nitrogen atoms
in the TaCxN1−x carbonitrides reveals increasing both a half-width of the XPS va-
lence-band spectrum and a relative intensity of the near-Fermi subband created
mainly by contributions of the Ta 5d-like states. When going from TaC0.98 to
TaN0.97 through the carbonitride of intermediate composition, the energy position
of the inflection point of the X-ray Ta LIII absorption edge shifts towards higher
energies, while the main maximum of the XPS valence-band spectrum shifts in
the direction opposite to the position of the Fermi level, EF. Additionally, in the
above sequence of compounds the XPS Ta 4f core-level binding energies increase.
Comparison on a common energy scale reveals that energy positions of the C Kα
and N Kα bands do not change with respect to EF when substituting carbon atoms
for nitrogen atoms in the TaCxN1−x carbonitrides.
The authors thank Prof. V.B. Shipilo and Dr E.M. Gololobov (Institute of
Solid State Physics and Semiconductors, National Academy of Sciences of Be-
larus, Minsk, Belarus Republuc) and Dr. V.A. Kolyagin (Institute of Physical
Chemistry, Russian Academy of Sciences, Moscow, Russian Federation) for syn-
thesis of the test TaCxN∼ 1−x samples.
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