First-principles band-structure calculations and X-ray photoelectron spectroscopy studies of the electronic structure of TlPb2Cl 5

Article abstract of DOI:10.1016/j.jallcom.2013.08.127

We report on first-principles calculations of total and partial densities of states of atoms constituting TlPb₂Cl₅ using the full potential linearized augmented plane wave (FP-LAPW) method. The calculations reveal that the valence band of TlPb₂Cl₅ is dominated by contributions of the Cl 3p-like states, which contribute mainly at the top of the valence band with also significant contributions throughout the whole valence-band region. In addition, the bottom of the conduction band of TlPb ₂Cl₅ is composed mainly of contributions of the unoccupied Pb 6p-like states. Our FP-LAPW data indicate that the TlPb₂Cl ₅ compound is an indirect-gap material with band gap of 3.42 eV. The X-ray photoelectron core-level and valence-band spectra for pristine and Ar ⁺ ion-irradiated surfaces of a TlPb₂Cl₅ polycrystalline sample were measured. The measurements reveal high chemical stability and confirm experimentally the low hygroscopicity of TlPb ₂Cl₅ surface.

Full text of DOI:10.1016/j.jallcom.2013.08.127

Accepted Manuscript
First-principles band-structure calculations and X-ray photoelectron spectro‐
scopy studies of the electronic structure of TlPb Cl5
2
O.Y. Khyzhun, V.L. Bekenev, N.M. Denysyuk, O.V. Parasyuk, A.O. Fedorchuk
PII:
S0925-8388(13)02003-3
DOI:
http://dx.doi.org/10.1016/j.jallcom.2013.08.127
JALCOM 29262
Reference:
To appear in:
Received Date:
Revised Date:
Accepted Date:
12 July 2013
18 August 2013
19 August 2013
Please cite this article as: O.Y. Khyzhun, V.L. Bekenev, N.M. Denysyuk, O.V. Parasyuk, A.O. Fedorchuk, First-
principles band-structure calculations and X-ray photoelectron spectroscopy studies of the electronic structure of
TlPb Cl , (2013), doi: http://dx.doi.org/10.1016/j.jallcom.2013.08.127
2
5
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First-principles band-structure calculations and X-ray photoelectron spectroscopy studies of the
electronic structure of TlPb Cl
2
5
a,*
a
a
b
c
O.Y. Khyzhun , V.L. Bekenev , N.M. Denysyuk , O.V. Parasyuk , A.O. Fedorchuk
a
Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanivsky
Street, Kyiv 03142, Ukraine
b
Department of Inorganic and Physical Chemistry, Eastern European National University, 13 Voli Avenue,
Lutsk 43025, Ukraine
c
Department of Inorganic and Organic Chemistry, Lviv National University of Veterinary Medicine and
Biotechnologies, Pekarska St., 50, 79010 Lviv, Ukraine
Abstract
We report on first-principles calculations of total and partial densities of states of atoms constituting
TlPb
that the valence band of TlPb
at the top of the valence band with also significant contributions throughout the whole valence-band region. In
addition, the bottom of the conduction band of TlPb Cl is composed mainly of contributions of the unoccupied
Pb 6p-like states. Our FP-LAPW data indicate that the TlPb
2
Cl
5
using the full potential linearized augmented plane wave (FP-LAPW) method. The calculations reveal
2
Cl is dominated by contributions of the Cl 3p-like states, which contribute mainly
5
2
5
2
Cl
5
compound is an indirect-gap material with band
+
gap of 3.42 eV. The X-ray photoelectron core-level and valence-band spectra for pristine and Ar ion-irradiated
surfaces of a TlPb
2
Cl
5
polycrystalline sample were measured. The measurements reveal high chemical stability
Cl surface.
and confirm experimentally the low hygroscopicity of TlPb
2
5
Keywords: Semiconductors; Electronic band structure; X-ray diffraction, Ab initio calculations.
*Corresponding author: khyzhun@ipms.kiev.ua,
Tel.: +38 044 390 11 23; Fax: +38 044 424 21 31

1
. Introduction
Since about 20 years, and the pioneering works by Bowman and his collaborators [1,2] who discovered
3
+
laser operation at 5.2 and 7.2 m in a moisture-sensitive Pr :LaCl
develop and study of the physical and chemical properties of new non-linear optical crystals for middle infrared
mid-IR) and long-wavelength IR laser sources. Because of the high hygroscopicity of LaCl and the difficulty to
use it as an effective laser source operating in ambient conditions, in recent years, attempts at growing crystals of
several moisture-unsensitive lead-based chlorides and bromides with the general formula APb and Tl PbX
A = K, Rb; X = Cl, Br) have led to alternative solutions [3,4]. In particular, these chlorides and bromides are
3
crystal, a lot of attempts have been made to
(
3
X
2 5
3
5
(
very attractive materials for their applications like free-space communication, optical remote sensing technology
LIDAR, military NRBC detection, optronic countermeasures, pollution detection, eye-safe solid-state lasers, and
so on [4-20]. Since thallium lead pentachloride, TlPb
TlPb Br compounds (crystallizing in the monoclinic NH
prospective host materials for their applications as solid state optical amplifiers operated in the mid-IR
wavelength range [3,4,21], one can expect that physical and chemical properties of TlPb Cl should resemble
somewhat those of the mentioned above chlorides and bromides. However, studies of the properties of TlPb
are scarce. The existence of the TlPb Cl
PbCl system [22,23], and its congruent melting point to be 420 °C [23]. The crystal structure of the TlPb
compound has been solved by Keller [24,25], who found that TlPb Cl crystallizes in the monoclinic NH Pb Cl
type structure, space group P2
0.04(4)°. These lattice parameters are close to those derived by X-ray single crystal investigations reported
comparatively recently on two natural samples of hephaistosite minerals found in a high-temperature fumarole
∼400°C) at the rim of La Fossa crater (Vulcano Island, Sicily, Italy) containing some small admixtures of F
and/or Br, namely a = 8.9477 Å, b = 7.9218 Å, c = 12.4955 Å, β= 90.092° for Tl0.94Pb2.01(Cl4.85Br0.14
26] and a = 9.0026 Å, b = 7.9723 Å, c = 12.5693 Å, β= 90.046°for Tl0.94Pb2.01(Cl4.91Br0.14 Σ= 5.05 [27].
To the best of our knowledge, the electronic structure of TlPb Cl has not yet been investigated. The
2
Cl
5
, is isostructural with KPb
2 5 2 5 2 5
Cl , RbPb Cl , KPb Br and
2
5
4
Pb Cl -type structure), which are recognized as very
2
5
2
5
2
Cl
5
2
5
compound was established when studying the pseudo-binary TlCl-
2
2
Cl
5
2
5
4
2
5
-
1
/c, with the lattice parameters a = 8.954 Å, b = 7.920 Å, c = 12.497 Å, and β=
9
(
0.07 Σ= 5.06
F )
[
)
2
5
knowledge of the band structure and peculiarities of the chemical bonding plays an essential role in
understanding the physical and chemical properties of solids [28]. Therefore, in the present work we make use of
the full potential linearized augmented plane wave (FP-LAPW) method as incorporated in the WIEN97 code
[
29] to elucidate the energy distribution of electronic states of different symmetries of the atoms constituting
TlPb Cl . Using the above package, we have fulfilled first-principles calculations of total density of states (DOS)
2
5

and partial densities of states of the monoclinic TlPb
4). In addition, we have employed the X-ray photoelectron spectroscopy (XPS) method to measure binding
energies of core-level electrons of atoms constituting TlPb Cl as well as to record the XPS valence-band
spectrum of the compound under study. The influence of middle-energy Ar ion-bombardment on the valence-
band and constituent element core-level XPS spectra of TlPb Cl surface is also within the scope of the present
work because this method of surface cleaning is widely applied in epitaxial technologies [30].
2 5 1
Cl compound crystallizing in the space group P2 /c (No.
1
2
5
+
2
5
2
. Experimental
.1. Sample preparation
The polycrystalline TlPb
Stockbarger method. The 10 g-batch was composed of the calculated amounts of the respective binary chlorides.
TlCl was produced by the reaction of the commercially available TlNO with the aqueous solution of
2
2
5
Cl sample for the present study was grown by the vertical Bridgman-
3
hydrochloric acid. Obtained precipitate was melted in an evacuated quartz ampoule and underwent 30-fold zone
melting. Then the middle part of the ingot was placed in the quartz ampoule with a conical bottom, evacuated,
transferred to a two-zone Bridgman growth furnace and lowered at the rate of 20 mm/day. The product obtained
was colorless. Commercially available lead chloride with the starting purity of 99.9 wt.% was first recrystallized
by the vertical Bridgman method. Its most impure upper part was removed, and the process was repeated. The
final stage of the purification was 20-fold zone melting in the horizontal furnace. The batch of TlPb
2
Cl
5
was
o
melted, homogenized and transferred to growth furnace. The temperature of the zones of the furnace (450 C /
o
3
70 C) yilded a temperature gradient at the solid-melt interface of 0.8 K/mm. The ampoule was lowered at the
rate of 12 mm/day. The annealing duration was 100 hrs. The furnace was cooled to room temperature in 240
hours. We have not yet managed to optimize the growth condition that would yield a single-crystal preparation,
2 5
and sample obtained was polycrystalline TlPb Cl . A possible reason for such cracking of the crystal is the
o
existence of a first-order phase transition at 396 C. The reconstruction of the crystal structure takes place during
cooling, and this leads to the cracking of the crystal. The maximum linear dimensions of the crystallites did not
exceed 1-2 mm. The crystal structure of the obtained TlPb
diffraction (XRD) data recorded employing a DRON 4-13 diffractometer equipped with a Cu Kαsource of X-ray
radiation. The results of XRD characterization of the synthesized TlPb Cl sample are shown in Fig. 1.
Conditions of the X-ray experiment are listed in Table 1. The diffraction pattern was indexed well in the
monoclinic NH Pb Cl -type structure, space group P2 /c, suggested by Ras et al. [31]. All computations were
2 5
Cl polycrystalline sample was determined by X-ray
2
5
4
2
5
1

performed using CSD software package [32]. The refinement of coordinates and of isotropic temperature
displacement parameters of atoms (Table 1) by full-profile Rietveld method yielded a good value of fit factor
R
I
=0.0757 and R
Employing the present lattice parameters, the experimental and calculated diffraction patterns of TlPb
) were found to be in excellent agreement.
P
=0.1300. The calculated lattice constants agree well with those reported in Refs. [24,26,27].
2
Cl (Fig.
5
1
2
.2. XPS measurements
2 5
XPS valence-band and core-level spectra of TlPb Cl were recorded at room temperature in an ion-
-10
pumped chamber having a base pressure less than 4 ×1 0 mbar of the UHV-Analysis-System assembled by
SPECS Surface Nano Analysis Company (Berlin, Germany) and equipped with a PHOIBOS 150 hemispherical
analyser. A Mg Kαsource of X-ray radiation (E=1253.6 eV) was used for spectra excitation. The XPS spectra
were recorded at a constant pass energy of 25 eV. The energy scale of the spectrometer was calibrated by setting
the measured Au 4f7/2 and Cu 2p3/2 binding energies to 84.00±0.05 eV and 932.66±0.05 eV, respectively, with
respect to the Fermi energy, E
F
. The XPS spectra were measured employing an electron flood gun to overcome
sample charging effects upon X-ray irradiation. With the aim of removing surface contaminations, bombardment
+
+
16
2
of crystal surface has been made by Ar ions with energy of 3.0 keV. A total Ar flux was ∼5 .3 ×1 0 ions/cm .
The method is completely the same as we used earlier in Refs. [33,34].
3
. Method of calculation
For calculations of the electronic structure of TlPb
LAPW method as incorporated in the WIEN97 code [29]. The basis function consists of the atomic orbitals of
Tl, Pb and Cl as listed in Table 2. The lattice parameters and positions of the atoms constituting TlPb Cl have
been chosen as listed in Tables 1 and 3, respectively. In the present calculations of the electronic structure of
TlPb Cl the muffin-tin (MT) sphere radii of the constituent atoms were assumed to be as follows: 2.75 a.u. for
2
5
Cl we use the first-principles self-consistent FP-
2
5
2
5
MT
MT
Tl and Pb atoms and 2.6 a.u. for Cl atoms (1 a.u. = 0.529177 Å). The Rmin kmax parameter, where Rmin denotes
the smallest MT sphere radius and kmax determines the value of the largest k vector in the plane wave expansion,
equals 7.0 (the charge density was Fourier expanded up to the value Gmax = 12). In the potential decomposition,
the valence wavefunctions inside the MT spheres were expanded up to lmax = 6. A total number of semi-core and
2 5
valence electrons (in addition to core electrons) per unit cell equals 376 in the present calculations of TlPb Cl .

The generalized gradient approximation (GGA) by Perdew et al. [35] has been used for calculations of the
exchange-correlation potential and integration through the Brillouin zone (BZ) has been made using the
tetrahedron method by Blöchl et al. [36]. The BZ sampling has been done using 2000 k-points within the
irreducible wedge of the zone. An additional sampling has been made using 4000 k-points within the irreducible
wedge of the BZ for calculations of densities of states. The iteration process was checked taking into account
changes of the integral charge difference q = ρ − ρ dr, where ρn−1 (r) and ρ (r) are input and output
∫
n
n−1
n
charge density, respectively. The calculations were interrupted in the case of q ≤ 0.0001.
4
. Results and discussion
Based on our XRD data, we have employed the Rietveld refinement method to define more precisely
the atomic parameters and inter-atomic Cl–Tl and Cl–Pb distances for the TlPb Cl compound (the data are
listed in Tables 3 and 4, respectively). Fig. 2a presents the crystal structure of TlPb Cl and the coordination
polyhedra of thallium, lead and chlorine atoms in this structure. The coordination environment of lead atoms is
reduced compared to the Pb surrounding in the structure of PbCl [37] (Fig. 2b). This can be seen as the result of
the isovalent substitution of lead atoms in the PbCl structure with thallium atoms to form TlPb Cl . The number
of atoms of the metallic component in relation to the chlorine atoms increases with this substitution. The
maximum value of this ratio is in the case of the Tl PbCl compound [38]. Reducing the number of ligand atoms
for lead with isovalent substitution of lead atoms with thallium atoms in the PbCl structure leads to a
redistribution of charge in the area of transfer of atoms to the Pb atoms in the case of Tl-bearing compounds
Tl PbCl and TlPb Cl . This is manifested primarily in the coordination environment of lead and thallium atoms
in the structure of Tl PbCl and TlPb Cl (Fig. 2c), making these compounds interesting for studies of the
2
5
2
5
2
2
2
5
3
5
2
3
5
2
5
3
5
2
5
electronic structure and related physical properties.
+
Survey XPS spectra of pristine and Ar ion-irradiated surfaces of the TlPb
2
Cl
5
sample are shown in Fig.
3
. It is obvious that all the spectral features, except the carbon and oxygen 1s levels and Auger KLL spectra for
pristine surface, can be assigned to the constituent element core-levels. However, Fig. 3 reveals that the relative
intensity of the C 1s core-level line for pristine surface of the TlPb Cl sample under study is comparatively
weak, and the C 1s line almost completely disappears after the Ar ion-bombardment of the surface. In addition,
the present XPS data show that no active chemical interaction with oxygen occurred when the TlPb Cl surface
2
5
+
2
5
contacted with air for a comparatively long time (a couple of weeks): the O 1s line is rather weak on the survey
+
XPS spectrum of the pristine TlPb
2
Cl
5
surface studied and no trace of the line is detected after the Ar ion-

bombardment (Fig. 3). Therefore, our XPS results reveal the low hygroscopicity of TlPb
extremely important for handling this material as an efficient laser source operating in ambient conditions. The
same conclusion has been made recently [39] in our XPS studies of the KPb Br compound, which is
isostructural to TlPb Cl . Additionally, the present XPS results indicate that no significant changes of binding
2 5
Cl . This property is
2
5
2
5
energy values of constituent element core-level electrons or shapes of the XPS core-level spectra occur as a
+
result of the Ar ion-bombardment of the TlPb
2
Cl
5
surface. The XPS core-level spectra of thallium and lead
+
atoms (Fig. 4) and of chlorine atoms (Fig. 5) measured for the pristine and Ar ion-bombarded TlPb
2
Cl
5
surface
+
2 5
indicate its high chemical stability. The high chemical stability of the TlPb Cl surface with respect to the Ar
ion-bombardment is confirmed by the fact that such a treatment does not cause any noticeable shifts of values of
the core-level binding energies for atoms constituting TlPb Cl (Table 5) and does not lead to any visible
2
5
changes of shapes of the XPS core-level (Figs. 4 and 5) and valence-band (Fig. 6) spectra. Our XPS data allow
+
also for concluding that the Ar ion-bombardment does not change the chemical stoichiometry and concentration
of individual elements of the TlPb
values of binding energies of the XPS Pb 4f core-level spectra measured in the present work for TlPb
close to those detected recently for isostructural KPb Br bromide [39] and KPb Cl and RbPb Cl chlorides
40]. Some visible differences in values of binding energies of the XPS Cl 2p core-level spectra derived for
2
Cl
5
surface. From comparison of data listed in Table 5, one can see that
2
Cl are
5
2
5
2
5
2
5
[
TlPb
2
Cl
5
in the present work and for KPb
2
Cl
5
and RbPb
2
Cl
5
compounds in Ref. [40] can be explained by the fact
+
that the XPS measurements in the latter work were made after Ar ion-beam cleaning the samples that induced
0
significant amount of lead atoms in the chemical state Pb on APb
2
Cl
5
(A = K, Rb) surfaces.
Cl within a 100-eV range.
Fig. 7 presents results of our theoretical calculations of total DOS of TlPb
2
5
The Fermi level is set to zero, and the zero of energy in Fig. 7 has been positioned at the top of the last occupied
band as it was suggested in a number of theoretical first-principles band-structure calculations made for
orthorhombic (P2
bromides and chlorides with the common formula Tl
the upper-core Tl 5p- and Tl 5d-like states form rather narrow bands, while the Pb 5p- and Pb 5d-like states
generate comparatively broad bands in the TlPb Cl compound. In addition, the Cl 3s-like states, which are
among the upper core-states, form a rather broad band ranging from –13.1 to –14.05 eV. Its width (0.95 eV) is
comparative with the width of the same band derived earlier in FP-LAPW calculations of HT-Tl PbCl (∼1.2 eV
41]) as well as with those of the bands generated by the Br 4s-like states in LT-Tl PbBr ( ∼0 .7 eV [42]) and HT-
Tl PbBr ( ∼1 .0 eV [43]) phases. Fig. 7 shows that some minor contributions of the electronic states associated
1
2
1
2
1
) low-temperature (LT) and tetragonal (P4
1
) high-temperature (HT) phases of thallium lead
3
PbX (X = Cl, Br) [41-43]). From Fig. 7, it is evident that
5
2
5
3
5
[
3
5
3
5

with Pb atoms into the Cl 3s-like band are also detected by the present FP-LAPW calculations of the electronic
structure of TlPb Cl
The valence band of TlPb
2
5
.
2 5
Cl can be divided on three sub-bands, marked as A, B, and C in Fig. 8a.
Contributions of the Pb 6s-like states dominate in the lower sub-band C ranging from –5.95 to –7.0 eV. The sub-
band is separated by a gap of about 1.8 eV from a comparatively narrow (width ∼0.6 eV) upper sub-band B,
which is formed almost exclusively from contributions of the Tl 6s-like states. It is worth mentioning that some
minor contributions of the Cl 3p-like states in the sub-bands B and C are also detected by the present FP-LAPW
2 5
calculations of the electronic structure of TlPb Cl (Fig. 8). Principal contributors in the upper sub-band A of the
valence band of TlPb Cl are the Cl 3p-like states. The sub-band A ranges from 0 to –3.3 eV and its bottom is
2
5
separated from the top of the sub-band B by 0.3 eV gap. Some smaller contributions of the Pb 6s- and Tl 6s-like
states in the upper portion of the sub-band A as well as Pb 6p-like states in its lower portion are also visible on
the curves of partial densities of states of TlPb
2
Cl
5
. However, as can be seen from Fig. 8, the Cl 3p-like states are
the principal contributors in the total valence-band region of the TlPb
2
Cl compound. In addition, the present FP-
5
LAPW calculations indicate that the Cl 3p-like states are hybridized in comparatively high degree with the Tl 6s-
and Pb 6s-states in the energy region corresponding to the upper portion of the sub-band A of the valence band of
TlPb
also characteristic of the electronic structure of the TlPb
result of hybridization of the above-mentioned electronic states, there is a significant contribution of the covalent
component into the chemical bonding of TlPb Cl in addition to the ionic component.
2
Cl
5
. Furthermore, hybridization of the Tl 6p- and Cl 3p-like states in the lower portion the sub-band A is
2
Cl compound. Our FP-LAPW data indicate that, as a
5
2
5
2 5
With respect to the occupation of the conduction band of the TlPb Cl compound, Fig. 8 reveals that the
*
bottom of the sub-band A ranging from 3.42 to about 6.8 eV is dominated by contributions of the unoccupied Pb
6
p-like states. Some smaller contributions of the unoccupied Cl 3p- and Tl 6p-like states into the lower portion
*
of the sub-band A are also detected by the present FP-LAPW calculations. As can be seen from Fig. 8, the upper
*
portion of the sub-band A is composed mainly from contributions of the unoccupied Pb 6p-, Tl 6p- and Cl 3p-
*
*
like states in almost equal proportions. Further, the top of sub-band A is separated from the upper sub-band B
*
by 1.0 eV gap. The main contributors into the sub-band B are the unoccupied p- and d-like states associated
with Pb and Cl atoms.
The above theoretical data allow for concluding that the electronic structure of TlPb
PbCl and Tl PbBr compounds (both the LT- and HT-phases) in which contributions of the Cl 3p- (Br
p-) like states dominate in the valence band, mainly at the top and in the central portion, with also significant
2 5
Cl resembles that
of the Tl
3
5
3
5
4

contributions throughout the whole valence-band region [41-43]. Similar to that in TlPb
Cl, Br) compounds the bottom of the conduction band is also composed mainly of contributions of the
unoccupied Pb 6p-like states [41-43]. The exception is that, in the case of the LT-Tl PbBr phase the bottom of
the conduction band is dominated by contributions of the unoccupied Pb 6p- and Tl 6p-like states in almost equal
proportions [42]. However, the main difference of the electronic structure of the TlPb Cl and Tl PbCl
compounds is that the Pb 6s-like sub-band is significantly broad compared to the Tl 6s-like sub-band in
TlPb Cl , while in Tl PbCl the curve of total DOS reveals a rather narrow Pb 6s-like sub-band, but the Tl 6s-
2 5 3 5
Cl , in the Tl PbX (X =
3
5
2
5
3
5
2
5
3
5
like sub-band is comparatively broad in this compound (with a width of about 0.95 eV). It is worth emphasizing
that, in spite of the fact that Pb and Tl are neighboring elements in the Periodic Table, the 6s-like states
associated with Pb and Tl atoms behave rather differently in the TlPb
as our FP-LAPW band-structure calculations reveal: the Pb 6s-like states generate a narrow semi-core sub-band
and the Tl 6s-like states participate in the chemical binding with the valence X p-like states in Tl PbX , while the
s-like states associated with the Pb and Tl atoms participate in the chemical bonding with the Cl 3p-like states
in TlPb Cl
Band dispersions in TlPb
2 5 3 5
Cl and Tl PbX (X = Cl, Br) compounds
3
5
6
2
5
.
2 5
Cl calculated for several symmetry directions of the monoclinic BZ are
shown in Fig. 9. We use the diagram of the monoclinic BZ, which is analogous to that reported previously in
Ref. [44]. From Fig. 9 it is evident that the dispersions of the curves near the valence-band maxima and
conduction-band minima are rather flat in TlPb
.0) and the valence-band maximum at k=(0.0938; 0.0; 0.0). These theoretical data allow for concluding that the
TlPb Cl compound is an indirect-gap material with band gap of E = 3.42 eV.
2 5
Cl . The conduction band minimum is located at k=(0.0; 0.1071;
0
2
5
g
5
. Conclusions
We have performed a complex study of the electronic structure of TlPb
2
5
Cl employing both theoretical
and experimental methods. By using X-ray photoelectron spectroscopy (XPS), we have measured the core-level
+
and valence-band spectra for the pristine and 3.0 keV Ar ion-irradiated TlPb
2
Cl
5
surfaces. The results indicate
+
that the Ar ion-bombardment of the TlPb
2
Cl
5
surface does not alter the shapes of the XPS core-level and
valence-band spectra as well as the binding energies of the constituent element core levels. In addition, the XPS
studies reveal the low hygroscopicity of TlPb Cl , a property that is extremely important for handling this
material in ambient conditions. From our first-principles FP-LAPW calculations, one can state that the valence
band of TlPb Cl is dominated by contributions of the Cl 3p-like states, which contribute mainly into the top and
2
5
2
5

2 5
the central portion of the valence band. Additionally, the bottom of the conduction band of TlPb Cl is
dominated by contributions of the unoccupied Pb 6p-like states. Our calculations indicate that the Cl 3p-like
states are hybridized in comparatively high degree with the Tl 6s- and Pb 6s-states in the energy region
corresponding to the upper portion of the valence band of TlPb
Cl 3p-like states is also characteristic of the electronic structure of the TlPb
calculations indicate that, as a result of hybridization of the above-mentioned electronic states, there is a
significant contribution of the covalent component into the chemical bonding of TlPb Cl in addition to the ionic
compound is an indirect-gap material with band
2
Cl
5
. Furthermore, hybridization of the Tl 6p- and
2
Cl compound. Our first-principles
5
2
5
component. The FP-LAPW calculations reveal that the TlPb
gap of 3.42 eV.
2 5
Cl
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2
3
4
.
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Table 1. Results of the refinement of the TlPb
2
Cl
5
crystal structure
TlPb Cl
mP32
Compound
2
5
Pearson symbol
Space group
P2
1
/c
Number of formula units per unit cell
4
Lattice parameters: a (Å)
8.9561(6)
7.9204(6)
b (Å)
c (Å)
β (°)
1
2.4908(7)
0.073(9)
9
3
Cell volume (Å)
886.0(2)
3
Calculated density (g/cm )
5.967(1)
Diffractometer
Powder DRON 4-13
Radiation and wavelength
α
CuK , λ=1.54185 Å
2θand sin θ/ λ(max)
99.60 and 0.495
Fit factors R and R
I
P
0.0757 and 0.1300
2 5
Table 2. Atomic orbitals used in the present FP-LAPW calculations of the electronic structure of TlPb Cl
Atom
Core electrons
Semi-core
electrons
Valence
Number of electrons involved
electrons
in the FP-LAPW calculations
2
2
6
2
6
10
2
6
10 14
2
6
10
2
1
Tl
1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d
6s 6p
19
20
7
2
2
6
2
6
10
2
6
10 14
2
6
10
2
2
Pb
Cl
1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d
6s 6p
2
2
6
2
5
1s 2s 2p
3s
3p

2 5
Table 3. Atomic positions and isotropic temperature displacement parameters site occupation in the TlPb Cl
structure
2
2
Atom
Wyckoff site
x/a
y/b
z/c
B ×1 0 , nm
isо
Tl
4e
0.4938(9)
0.0119(5)
0.3261(4)
1.04(8)
Pb1
Pb2
Cl1
Cl2
Cl3
Cl4
Cl5
4e
4e
4e
4e
4e
4e
4e
-0.0142(7)
0.2449(10)
0.050(3)
0.276(4)
0.268(5)
0.259(6)
0.553(4)
0.0539(5)
0.0660(5)
0.691(4)
0.456(3)
0.840(3)
0.318(3)
0.153(4)
0.3340(4)
0.0074(4)
0.411(3)
-0.002(3)
0.188(2)
0.280(2)
0.094(3)
1.19(9)
1.05(8)
1.3(8)
0.5(6)
0.7(6)
0.7(6)
1.4(7)

Table 4. Inter-atomic distances (δ) and coordination numbers of atoms (C.N.) in the TlPb
2
Cl
5
structure
Atoms
, Å
C.N.
Atoms
, Å
C.N.
Pb1-1Cl1
3.09(3)
3.21(4)
3.22(3)
3.24(3)
3.26(3)
3.29(4)
3.31(3)
2.81(3)
2.89(2)
2.94(3)
2.99(3)
3.00(3)
3.04(3)
3.10(2)
2.94(3)
3.00(3)
3.09(3)
3.26(3)
2.81(3)
3.04(3)
3.04(3)
3.15(3)
Tl1-1Cl2
1Cl3
2.92(3)
2.98(3)
3.00(4)
3.04(3)
3.04(3)
3.15(3)
3.26(4)
3.36(3)
2.92(3)
3.04(3)
3.10(2)
3.24(3)
3.31(3)
2.89(2)
2.98(3)
3.22(3)
3.36(3)
2.99(3)
3.00(4)
3.21(4)
3.26(4)
3.29(4)
1
1
1
1
1
1
Cl4
Cl3
Cl2
Cl1
Cl4
Cl2
1Cl4
7
1Cl5
8
1Cl2
1Cl5
1Cl4
Pb2-1Cl5
1Cl3
1
1
1
1
1
1
Cl3
Cl1
Cl4
Cl1
Cl5
Cl2
Cl2-1Tl1
1Tl1
7
1Pb2
5
4
5
1Pb1
1Pb1
Cl3-1Pb2
1Tl1
Cl1-1Pb2
1Pb2
1Pb1
1Pb1
1Pb1
4
4
1Tl1
Cl4-1Pb2
1Tl1
Cl5-1Pb2
Pb2
1
1Pb1
1
Tl1
Tl1
1Tl1
1
1Pb1

*
+
Table 5. Binding energies (in eV ) of constituent element core levels of pristine and Ar ion-irradiated surfaces
of the TlPb
Core-level
2 5
Cl polycrystalline sample under study.
TlPb
pristine
surface
2
Cl
5
TlPb
2
Cl
5
KPb
2
Br
5
KPb
2
Cl
5
2 5
RbPb Cl
+
/
/Ar ion-
irradiated
surface
/pristine
surface
*
*
**
Valence band
maximum
Tl 5d5/2
Tl 5d3/2
Pb 5d5/2
Pb 5d3/2
Tl 4f7/2
4.3
4.5
3.20
-
-
13.37
13.32
-
-
-
-
-
-
-
-
-
15.55
15.48
-
-
19.66
19.73
19.41
-
22.22
22.28
21.95
-
118.59
123.08
130.13
138.50
143.35
198.39
268.75
385.83
406.52
413.66
435.93
118.62
123.01
130.18
138.59
143.42
198.31
268.69
385.91
406.65
413.80
436.05
This work
-
-
Tl 4f5/2
-
-
Pb 5s
-
-
Pb 4f7/2
Pb 4f5/2
Cl 2p3/2
Cl 2s
138.33
138.3
138.3
143.16
143.2
143.2
-
197.6
197.5
-
-
-
Tl 4d5/2
Tl 4d3/2
Pb 4d5/2
Pb 4d5/2
-
-
-
-
-
-
413.59
435.84
[39]
-
-
-
-
Reference
This work
[40]
[40]
*
Uncertainty of the measurements is ±0 .05 eV

*
*
Uncertainty of the measurements is ±0 .1 eV.
Figure captions
2 5
Figure 1. Experimental and calculated X-ray diffraction patterns and their difference for the TlPb Cl specimen
under study.
Figure 2. (a) Location and coordination environment of atoms in the structure of TlPb
2
Cl
5
(Tl–blue balls, Pb–
, and (c)
–green) in the structure of TlPb Cl and
green balls, Cl–yellow balls), (b) polyhedra of chlorine atoms around Pb atoms in the structure of PbCl
2
polyhedra of chlorine atoms around the metal atoms (TlCl
Tl PbCl compounds.
x
–blue, PbCl
x
2
5
3
5
+
Figure 3. Survey XPS spectra recorded for (1) pristine and (2) Ar ion-bombarded surface of the TlPb
2
Cl
5
compound.
Figure 4. Detailed XPS (a) Tl 4f and Pb 4f and (b) Tl 4d and Pb 4d core-level spectra recorded for (1) pristine
+
and (2) Ar ion-bombarded surface of the TlPb
2
Cl
5
compound.
+
Figure 5. Detailed XPS (a) Cl 2p and (b) Cl 2s core-level spectra recorded for (1) pristine and (2) Ar ion-
bombarded surface of the TlPb Cl compound.
2
5
Figure 6. XPS valence-band spectra (including upper Pb 5d and Tl 5d core-levels) recorded for (1) pristine and
+
(
2) Ar ion-bombarded surface of the TlPb
2
Cl
5
compound.
Figure 7. Plot of total DOS including upper core, valence-band (VB) and conduction-band (CB) states of
TlPb Cl (note: the upper core states are labelled with respect to their dominant atomic contributions).
2
5
2 5
Figure 8. (a) Total DOS, total and partial densities of states of (b) Tl, (c) Pb, and (d) Cl atoms of TlPb Cl .
2 5
Figure 9. Electronic bands along selected symmetry paths within the first Brillouin zone of TlPb Cl .

Figure 1

(
a)
Figure 2a
(
b)
Figure 2b

(
c)
Figure 2c

Figure 3

Figure 4a
Figure 4b

Figure 5

Figure 6

Figure 7

Figure 8a
Figure 8b

Figure 8c
Figure 8d

Figure 9

Graphical abstract

Highlights
►Electronic structure of TlPb
2
Cl
5
is calculated by the FP-LAPW method. ► The valence band is dominated by
*
contributions of Cl 3p states. ► Contributions of Pb 6p states dominate at the bottom of the conduction band.
►
2 5
The FP-LAPW data allow concluding that TlPb Cl is an indirect-gap material. ► XPS core-level and
valence-band spectra of polycrystalline TlPb Cl are measured.
2
5