First-principles band-structure calculations and X-ray photoelectron spectroscopy studies of the electronic structure of TlPb2Cl5

Accepted Manuscript First-principles band-structure calculations and X-ray photoelectron spectro‐ scopy studies of the electronic structure of TlPb2Cl5 O.Y. Khyzhun, V.L. Bekenev, N.M. Denysyuk, O.V. Parasyuk, A.O. Fedorchuk PII: DOI: Reference:

S0925-8388(13)02003-3 http://dx.doi.org/10.1016/j.jallcom.2013.08.127 JALCOM 29262

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, Firstprinciples band-structure calculations and X-ray photoelectron spectroscopy studies of the electronic structure of TlPb2Cl5, (2013), doi: http://dx.doi.org/10.1016/j.jallcom.2013.08.127

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First-principles band-structure calculations and X-ray photoelectron spectroscopy studies of the electronic structure of TlPb2Cl5

O.Y. Khyzhuna,*, V.L. Bekeneva, N.M. Denysyuka, O.V. Parasyukb, A.O. Fedorchukc

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 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 TlPb2Cl5 is composed mainly of contributions of the unoccupied Pb 6p-like states. Our FP-LAPW data indicate that the TlPb2Cl5 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 TlPb2Cl5 surface.

Keywords: Semiconductors; Electronic band structure; X-ray diffraction, Ab initio calculations.

*Corresponding author: [email protected], 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 laser operation at 5.2 and 7.2 m in a moisture-sensitive Pr3+:LaCl3 crystal, a lot of attempts have been made to 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 LaCl3 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 APb2X5 and Tl3PbX 5 (A = K, Rb; X = Cl, Br) have led to alternative solutions [3,4]. In particular, these chlorides and bromides are 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, TlPb2Cl5, is isostructural with KPb2Cl5, RbPb2Cl5, KPb2Br5 and TlPb2Br5 compounds (crystallizing in the monoclinic NH4Pb2Cl5-type structure), which are recognized as very 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 TlPb2Cl5 should resemble somewhat those of the mentioned above chlorides and bromides. However, studies of the properties of TlPb2Cl5 are scarce. The existence of the TlPb2Cl5 compound was established when studying the pseudo-binary TlClPbCl2 system [22,23], and its congruent melting point to be 420 °C [23]. The crystal structure of the TlPb2Cl5 compound has been solved by Keller [24,25], who found that TlPb2Cl5 crystallizes in the monoclinic NH4Pb2Cl5type structure, space group P21/c, with the lattice parameters a = 8.954 Å, b = 7.920 Å, c = 12.497 Å, and β = 90.04(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 F0.07)Σ=5.06 [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 TlPb2Cl5 has not yet been investigated. The 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 TlPb2Cl5. Using the above package, we have fulfilled first-principles calculations of total density of states (DOS)

and partial densities of states of the monoclinic TlPb2Cl5 compound crystallizing in the space group P2 1/c (No. 14). In addition, we have employed the X-ray photoelectron spectroscopy (XPS) method to measure binding energies of core-level electrons of atoms constituting TlPb2Cl5 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 valenceband and constituent element core-level XPS spectra of TlPb2Cl5 surface is also within the scope of the present work because this method of surface cleaning is widely applied in epitaxial technologies [30].

2. Experimental 2.1. Sample preparation The polycrystalline TlPb2Cl5 sample for the present study was grown by the vertical BridgmanStockbarger 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 TlNO3 with the aqueous solution of 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 TlPb2Cl5 was melted, homogenized and transferred to growth furnace. The temperature of the zones of the furnace (450 oC / 370 oC) 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, and sample obtained was polycrystalline TlPb2Cl5. A possible reason for such cracking of the crystal is the existence of a first-order phase transition at 396 oC. 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 TlPb2Cl5 polycrystalline sample was determined by X-ray 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 TlPb2Cl5 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 NH4Pb2Cl5-type structure, space group P21/c, suggested by Ras et al. [31]. All computations were

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 RI =0.0757 and RP =0.1300. The calculated lattice constants agree well with those reported in Refs. [24,26,27]. Employing the present lattice parameters, the experimental and calculated diffraction patterns of TlPb2Cl5 (Fig. 1) were found to be in excellent agreement.

2.2. XPS measurements XPS valence-band and core-level spectra of TlPb2Cl5 were recorded at room temperature in an ionpumped chamber having a base pressure less than 4×10-10 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, EF. 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 of crystal surface has been made by Ar+ ions with energy of 3.0 keV. A total Ar+ flux was ∼5.3×1016 ions/cm2. 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 TlPb2Cl5 we use the first-principles self-consistent FPLAPW 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 TlPb2Cl5 have been chosen as listed in Tables 1 and 3, respectively. In the present calculations of the electronic structure of TlPb2Cl5 the muffin-tin (MT) sphere radii of the constituent atoms were assumed to be as follows: 2.75 a.u. for MT

MT

Tl and Pb atoms and 2.6 a.u. for Cl atoms (1 a.u. = 0.529177 Å). The Rmin k max parameter, where Rmin denotes the smallest MT sphere radius and k max 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 valence electrons (in addition to core electrons) per unit cell equals 376 in the present calculations of TlPb2Cl5.

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

n

− ρ n−1 dr , where ρ n−1 (r ) and ρ n (r ) are input and output

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 TlPb2Cl5 compound (the data are listed in Tables 3 and 4, respectively). Fig. 2a presents the crystal structure of TlPb2Cl5 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 PbCl2 [37] (Fig. 2b). This can be seen as the result of the isovalent substitution of lead atoms in the PbCl2 structure with thallium atoms to form TlPb2Cl5. 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 Tl3PbCl5 compound [38]. Reducing the number of ligand atoms for lead with isovalent substitution of lead atoms with thallium atoms in the PbCl2 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 Tl3PbCl5 and TlPb2Cl5. This is manifested primarily in the coordination environment of lead and thallium atoms in the structure of Tl3PbCl5 and TlPb2Cl5 (Fig. 2c), making these compounds interesting for studies of the electronic structure and related physical properties. Survey XPS spectra of pristine and Ar+ ion-irradiated surfaces of the TlPb2Cl5 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 TlPb2Cl5 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 TlPb2Cl5 surface 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 TlPb2Cl5 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 TlPb2Cl5. This property is 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 KPb2Br5 compound, which is isostructural to TlPb2Cl5. Additionally, the present XPS results indicate that no significant changes of binding 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 TlPb2Cl5 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 TlPb2Cl5 surface indicate its high chemical stability. The high chemical stability of the TlPb2Cl5 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 TlPb2Cl5 (Table 5) and does not lead to any visible 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 TlPb2Cl5 surface. From comparison of data listed in Table 5, one can see that values of binding energies of the XPS Pb 4f core-level spectra measured in the present work for TlPb2Cl5 are close to those detected recently for isostructural KPb2Br5 bromide [39] and KPb2Cl5 and RbPb2Cl5 chlorides [40]. Some visible differences in values of binding energies of the XPS Cl 2p core-level spectra derived for TlPb2Cl5 in the present work and for KPb2Cl5 and RbPb2Cl5 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 significant amount of lead atoms in the chemical state Pb0 on APb2Cl5 (A = K, Rb) surfaces. Fig. 7 presents results of our theoretical calculations of total DOS of TlPb2Cl5 within a 100-eV range. 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 12121) low-temperature (LT) and tetragonal (P41) high-temperature (HT) phases of thallium lead bromides and chlorides with the common formula Tl 3PbX5 (X = Cl, Br) [41-43]). From Fig. 7, it is evident that 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 TlPb2Cl5 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-Tl3PbCl5 (∼1.2 eV [41]) as well as with those of the bands generated by the Br 4s-like states in LT-Tl3 PbBr5 (∼0.7 eV [42]) and HTTl3PbBr5 (∼1.0 eV [43]) phases. Fig. 7 shows that some minor contributions of the electronic states associated

with Pb atoms into the Cl 3s-like band are also detected by the present FP-LAPW calculations of the electronic structure of TlPb2Cl5. The valence band of TlPb2Cl5 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 subband 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 calculations of the electronic structure of TlPb2Cl5 (Fig. 8). Principal contributors in the upper sub-band A of the valence band of TlPb2Cl5 are the Cl 3p-like states. The sub-band A ranges from 0 to –3.3 eV and its bottom is 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 TlPb2Cl5. 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 TlPb2Cl5 compound. In addition, the present FPLAPW calculations indicate that the Cl 3p-like states are hybridized in comparatively high degree with the Tl 6sand Pb 6s-states in the energy region corresponding to the upper portion of the sub-band A of the valence band of TlPb2Cl5. Furthermore, hybridization of the Tl 6p- and Cl 3p-like states in the lower portion the sub-band A is also characteristic of the electronic structure of the TlPb2Cl 5 compound. Our FP-LAPW data 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 TlPb2Cl5 in addition to the ionic component. With respect to the occupation of the conduction band of the TlPb2Cl5 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 6p-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 3plike 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 TlPb2Cl5 resembles that of the Tl3PbCl5 and Tl3PbBr5 compounds (both the LT- and HT-phases) in which contributions of the Cl 3p- (Br 4p-) like states dominate in the valence band, mainly at the top and in the central portion, with also significant

contributions throughout the whole valence-band region [41-43]. Similar to that in TlPb2Cl5, in the Tl3PbX5 (X = 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-Tl3PbBr5 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 TlPb2Cl5 and Tl3PbCl5 compounds is that the Pb 6s-like sub-band is significantly broad compared to the Tl 6s-like sub-band in TlPb2Cl5, while in Tl3PbCl5 the curve of total DOS reveals a rather narrow Pb 6s-like sub-band, but the Tl 6slike 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 TlPb2Cl5 and Tl 3PbX5 (X = Cl, Br) compounds 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 Tl3PbX5, while the 6s-like states associated with the Pb and Tl atoms participate in the chemical bonding with the Cl 3p-like states in TlPb2Cl5. Band dispersions in TlPb2Cl5 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 TlPb2Cl5. The conduction band minimum is located at k=(0.0; 0.1071; 0.0) and the valence-band maximum at k=(0.0938; 0.0; 0.0). These theoretical data allow for concluding that the TlPb2Cl5 compound is an indirect-gap material with band gap of Eg = 3.42 eV.

5. Conclusions We have performed a complex study of the electronic structure of TlPb2Cl5 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 TlPb2Cl5 surfaces. The results indicate that the Ar+ ion-bombardment of the TlPb2Cl5 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 TlPb2Cl5, 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 TlPb2Cl5 is dominated by contributions of the Cl 3p-like states, which contribute mainly into the top and

the central portion of the valence band. Additionally, the bottom of the conduction band of TlPb2Cl5 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 TlPb2Cl5. Furthermore, hybridization of the Tl 6p- and Cl 3p-like states is also characteristic of the electronic structure of the TlPb2Cl5 compound. Our first-principles 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 TlPb2Cl5 in addition to the ionic component. The FP-LAPW calculations reveal that the TlPb2Cl5 compound is an indirect-gap material with band gap of 3.42 eV.

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Table 1. Results of the refinement of the TlPb2Cl5 crystal structure Compound

TlPb2Cl5

Pearson symbol

mP32

Space group

P2 1/c

Number of formula units per unit cell

4

Lattice parameters: a (Å)

8.9561(6)

b (Å)

7.9204(6)

c (Å)

12.4908(7)

β (°)

90.073(9)

Cell volume (Å3)

886.0(2)

Calculated density (g/cm3)

5.967(1)

Diffractometer

Powder DRON 4-13

Radiation and wavelength

CuKα, λ=1.54185 Å

2θand sinθ/λ(max)

99.60 and 0.495

Fit factors RI and RP

0.0757 and 0.1300

Table 2. Atomic orbitals used in the present FP-LAPW calculations of the electronic structure of TlPb2Cl5 Atom

Core electrons

Semi-core

Valence

Number of electrons involved

electrons

electrons

in the FP-LAPW calculations

Tl

1s22s22p63s23p63d104s24p64d104f145s2

5p65d10

6s26p1

19

Pb

1s22s22p63s23p63d104s24p64d104f145s2

5p65d10

6s26p2

20

Cl

1s22s22p6

3s2

3p5

7

Table 3. Atomic positions and isotropic temperature displacement parameters site occupation in the TlPb2Cl5 structure

Wyckoff site

x/a

y/b

z/c

Bisо×102, nm2

Tl

4e

0.4938(9)

0.0119(5)

0.3261(4)

1.04(8)

Pb1

4e

-0.0142(7)

0.0539(5)

0.3340(4)

1.19(9)

Pb2

4e

0.2449(10)

0.0660(5)

0.0074(4)

1.05(8)

Cl1

4e

0.050(3)

0.691(4)

0.411(3)

1.3(8)

Cl2

4e

0.276(4)

0.456(3)

-0.002(3)

0.5(6)

Cl3

4e

0.268(5)

0.840(3)

0.188(2)

0.7(6)

Cl4

4e

0.259(6)

0.318(3)

0.280(2)

0.7(6)

Cl5

4e

0.553(4)

0.153(4)

0.094(3)

1.4(7)

Atom

Table 4. Inter-atomic distances (δ) and coordination numbers of atoms (C.N.) in the TlPb2Cl5 structure Atoms

,Å

C.N.

Atoms

,Å

Pb1-1Cl1

3.09(3)

Tl1-1Cl2

2.92(3)

1Cl4

3.21(4)

1Cl3

2.98(3)

1Cl3

3.22(3)

1Cl4

3.00(4)

1Cl2

3.24(3)

1Cl5

3.04(3)

1Cl1

3.26(3)

1Cl2

3.04(3)

1Cl4

3.29(4)

1Cl5

3.15(3)

1Cl2

3.31(3)

1Cl4

3.26(4)

Pb2-1Cl5

2.81(3)

1Cl3

3.36(3)

1Cl3

2.89(2)

Cl2-1Tl1

2.92(3)

1Cl1

2.94(3)

1Tl1

3.04(3)

1Cl4

2.99(3)

1Pb2

3.10(2)

1Cl1

3.00(3)

1Pb1

3.24(3)

1Cl5

3.04(3)

1Pb1

3.31(3)

1Cl2

3.10(2)

Cl3-1Pb2

2.89(2)

Cl1-1Pb2

2.94(3)

1Tl1

2.98(3)

1Pb2

3.00(3)

1Pb1

3.22(3)

1Pb1

3.09(3)

1Tl1

3.36(3)

1Pb1

3.26(3)

Cl4-1Pb2

2.99(3)

Cl5-1Pb2

2.81(3)

1Tl1

3.00(4)

1Pb2

3.04(3)

1Pb1

3.21(4)

1Tl1

3.04(3)

1Tl1

3.26(4)

1Tl1

3.15(3)

1Pb1

3.29(4)

7

7

C.N.

8

5

4

4

4

5

Table 5. Binding energies (in eV* ) of constituent element core levels of pristine and Ar+ ion-irradiated surfaces of the TlPb2Cl5 polycrystalline sample under study. Core-level

TlPb2Cl5

TlPb2Cl5 +

KPb2Br5

/pristine

/Ar ion-

/pristine

surface

irradiated

surface

KPb2Cl5

RbPb2Cl5

surface 4.3**

4.5**

3.20

-

-

Tl 5d5/2

13.37

13.32

-

-

-

Tl 5d3/2

15.55

15.48

-

-

-

Pb 5d5/2

19.66

19.73

19.41

-

-

Pb 5d3/2

22.22

22.28

21.95

-

-

Tl 4f7/2

118.59

118.62

-

-

-

Tl 4f5/2

123.08

123.01

-

-

-

Pb 5s

130.13

130.18

-

-

-

Pb 4f7/2

138.50

138.59

138.33

138.3

138.3

Pb 4f5/2

143.35

143.42

143.16

143.2

143.2

Cl 2p3/2

198.39

198.31

-

197.6

197.5

Cl 2s

268.75

268.69

-

-

-

Tl 4d5/2

385.83

385.91

-

-

-

Tl 4d3/2

406.52

406.65

-

-

-

Pb 4d5/2

413.66

413.80

413.59

-

-

Pb 4d5/2

435.93

436.05

435.84

-

-

Reference

This work

This work

[39]

[40]

[40]

Valence band maximum

*

Uncertainty of the measurements is ±0.05 eV

**

Uncertainty of the measurements is ±0.1 eV. Figure captions

Figure 1. Experimental and calculated X-ray diffraction patterns and their difference for the TlPb2Cl5 specimen under study.

Figure 2. (a) Location and coordination environment of atoms in the structure of TlPb2Cl5 (Tl–blue balls, Pb– green balls, Cl–yellow balls), (b) polyhedra of chlorine atoms around Pb atoms in the structure of PbCl2, and (c) polyhedra of chlorine atoms around the metal atoms (TlClx–blue, PbClx–green) in the structure of TlPb2Cl5 and Tl3PbCl5 compounds.

Figure 3. Survey XPS spectra recorded for (1) pristine and (2) Ar+ ion-bombarded surface of the TlPb2Cl5 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 TlPb2Cl5 compound.

Figure 5. Detailed XPS (a) Cl 2p and (b) Cl 2s core-level spectra recorded for (1) pristine and (2) Ar+ ionbombarded surface of the TlPb2Cl5 compound.

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 TlPb2Cl5 compound.

Figure 7. Plot of total DOS including upper core, valence-band (VB) and conduction-band (CB) states of TlPb2Cl5 (note: the upper core states are labelled with respect to their dominant atomic contributions).

Figure 8. (a) Total DOS, total and partial densities of states of (b) Tl, (c) Pb, and (d) Cl atoms of TlPb2Cl5.

Figure 9. Electronic bands along selected symmetry paths within the first Brillouin zone of TlPb2Cl5.

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 TlPb2Cl5 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. ► The FP-LAPW data allow concluding that TlPb2Cl5 is an indirect-gap material. ► XPS core-level and valence-band spectra of polycrystalline TlPb2Cl5 are measured.