Experimental and Computational Insight into the Chemical Bonding

Article pubs.acs.org/IC

Experimental and Computational Insight into the Chemical Bonding and Electronic Structure of Clathrate Compounds in the Sn−In−As−I System Lada V. Yashina,†,‡ Andrey A. Volykhov,†,‡,§ Vera S. Neudachina,†,‡ Nadezhda V. Aleksandrova,† Liudmila N. Reshetova,† Marina E. Tamm,† Virginia Pérez-Dieste,∥ Carlos Escudero,∥ Denis V. Vyalikh,⊥,# and Andrei V. Shevelkov*,† †

Department of Chemistry, Lomonosov Moscow State University, Leninskie gory, 119991 Moscow, Russia Federal State Research and Design Institute of Rare Metal Industry “GIREDMET”, B. Tolmachevsky lane 5, 119017 Moscow, Russia § Kurnakov Institute of General and Inorganic Chemistry RAS, Leninsky Av 31, 119991 Moscow, Russia ∥ ALBA Synchrotron Light Source, Carretera BP 1413 Km. 3.3, 08290 Cerdanyola del Vallés, Barcelona, Spain ⊥ Institute of Solid State Physics, Dresden University of Technology, Zellescher Weg 16, D-01062 Dresden, Germany # St. Petersburg State University, Ulyanovskaya street 1, St. Petersburg 198504, Russia ‡

S Supporting Information *

ABSTRACT: Inorganic clathrate materials are of great fundamental interest and potential practical use for application as thermoelectric materials in freon-free refrigerators, wasteheat converters, direct solar thermal energy converters, and many others. Experimental studies of their electronic structure and bonding have been, however, strongly restricted by (i) the crystal size and (ii) essential difficulties linked with the clean surface preparation. Overcoming these handicaps, we present for the first time a comprehensive picture of the electronic band structure and the chemical bonding for the Sn24−x−δInxAs22−yI8 clathrates obtained by means of photoelectron spectroscopy and complementary quantum modeling.

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composing the clathrate framework as well as guest species.8,9 Symmetrically, transport of charge carriers can be optimized without compromising low thermal conductivity3,4,6 Clathrates with positively charged frameworks composed mainly of group 14 atoms, which trap electronegative guests such as iodine or tellurium, are known as cationic or inverse clathrates.10 Recently designed clathrate compounds of this type demonstrate high values of the thermoelectric figure-ofmerit, ZT, combined with such properties as thermal and chemical stability at high temperatures.11,12 Particularly, several newly studied clathrates of the Sn−In−As−I type demonstrate prospective thermoelectric properties.13,14 It was shown that, for these compounds, the absolute values of the Seebeck coefficient reach 600 μV·K−1; at the same time, their thermal conductivity is low, usually below 0.4 W·m−1 K−1 at room temperature, which is typical for amorphous solids, with the total thermal conductivity being almost entirely phononic. However, because of the not high enough electrical conductivity, the ZT is only 0.041 at room temperature and extrapolated to 0.4 at 600 K. These values are low for any

INTRODUCTION Nowadays, the growing need for alternative energy sources initiates a new quest of materials for energy generation, storage, and conversion. Under these circumstances, advanced thermoelectric materials with high electrical conductivity, low thermal conductivity, and, hence, good thermoelectric figures-of-merit are of high interest owing to their potential application in freonfree refrigerators, waste-heat converters, and direct solar thermal energy converters.1,2 A possible pathway to decrease the thermal conductivity is to exploit the unique structural features of inorganic/intermetallic clathrate materials. Their architecture includes a nanoscale framework of ordered cages, with large electropositive or electronegative atoms filling them and, therefore, compensating for the framework charge.3 The unique feature of clathrates is their abnormally low thermal conductivity because of the localized motion, so-called “rattling”, of the guest atoms inside the oversized cavities of the framework, which leads to either scattering or low group velocity of heat-carrying phonons.4−7 In the first approximation, this effect is independent of the charge carrier transport through the framework. Consequently, the lattice thermal conductivity can be optimized by tuning the host−guest mismatch and by increasing the atomic mass of the atoms © XXXX American Chemical Society

Received: September 28, 2015

A

DOI: 10.1021/acs.inorgchem.5b02223 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Crystal structure of the Sn24−x−δInxAs22−yI8 clathrates: (a) two types of the iodine guest atom coordination in idealized polyhedral cages, (b) polyhedral presentation of the crystal structure, (c) a view of the unit cell, (d) photo of a crystal grown for XPS experiments.

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application, but indicate that the Sn−In−As−I clathrates provide a solid ground for the development of thermoelectric materials for midtemperature applications. Apart from the application issues, the fundamental properties of the Zintl phases also ensure further development of theoretical chemistry. Cationic clathrates represent a particular family of Zintl phases known as inverse Zintl phases, where guest atoms serve as anions withdrawing excess electrons from a cationic four-bonded framework in order to complete the electron octet for each atom in the framework.3,10 It has been already established that the clathrate compounds formed in the Sn−In−As−I system display crystal structure diversity. They can be described by a general formula Sn24−x−δInxAs22−yI8, where indium content, x, varies between 1 and 12, and depending on it, vacancy concentration in tin, δ, and arsenic, y, substructures also vary in such a way that the overall electroneutrality is maintained. Application of the Zintl concept enables calculating the strict relationship between x, y, and δ, taking into account that four-bonded tin, indium, and arsenic bare formal oxidation numbers 0, −1, and +1, respectively, whereas each vacancy consumes four electrons.3 These clathrates belong to the clathrate-I structure type, but owing to varying indium and vacancy concentrations, they show striking changes in atomic positions, vacancy distribution, and even superstructure formation upon changing the Sn/In ratio.13 However, the understanding of the electronic properties and chemical bonding in this system is still far from being clear and comprehensive. In particular, the shallow bands between the Fermi level (EF) and 2−5 eV are of great interest for unraveling the unique physical properties of the inverse Zintl phases, i.e., the origin of their low thermal and high electrical conductivity. Up to now, such kind of research has been strongly restricted by the crystal size and difficulties related to the clean surface preparation. As a consequence, apart from the limited number of local electronic structure studies by means of 119Sn Mö ssbauer spectroscopy,15,16 insight into the electronic structure of clathrates was provided by various types of calculations only.17−20 Here, we focus on the experimental study of the band structure and chemical bonding for the Sn24−x−δInxAs22−yI8 clathrates as probed by valence band and core-level photoemission for the first time, supplemented by the complementary quantum modeling.

EXPERIMENTAL SECTION

The Sn24−x−δInxAs22−yI8 clathrate single crystals (x = 5 and 9.5) were grown using chemical vapor transport as described in detail elsewhere.13 In short, polycrystalline samples of the desired stoichiometry were prepared by a standard ampule method from SnAs, InAs, As, and SnI4 and further used for the crystal growth. For this, they were sealed into silica tubes about 100 mm long together with about 5 mg of SnI4 to facilitate transport through the gas phase. The ampule was placed into a two-zone furnace with a reversed gradient with temperatures T1 = 820 K (of the load) and T2 = 835 K (of the empty side). After 36 h, the temperatures were slowly adjusted to T1 = 840 K and T2 = 825 K. Large clathrate crystals, several mm in size, were grown in 12 days. The obtained crystals were characterized by XRD, X-ray fluorescence, and EDX and confirmed to be phasepure, with no gradient in composition across the volume and of the same Sn/In ratio as in the load, Sn17.1(3)In5.0(1)As21.8(1)I8 (x = 5) and Sn14.0(2)In9.5(1)As21.4(1)I8 (x = 9.5). The X-ray photoemission spectra for the surfaces of the obtained crystals were recorded at the dipole Russian-German beamline (RGBL) of the synchrotron radiation facility BESSY II (Berlin, Germany) for low photon energies and at the NAPP branch from the undulator CIRCE beamline at ALBA Synchrotron Light Source (Cerdanyola del Vallès, Spain) for the relatively high photon energy. The exact photon energy was determined by registering the core-level spectra excited by the 1st and 2nd order radiation reflected from the diffraction grating. Acquisition of X-ray photoemission spectra was performed using a PHOIBOS 150 and PHOIBOS 150 NAP electron energy analyzers (SPECS GmbH) at a base pressure of (2−4)·10−10 Torr and at 0.1 mBar O2. The spectra were acquired at photon energies (hν) of 60−727 eV to provide variable surface sensitivity with the overall (monochromator and analyzer) energy resolution of approximately 40−150 meV full width at half-maximum (fwhm). Generally, the photoemission cross sections for shallow Sn, In, I 4d, and As 3d core levels vary strongly in the photon energy range of 75− 150 eV (see Figure S1 of the Supporting Information). The photon energy of 100 eV was found to be the optimal choice in order to provide high intensity for the shallow core levels for all elements (In, Sn, I 4d and As 3d), thus allowing us to record the spectra with the ultimate resolution. The surface composition was calculated using the spectra obtained at fixed kinetic energies of 200 and 300 eV. The detection angle was close to the normal emission. The diameter of the X-ray spot was estimated to be in the order of 100 μm. For the data analysis, the spectra were fitted by the Gaussian/Lorentzian convolution functions with the simultaneous optimization of the background parameters using the Unifit2014 software.21 The background was modeled by a combination of Shirley and Tougaard background functions. B


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RESULTS AND DISCUSSION Surface Preparation. In the Sn−In−As−I system, clathrate compounds have a general formula Sn24−x−δInxAs22−yI8, with x varying between 1 and 12, while the δ and y parameters are calculated based on the x value to maintain the overall electroneutrality.14 They crystallize in the cubic space group Pm3̅n (#223). The unit cell parameter follows the Vegard’s law slightly decreasing from a = 11.343 Å to a = 11.088 Å upon decreasing the In content from x = 10.7 to x = 0.13 In their crystal structure, two types of guest iodine atoms, I1 and I2, are located in the centers of polyhedral cages of different sizes formed by 20 and 24 framework atoms, respectively (Figure 1a). The 20-vertex dodecahedra and 24-vertex tetrakaidecahedra composed of tin, indium, and arsenic atoms alternate in the crystal structure in the 1:3 ratio, as illustrated in Figure 1b. The decrease in indium content, x, in the unit cell (Figure 1c) results in (i) the positional disorder of both positions of arsenic atoms within the framework, (ii) the increase of the metal vacancy density and decrease in the number of the As vacancies, and (iii) the redistribution of the host−guest bond distances. The as-grown crystals possess a cubooctahedral habit owing to the highly symmetric cubic structure (Figure 1d). The surface of the as-grown crystals is polluted by the transport agent and essentially oxidized due to air exposure, and therefore, it required cleaning for the purposes of our studies. Two different procedures were employed. First, as the crystals have a distinct polyhedral habit with mm-sized growth planes, we tried to clean up the as-grown surface using the procedure based on ion sputtering/annealing cycles, which has been proven to be efficient for multiple systems. Sputtering with Ar+ ions (1 keV, 30 min) allowed us to completely remove the surface pollutions and the oxidized layer. The sputtered surface spectrum obtained at the photon energy of 100 eV is presented in Figure 2a. After sputtering, the surface is essentially enriched in tin due to the preferential sputtering effect. Subsequent annealing at different temperatures (300−350 °C) with varied duration (2−4.5 h), which was aimed at crystallizing the mixing layer that appeared at the surface after sputtering, resulted in essential surface depletion in iodine accompanied by tin arsenide formation, as clearly follows from Figure 2b. Therefore, for such a multicomponent system, the sputtering/annealing approach explicitly failed due to high variability of the system under study. The second approach we used was vacuum cleavage of the bulk crystal. The resulting surface is very rough due to the structural peculiarities (i.e., the 3D nature and absence of cleavability) of the clathrate crystals. Nevertheless, the surface composition coincides with the bulk one in this case within the accuracy. The corresponding spectrum is presented in Figure 2c. Charge State of the Constituent Atoms. To probe the charge state of the atoms in the Sn24−x−δInxAs22−yI8 clathrates of different compositions, we analyzed the respective photoemission core-level peak, specifically, its line shape and binding energy (BE) position. The discussed spectra are shown in Figure 2d−g, while the corresponding peak positions are summarized in Table 1. In our discussion of the line shape analysis, we mostly focus on shallow core levels Sn 4d and In 4d because of their lowest natural width (highest lifetime of core shell). At the same time, we need to compare peak positions with the literature data, which are mostly available for the

Figure 2. Core-level photoemission spectra for the Sn24−x−δInxAs22−yI8 (x = 8) clathrates obtained at a photon energy of 100 eV: (a) survey spectra for the sputtered surface of the growth (100) plane, (b) spectra for the same surface after annealing at 300 C for 30 min, (c) vacuum cleaved surface, (d−g) detailed photoemission spectra for the vacuum cleaved surface with the experimental data points and their fitting given as solid line, (h, i) Auger lines. C


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Inorganic Chemistry

approach using the PW-GGA method. Core electrons were modeled by using the standard PAW pseudopotentials. A (4 × 4 × 4) k-points mesh in the first Brillouin zone was used. For the calculations, we used the Vienna Ab initio Simulation Package (VASP).23−25 After the optimization, the initial cubic lattice appeared to be slightly distorted by 0.2% with a “mean” lattice constant of 11.70 Å. We also simulated Sn14In10As22I8, Sn17In7As22I8, and Sn19In5As22I8 taking more tin atoms in the frameworks and, finally, a virtual composition beyond experimental observations, Sn10In14As22I8, which satisfies the Zintl−Klemm rule; in the latter case, the Fermi level expectedly falls in the band gap. The results of the binding energy simulation in the initial state approximation are presented in Figure 2c as vertical lines. The experimental and simulated values are in excellent agreement with each other. In detail, the calculation results suggest that there should be two components in the I 4d photoemission spectrum with a reasonably low splitting of 0.35 eV. The simulated As 3d binding energies are in the range of 48.5−49.1 eV, which is consistent with notable broadening of the experimentally measured peak. In addition, we found no composition effects on the BE values upon x variation in our simulation, which is in line with the experimental results (see the Supporting Information). It is well-known that the final state effect, which was neglected in our simulations, can contribute significantly, especially in the case of cations.26 However, a conclusion concerning the chemical bonds should be done on the basis of the initial state contribution only. To figure out the contribution of the final state effect to the experimental binding energy, we calculated the relaxation energy using the so-called Wagner Auger parameter α′26,27 obtained from the corresponding Auger lines (Figure 2h,i) using the following equation

Table 1. Summary of the Experimental and Simulated Peak Positions (Given in eV and Calculated Relative to the Vacuum Level) for the Studied Clathrates and Reference Materials core level

reference material

experimental peak position for the reference

Sn 4d In 4d As 3d I 4d

SnSe InSe GaAs Bi3TeI

24.87 ± 0.05 17.35 ± 0.05 41.2 49.01 ± 0.05

experimental peak position for the clathrate

simulated peak position in clathrate In12Sn12As22I8

± ± ± ± ±

24.97 17.34 41.05−41.43 49.08 48.44

24.78 17.44 41.21 49.15 48.69

0.03 0.02 0.06 0.03 0.04

corresponding 3d peaks. In general, we supposed the same chemical shifts for 3d and 4d lines, which is fulfilled commonly with the accuracy not worse than 0.05 eV. The spectra of shallow and hence the most narrow lines of Sn 4d and In 4d as well as Sn 3d and In 3d are ideally described with one spin−orbit doublet each, with the peak width, however, notably larger than that for the available reference materials such as SnSe and InSe (see the Supporting Information for details). The I 4d and I 3d as well as As 3d spectra are even broader and complex. The BE values obtained upon subsequent cleavage procedures for the same crystal as well as for the crystals of different compositions (x = 5.0 and 9.5) are well reproduced. This fact evidences the reproducibility of the surface structure obtained by cleavage. Next, we modeled the BEs using the following approach. The core-level shifts were calculated in the initial state approximation as a variation of the electrostatic potentials at the atomic centers, as described in the literature,22 relative to the energies available for the reference compounds. Then, we calculated the BEs using the experimental values and the calculation data for the reference surfaces. In our simulations of the chemical shifts, we considered the idealized structure of the ultimate composition Sn12In12As22I8, with In and Sn atoms being randomly distributed over the clathrate framework, though avoiding Sn−Sn or In−In bonds. Arsenic vacancies were disregarded at this stage. Throughout the calculations, we used a unit cell composed by 12Sn + 12In + 22As + 8I atoms without any superlattice, which was achieved upon lowering the symmetry down to the space group P1. The starting geometry was chosen in accordance with the structural data. The geometry was fully optimized within the DFT

α′ = BE (3d5/2) + KE (M4N45N45)

The total chemical shift in photoelectron spectra of any compound relative to free atom according to the simplest approximation27 can be considered as three contributions ΔBE = ΔEini + qRa − Rea

Here, ΔEini is the shift of the core level in the initial state; it consists of kq (the change in the core potential due to removal of the valence electron) and V (Madelung potential). The quantities qRa (atomic relaxation energy) and Rea (the extra

Table 2. Summary of the Final State Contribution to the Chemical Shifts

clathrate SnSe SnS28 SnO2 clathrate InSe29 InP21 In2Te321 In2Se321 clathrate LiI21 KI21 AgI21

BE (3d5/2), eV ± 0.1

KE (M4N45N45), eV ± 0.2

α′, eV ± 0.1

485.9 485.9 485.7 486.6 444.6 445.0 444.6 444.5 444.5 619.1 619.7 618.7 619.4

435.75 435.6 435.3 432.6 409.05 407.6 408.0 408.9 408.0 519.5 517.0 517.0 518.3

921.7 921.5 921.1 919.2 853.3 852.6 852.6 853.4 852.5 1138.6 1136.7 1135.7 1137.7

Δα′/2, eV ± 0.1

experimental ΔBE, eV

experimental ΔEini, eV

calculated ΔEini, eV

0.1 0.3 1.3

0.0 ± 0.1 0.2 −0.7

−0.1 ± 0.1 −0.1 −2.0

0.10 ± 0.1

0.4 0.4 0.05 0.4

0.4 ± 0.1 0 0.1 0.1

0 ± 0.1 −0.4 0.05 −0.3

−0.01 ± 0.1

1.0 1.4 0.4

−0.6 ± 0.2 −0.7 ± 0.2 −0.3

−1.6 ± 0.2 −1.7 ± 0.2 −0.7

D


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Inorganic Chemistry

Figure 3. Band structure of Sn24−x−δInxAs22−yI8 for x = 12: (a) calculated band dispersion and total DOS; the Fermi level is set at zero energy. (b) Photon energy variation of the valence band spectrum (experimental data).

As 3d peak is broad owing to the fact that there are two arsenic positions in the crystal structure that are slightly different in their coordination. The As 3d peak position is close to that for covalent GaAs (see Table 1). As for iodine, the spectrum has a distinct shoulder and it was fitted with two slightly split components. The intensity ratio is approximately 1:3, and it remains unchanged with the photon energy variation. Thus, both of these peaks are assumed to be related to the bulk, where the ratio of two crystallographically different iodine atoms is also 1:3. The chemical shifts in the initial state show the effective negative charge, which is even higher than that typical for ionic compounds like LiI and KI; this proves the electrostatic nature of the host−guest chemical bonds. Band Structure. Recently, it was established by four-probe measurements on large crystals that Sn24−x−δIn xAs 22−y I8 clathrates behave as n-type semiconductors.13,14 In this work, we probed the band structure both experimentally and by quantum modeling. The band structure for the clathrates of different compositions was calculated using the Green function method in the simplest G0W0 approximation using (3 × 3 × 3) and (4 × 4 × 4) k-point meshes of the first Brillouin zone. The geometry was taken from the results of the above-described DFT calculations and fixed for the band structure calculations. The density of electron states (DOS) contributed by different atomic orbitals in the valence and the conduction band features were artificially broadened by a Gaussian with a 0.05 eV width. The band dispersion and the total DOS data obtained for the Sn12In12As22I8 crystal are presented in Figure 3a. Experimentally, we measured photoemission spectra in the range of the valence band to characterize the occupied electronic states below EF. We used different excitation energies from 60 to 150 eV as well as 727 eV (which is more bulk

atomic contribution, which is the relaxation energy associated with the flow of electron density from the surrounding atoms toward the core-ionized atoms) are related to the final state effect. Usually, it is assumed that Rea ≫qRa

so that qRa will be neglected in the following consideration. Within the assumption that equal chemical shifts for all core levels involved in the definition of the Wagner Auger parameter are verified, the relaxation energy can be written as Rea = Δα′/2

with Δα′ = α′(sample) − α′(free atom). Thus the total chemical shift becomes ΔBE = ΔEini − Δα′/2

We calculate chemical shifts and a variation of the relaxation energy relative to reference compounds. Here, we choose InSe and SnSe again as references; for iodine we used the literature data21 on LiI. All necessary values are listed in Table 2. After the correction for the relaxation energy, one can obtain the experimental values of the initial state contribution ΔEini and then compare with the calculated ones. One can see from Table 2 that, when the contribution of the final state effect was taken into account, quantitative correspondence of experimental and calculated data is achieved for both indium and tin atoms. Finally, the experimental In 3d chemical shifts in the initial state for the clathrate surface coincides with the values typical for covalent compounds of formal In2+ InSe29 and In3+ in In2Te3 and even less positive than In3+ in InP and In2Se3.21 For the Sn 3d initial state, chemical shifts correspond to Sn2+ in relatively covalent compounds like SnS, SnSe, and SnTe.28 The E


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Inorganic Chemistry

Figure 4. (a−e) Partial density of states for the clathrate compounds Sn24−x−δInxAs22−yI8 of different x and y (y = 0 for a−d). (f) Band structure parameters for clathrate compounds Sn24−x−δInxAs22−yI8 of different x (filled symbols for DFT results and empty symbols for G0W0 results).

Figure 5. Time resolved VB (a), I 4d (b), and I 3d (c) spectra (hν = 727 eV) for Sn24−x−δInxAs22−yI8 (x = 8) during in situ reaction with oxygen (P(O2) = 0.1 mBar). Brown curve corresponds to clean surface; blue curve shows the final point of the oxidation.

According to our calculations, ideally ordered and defect-free Sn12In12As22I8 is a degenerate semiconductor with an indirect band gap of 0.79 eV. The valence band has a maximum at the Γ point, whereas the conduction band minimum is between the Γ and X points. The Fermi level is 0.16 eV upshifted relative to the bottom of the conduction band. The partial density of states (PDOS) is presented in Figure 4. The bands positioned at binding energies up to −5 eV are presumably composed by p orbitals of constituent atoms. From the calculation, we learn that the states in the p bands are strongly mixed partially due to the covalent nature of the bonds in the cage and partially due to their overlapping in the energy scale. Here, the well-known approach to probe the partial DOS experimentally, by photon

sensitive) to check whether the possible surface contribution to the valence band spectrum can be revealed. We believe that, with the photon energy variation, we change the photoionization cross section only. As the lattice constant is quite large, the contribution of the k perpendicular variation cannot be resolved. Indeed, all spectra show the same features, but with different intensity ratios. In addition, the spectrum obtained at high photon energy is in good agreement with the calculated DOS for the bulk crystal and taking into account broadening of the experimental spectra, explained by the experimental resolutions and the lifetime of the excited photoelectrons. This conclusively verifies our model calculations. F


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CONCLUDING REMARKS In summary, the chemical bonding and electronic properties of the clathrates Sn24−x−δInxAs22−yI8 of different compositions (x) were explored by means of photoemission spectroscopy and theoretical modeling. Using the Auger parameter concept, the initial state chemical shifts were determined and compared with those simulated by DFT calculations. From this comparison, one can conclude a covalent chemical bonding in the cages composed of Sn, In, and As and strongly ionic bonds between host and guest atoms, with clear charge transfer from the framework atoms to the guest iodine orbitals. The defect-free Sn24−x−δInxAs22−yI8 clathrate crystals are n-type semiconductors with the estimated band gap of 0.3−0.8 eV depending on the x value. This contradicts obviously with the experimental data on their transport properties. The obstacle is overcome if arsenic vacancies are taken into account; then, the band structure essentially modifies owing to a new energy level appearing in the gap that makes the gap width much smaller (∼0.05 eV) and closer to the experimental data on charge carrier activation energy and photoemission valence band spectra. To estimate partial contributions of the certain orbitals, both the approach based on photon energy variation and the alteration of crystal composition failed. At the same time, the chemical modification of the surface reveals the contribution of I 5p orbitals in DOS in the vicinity of the Fermi level. This is in line with the calculations, according to which the valence band is composed by shallow mixed I 5p, As 4p, In 5p, and Sn 5p orbitals and two lower bands of presumably s character.

energy variation (see, for example, ref 30), fails because the photoionization cross sections of these levels vary symbatically. The maximum at BE ∼ −2 eV (feature A in Figure 3b) is due to the I 5p contribution (the detailed calculated partial DOS near Fermi level can be found in the Supporting Information, Figure S2). This confirms the charge transfer from the framework atoms to the guest iodine orbital. Additional evidence of the assignment is provided by the surface oxidation experiments. During the surface oxidation, the feature A disappears, as shown in Figure 5a, since the surface loses iodine (Figure 5b,c) when framework cages open. Feature B (Figure 3b) is composed almost equally by all p orbitals. Two lower bands placed at −(5−13) eV, i.e., which reveal CDE features and an F feature, are split from the valence band and are practically nondispersive. According to calculations, they are of s character. The clathrate composition variation results in corresponding changes in the density of states, as follows from Figure 4a−d. The shape of the shallow band at BE > −5 eV does not change essentially, whereas the In 5p and Sn 5p contributions change proportionally to the composition. The most prominent variations are related to the band positioned between −4.5 and −9 eV, which is composed presumably by Sn 5s and In 5s orbitals. The increase in the indium concentration leads to the variation of the band gap from 0.31 for x = 5 to 0.79 for x = 12. The Fermi level for these idealized structures is upshifted relative to the conduction band bottom with x decreasing, with the work function being constant. This leads to the corresponding growth of the DOS at the Fermi level and should provide higher electrical conductivity. For the virtual composition x = 14, the Fermi level drops inside the band gap. These data are summarized in Figure 4f and listed in Table S1 of the Supporting Information. In contrast to the model, the composition of clathrates of the Sn−In−As−I series deviates from ideal stoichiometry to satisfy electroneutrality because each vacancy removes four electrons from the formula unit. Sn12In12As22I8, for instance, has two electrons more than required by overall electroneutrality as can be calculated from formal oxidation states Sn0, In−1, and As1+ for four-bonded atoms composing the framework and I−1 for guest iodine. Such deviation can be realized via Sn vacancies giving Sn 11. 5 In 1 2 As 22 I 8 or As vacancies leading to Sn12In12As21.6I8. To calculate the band structure for such compositions is hardly possible due to the huge unit cell needed for the simulation. To grasp trends, we model two systems, Sn12In12As21I8 and Sn11In12As22I8, correspondingly. In these structures, we positioned vacancies in the Sn1 and As2 sites, respectively, using the model built in the space group P1. The geometry was fully optimized. In the case of the Sn vacancy, the band structure shows minor modifications, with the band gap being more than 0.5 eV and the Fermi level staying in the band gap. Contrarily, introducing an As vacancy strongly modifies the electronic structure of the clathrate, as clearly seen in Figure 4e. For this crystal, a new energy level in the vicinity of the valence band in the gap appears, resulting in the band gap of 0.07 eV, with the Fermi level again appearing in the band gap. This is in line with the experimental observation of the charge carrier activation energy of 0.033−0.051 for different compositions of clathrates of the Sn−In−As−I series.13 Some discrepancy appears due to unrealistically high vacancy concentration in our model structure. However, we note that the corresponding shoulder is visible at lower BE of the feature A in the experimental spectra presented in Figure 3.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02223. XP spectra for the crystal with x = 9.5 at different photon energies, calculated partial DOS for clathrate with x = 12, and a summary of the electron structure modeling (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Russian Foundation for Basic Research (grants 13-03-01187 and 13-03-00571) and the Russian-German Laboratory at BESSY II for the financial support. We thank HZB (RGLB) and ALBA (CIRCE beamline) Synchrotron Light Source for the allocation of synchrotron radiation beam times. The calculations were performed using SKIF and “Lomonosov” supercomputers, Supercomputing Center of Lomonosov Moscow State University. The authors thank Anna Sirotina, Oleg Vilkov, Daniil Itkis, Elmar Kataev, and Alina Belova for the participation in experiments, and Evgeny Kelm for helping in preparation of crystals. L.V.Y., A.A.V., and V.S.N. acknowledge a partial support of the Federal Target Grant “Research and development on priority Areas of Science and Technology Complex of Russia for 2014-2020” (government contract G


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(27) Moretti, G. J. Electron Spectrosc. Relat. Phenom. 1998, 95, 95− 144. (28) Yashina, L. V.; Volykhov, A. A.; Vassiliev, S. Y.; Semenenko, D. A.; Itkis, D. M.; Eliseev, A. A.; Kharlamova, M. V.; Verbitsky, N. I.; Zyubina, T. S.; Belogorokhov, A. I. Mater. Elektron. Tekh. 2009, 4, 50− 55. (29) Volykhov, A. A.; Neudachina, V. S.; Kharlamova, M. V.; Itkis, D. M.; Yashina, L. V.; Belogorokhov, A. I. Russ. Microelectron. 2012, 41, 521−526. (30) Makarova, A. A.; Grachova, E. V.; Neudachina, V. S.; Yashina, L. V.; Blüher, A.; Molodtsov, S. L.; Mertig, M.; Ehrlich, H.; Adamchuk, V. K.; Laubschat, C.; Vyalikh, D. V. Sci. Rep. 2015, 5, 8710.

14.576.21.0029, unique identifier for Applied Scientific Research RFMEFI57614X0029).

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