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Article

Novel Quaternary TlGaSn2Se6 Single Crystal as Promising Material for Laser Operated Infrared Nonlinear Optical Modulators

by
Oleh V. Parasyuk
1,
Volodymyr S. Babizhetskyy
2,
Oleg Y. Khyzhun
3,
Volodymyr O. Levytskyy
2,
Iwan V. Kityk
4,*,
Galyna L. Myronchuk
5,
Oksana V. Tsisar
1,
Lyudmyla V. Piskach
1,
Jaroslaw Jedryka
4,
Artur Maciag
4 and
Michal Piasecki
6,*
1
Department of Inorganic and Physical Chemistry, Lesya Ukrainka Eastern European National University, Voli Ave. 13, 43025 Lutsk, Ukraine
2
Department of Inorganic Chemistry, Ivan Franko National University of Lviv, Kyryla and Mefodiya St. 6, 79005 Lviv, Ukraine
3
Frantsevych Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Krzhyzhanivsky St. 3, 03142 Kyiv, Ukraine
4
Institute of Optoelectronics and Measuring Systems, Faculty of Electrical Engineering, Czestochowa University of Technology, Armii Krajowej 17, PL-42-201 Czestochowa, Poland
5
Physics Department, Lesya Ukrainka Eastern European National University, Voli Ave. 13, 43025 Lutsk, Ukraine
6
Institute of Physics, J. Dlugosz University, Armii Krajowej 13/15, PL-42201 Częstochowa, Poland
*
Authors to whom correspondence should be addressed.
Crystals 2017, 7(11), 341; https://doi.org/10.3390/cryst7110341
Submission received: 30 September 2017 / Revised: 2 November 2017 / Accepted: 4 November 2017 / Published: 7 November 2017
(This article belongs to the Section Crystalline Materials)
Browse Figures
Versions Notes

Abstract

:
The studies of the laser operated third order nonlinear optical features of novel TlGaSn2Se6 crystal were done. The main efforts were devoted to a search of a possibility to apply these crystals as laser operated optoelectronic material. For this reason, the third harmonic generation of the Nd:YAG pulse laser 1064 nm as the fundamental beam with varied energy density of up to 200 J/m2 was studied. As a source of laser operated light, we have used the cw laser (532 nm), exciting the material above the energy gap. Additionally, the influence of middle-energy Ar+ ions on the XPS spectra of the TlInSn2Se6 surface has been explored. We have shown that the main contribution of the Se4p states is manifested in the upper part of the valence band of TlInSn2Se6 We have established that for the TlGaSn2Se6 crystal there exists a possibility of variation of the third harmonic generation efficiency using illumination by external continuous wave laser beam. The discovered effect makes it possible to utilize TlGaSn2Se6 crystal in advanced optoelectronic laser operated devices.

Graphical Abstract

1. Introduction

Before the consideration of the TlGaSn2Se6 compound, it is necessary to discuss the similar well studied ternary thallium gallium selenide TlGaSe2, which belongs to a known class of TlCIIIXVI2 ferroelectrics semiconductors with layered structure. The later causes strong anisotropy of their parameters [1,2]. The properties of TlGaSe2 are currently quite well studied (see, e.g., [3,4,5,6,7,8]). The material has high photosensitivity, for instance, being used in low-inertia photoresistors as well as in optical analyzers, detectors for visible and IR spectral regions [1,9]. The detector’s properties of TlGaSe2 were studied in [7,8], and it was shown as a good opportunity to use them for γ- and X-ray radiation (with principal parameters μτe = 6.0 × 10−5 cm2 V−1, μτh = 9.2 × 10−6 cm2 V−1) [8].
It is well known that the improvement of the parameters of such kinds of compounds and consequently their wider application can be attained by the modification with various dopants. This is best achieved by the systematic studies of phase diagrams based on these compounds. When such an approach was applied to the systems AgGaS2–GeS2 and AgGaSe2–GeSe2 [10,11,12,13,14,15] to improve non-linear optical parameters of the ternary compounds AgGaS2 and AgGaSe2, it resulted in the synthesis of quaternary chalcogenides AgGaGeS4 and AgxGaxGe1–xSe2. Like their ternary analog, they also crystallize in the acentric structure, but they have wider transparency windows, higher laser damage threshold (which is particularly important as they are intended for the operating range of the powerful CO- and CO2-pulsed lasers), and a simpler crystal growth technique.
As another example, the study of the CuInSe2–CuGaSe2 system determining the formation of continuous solid solutions series CuIn1–xGaxSe2 (CIGS) is of particular interest. It leads to a significant increase of the photovoltaic parameters of thin film solar cells [16] when compared to those using CuInSe2 (CIS). Interesting results were also obtained by us in the study of several TlCIIIXVI2-based systems. For instance, the TlInSe2–DIVSe2 systems (DIV = Si, Ge, Sn) [17,18] exhibit larger solid solution ranges of TlInSe2, the enhanced transport, and better photoelectric and non-linear optical parameters [19,20,21,22]. The fundamental difference of the TlInSe2–DIVSe2 systems with respect to many other chalcogenide crystals is the presence of heavy Tl ions which determine the high polarization of the compounds. In addition, chalcogenide complexes provide significant phonon anharmonicities [23] that play an important role in photoinduced IR optical second-order susceptibilities [24]. The particular role for nonlinear optics play is also Li-based ternary/quaternary chalcogenides, which possess high laser damage [25,26].
The chalcogenide crystals studied by us possess a laser damage threshold up to 0.85 GW/cm2. At the same time, laser radiation causes irreversible changes that do not exceed 0.3%. Comprehensive studies of photoconductivity, particularly for thallium-bearing compounds, along with calculations of band structure, have shown an essential role of intrinsic cationic defects in the observed effects [19,20,21,22] with energy levels within the forbidden energy gap.
In addition to the formation of solid solutions, the TlCIIIX2VI–DIVX2VI systems feature intermediate compounds of several different compositions. The most numerous group are the 1-1-1-4 compounds with the equimolar ratio of the system components. Currently, six such sulphide compounds are known: TlInSiS4 (SG Pbmn) [25], TlInGeS4 (dimorphous, SG Pa3 and Pnma) [27], TlInSnS4 (SG P63/mmc) [28,29], TlAlSiS4, TlGaSiS4, and TlGaGeS4 (SG Pbmn) [30]. The crystal structure is fully determined only for the first three of these compounds. Selenium-containing compounds of equimolar composition exist for all of the TlGa(In)Se2–DIVSe2 cases [17], but for the TlInSe2–SnSe2 system [18] no intermediate quaternary compounds were found. When testing the formation of the phases of other compositions, a 1-1-2-6 series of compounds was established. The TlInSe2–GeSe2 system features the TlInGe2Se6 compound that crystallizes in its own structure type (SG R3) [31]. Two other compounds were found in the similar sulfur-containing system, TlInGe2S6 (SG R3) [32] and a 1-1-3-8 composition (TlInGe3S8 (SG P21/c) [33]). The substitution of tin by germanium in the selenide system yielded a new compound, TlGaSn2Se6.
In the present work, we present the results of investigations of crystal structure for TlGaSn2Se6 performed by the X-ray powder method, its electronic structure using X-ray spectroscopy methods, as well as its optical and non-linear optical properties. Studies of the third order nonlinear optical properties of the TlGaSn2Se6 crystal have shown that the third harmonic generation is sensitive for external photoinducing radiation giving unique material properties, namely an opportunity to manage the third harmonic generation (THG) magnitude by photoinduced radiation.

2. Materials and Methods

TlGaSn2Se6 single crystal was grown by melting the batches of high-purity elements (Tl, 99.99 wt. %, Ga, 99.9997 wt. %, Sn, 99.99 wt. %, Se, 99.999 wt. %). The total batch mass was 5 g. The alloy was synthesized in vacuum evacuated quartz containers (inner diameter 9 mm, length about 100 mm) in a shaft-type furnace by heating to the maximum temperature 1070 K at the rate of 20 K/h, exposure at 1070 K for 6 h, cooling to 670 K at the rate of 10–20 K/h, annealing for 240 h, followed by quenching in air.
A DRON 4-13 diffractometer (Bourevestnik, St. Petersburg, Russia) was used for X-ray powder diffraction (PXRD) data collection (45 kV and 30 mA operation conditions, Cu Kα-radiation, Bragg-Brentano geometry). Dark-gray pieces with metallic clusters were ground into fine powder of maroon color using an agate mortar. XRD data were analysed with WinCSD program package [34].
Spectral dependence of the absorption coefficient near the energy gap was measured using non-polarized light of MDR-206 monochromator (LOMO Photonics, St. Petersburg, Russia) in the spectral wavelength range 360–1100 nm. The monochromator resolution was 0.3 nm. The absorption coefficient was calculated similarly to the technique described in [20].
As-grown TlInSn2Se6 crystal was controlled by the X-ray photoelectron spectroscopy (XPS) technique to clarify the main peculiarities of its electronic structure and chemical bonding. The general features of the present XPS measurements are similar to that employed earlier when studying the related quaternary selenide, TlInGe2Se6 [29]. Briefly, the present XPS experiments were carried out with UHV-Analysis-System (SPECS Surface Nano Analysis Company, Berlin, Germany). The UHV-Analysis-System is supplied with a hemispherical PHOIBOS 150 analyzer (SPECS Surface Nano Analysis Company, Berlin, Germany). The XPS core-level and valence-band spectra of the TlInGe2Se6 alloy were excited by a Mg Kα source of X-ray radiation (E = 1253.6 eV) and were registered at fixed pass energy of 30 eV in an ion-pumped chamber having a base pressure of less than 6 × 10−10 mbar. The spectrometer energy scale was calibrated, as described elsewhere [35]. In order to take into account the charging effects, we use the reference C 1s line of adventitious carbon, which the binding energy (BE) was set to be 284.6 eV as it is proposed for respective quaternary Tl-, Ga- and/or Sn-bearing chalcogenides [20,36,37,38]. We have also measured the X-ray emission (XE) Se Kβ2 band (transition K → MII,III) giving information regarding the energy distribution of the valence Se p states because it is well known that the electronic structure of the quaternary Tl-, Ga-, and/or Sn-bearing selenides is determined substantively by contributions of the Se 4p states forming their valence band region (see, e.g., Refs. [37,38,39]). The XE Se Kβ2 band was acquired with an energy resolution of about 0.3 eV using a Johann-type DRS-2M spectrograph, following the technique described in detail in Ref. [40]. In addition, the impact of bombardment with middle-energy Ar+ ions on the XPS spectra of the TlInGe2Se6 alloy surface was also within the scope of the present work, because such treatment is regularly used in epitaxial technologies [31].

3. Results and Discussion

3.1. Experimental

3.1.1. Crystal Growth and Structure Determination

Preliminary results on the XRD structure analysis for TlGaSn2Se6 were presented in the abstract of the presentation at XIII International Conference on Crystal Chemistry of Intermetallic Compounds (Lviv, Ukraine, 2017) [41]. Here, we used more precise equipment with better resolution allowing us to gather statistics for more experimental data. The crystal structure of TlGaSn2Se6 was refined using initial atomic coordinates of structural type TlInGe2Se6 (space group R3) [42]. Experimental, calculated, and difference powder XRD profiles of the sample with nominal composition corresponding to TlGaSn2Se6 stoichiometry are shown in Figure 1. Details of the refinement are listed in Table 1. Atomic coordinates and isotropic thermal parameters of the TlGaSn2Se6 structure are presented in Table 2. Two types of metal atoms arrangement occur in the structure (see Figure 2). The selected inter-atomic distances indicate bonding between the respective atoms and they are shown in Table 3. The charge-balanced electronic formula of TlGaSn2Se6 reveals the following form: Tl+Ga3+Sn4+2Se2−6. The ionic radii were used when drawing Figure 2 by the DIAMOND program [43].

3.1.2. XPS and XES Data

Survey XPS spectra recorded for as-derived and Ar+ ion-irradiated surfaces of the TlGaSn2Se6 alloy are presented in Figure 3. From this figure, it evident that all of the spectral peculiarities of the survey XPS data, except for the carbon and oxygen 1s core levels and Auger KLL lines, are well assigned to the core-levels or Auger lines of atoms, which compose the TlGaSn2Se6 alloy. It is worth mentioning that the relative intensities of the XPS C 1s and O 1s core-level spectra for the pristine surface are relatively small, as Figure 3 depicts. Their origin is a result of adsorption from the air of hydrocarbons and oxygen-containing species because prior to our XPS experiments, the surface of the synthesized TlGaSn2Se6 alloy was exposed to air over several weeks. It should be mentioned that in the case of the TlGaSn2Se6 crystal, the Auger Ga L3M23M45 line is superimposed on the C 1s core-level spectrum, as can be seen from Figure 3. Therefore, in order to overcome the effects of the sample charging during X-ray radiation, we also monitored the binding energy values of measuring core-level spectra employing an electron flood gun, as is recommended in such an occurrence [42,43,44]. The binding energy of the XPS O 1s core-level spectrum is equal to 531.9 ± 0.1 eV for the as-derived TlGaSn2Se6 alloy surface, and this binding energy value corresponds to adsorbed oxygen-containing species [43]. However, as can be seen from Figure 3, the Ar+ ion-irradiation of the TlGaSn2Se6 alloy surface causes substantial decreasing of the relative intensities of the XPS C 1s and O 1s core-level spectra.
Figure 4 compiles the most essential XPS core-level spectra related to atoms forming the quaternary selenide under consideration, while binding energy values of the core-level electrons recorded for both surfaces, as-derived and Ar+ ion-irradiated, are collected in Table 4. The present XPS results confirm that irradiation of the TlGaSn2Se6 alloy with 3.0 keV Ar+ ions during 5 min (at ion current density installed to be equal to 14 µA/cm2) does not significantly change the values of the binding energies of the core-level electrons related to thallium, gallium and selenium atoms. However, the binding energies of the XPS Sn 3d and Sn 4d core-level electrons decrease by about 0.25 eV in such cases (Table 4). A similar effect was observed previously when studying the influence of the middle-energy Ar+ ion-bombardment on other Sn-bearing quaternary selenides.
Tl1–xIn1–xSnxSe2 (x = 0.1, 0.2, and 0.3), namely Ag2In2SiSe6 and Ag2In2GeSe6 [20]. In our opinion, the above-mentioned effect of decreasing the binding energies of the Sn 3d and Sn 4d core-level electrons can be attributed to the removing of a very thin tin oxide species that are formed on the pristine TlGaSn2Se6 alloy surface due to its exposure to air. Taking into consideration the relative intensities of the XPS Tl 4f7/2, Ga 3p3/2, Sn 3d5/2, and Se 3d core-level lines and the related atomic sensitivity factors (ASF) reported in Ref. [45], we can conclude that in the case of ignoring the presence of carbon and oxygen adsorbed species the Ar+ ion-bombardment does not cause visible changes of the stoichiometry of the TlGaSn2Se6 topmost surface layers. Literature data indicate that the binding energy values for the Sn 3d5/2 core-level electrons in highest tin oxide, SnO2, correspond to 286.8–287.0 eV [46,47], which are rather close to those in the TlGaSn2Se6 alloy under study (see Table 4). As can be seen from the survey spectra presented in Figure 4, the Ar+ ion-irradiation induces almost complete elimination of the O 1s core-level line from the TlGaSn2Se6 alloy surface. The effect of such elimination of a very thin SnO2 species is detected in the present XPS experiments, as the above-mentioned decreasing the binding energies of the XPS Sn 3d and Sn 4d core-level electrons in the case of the Ar+ ion-bombardment of the TlGaSn2Se6 alloy surface. Our XPS measurements unambiguously show that the Ar+ ion-irradiation does not bring visible changes of the shapes of the XPS core-level spectra (Figure 4), as well as energy distribution of the electronic states within the valence-band region of the TlGaSn2Se6 alloy (Figure 5). This fact allows us to conclude that the TlGaSn2Se6 crystalline alloy surface is rather rigid with respect to the influence on it the X-ray irradiation.
For the TlGaSn2Se6 alloy, we have also measured the XE Se Kβ2 band, bringing information regarding the energy distribution of the Se 4p states and matching it on a common energy scale with the XPS valence band spectrum. The results of such matching the above X-ray photoelectron and emission spectra measured for the TlGaSn2Se6 alloy following technique [48] are depicted in Figure 6. Following this figure, the spectral maximum of the XES Se Kβ2 band is positioned mainly at the upper portion of the XPS valence-band spectrum. Therefore, the principal contributions of the Se 4p states are detected at the upper part of the valence band for the TlGaSn2Se6 crystal, with the lesser contributions in other parts of the band. Similar peculiarity of the valence band by the Se 4p states is typical for the related quaternary selenide, TlInGe2Se6, as it established in Ref. [31]. It is worth mentioning that one can also expect substantial contributions of the Tl s, Sn d, and Ga p electronic states to the valence band of the TlGaSn2Se6 compound, however, the present available facilities do not allow for recording the energy distribution of these states by our group.
It is well-known that when recording the XPS spectra for chemical elements that constitute semiconductors, they are very sensitive to methods of calibrations of the surface charging effects [45,49]. In such a case, the binding energy difference parameter, e.g., ΔTl–Se, which is a difference of the binding energies of the Tl 4f7/2 and Se 3d core levels, is not-sensitive to the charging effects [49]. Such a binding energy difference parameter can be effectively used for the determination of iconicity degree of the Tl–Se chemical bonds: a higher value of the Δ difference parameter induces an enhanced degree of the ionic component for this chemical bond. From the data listed in Table 4, the difference parameter ΔTl–Se in the TlGaSn2Se6 alloy is equal to 64.19 ± 0.5 eV. This value is very close to that of the difference parameter ΔTl–Se for the related selenide TlInGe2Se6, namely 64.1 eV [31]). This means that the iconicity degree of the chemical Tl–Se bonds in the TlGaSn2Se6 compound corresponds to that in TlInGe2Se6.

3.1.3. Optical Properties

The spectral distribution of the absorption coefficient is presented in Figure 7а. To obtain the detailed information about the energy band gaps in the crystal, the absorption band edge α vs. photon energy is analyzed in the high energy absorption regions, where Eg can be presented by an equation [50]:
α h ν = A ( h ν − E g ) n
where A is a constant that is determined by the inter-band transition probabilities, n is a power index characterizing the optical absorption process that (for perfect crystals) is equal to 2 and 1/2 for indirect and direct allowed transitions, respectively. However, this rule is substantially disturbed for the chalcogenide crystals due to presence of a large number of intrinsic cationic defects and exact application of this equation is limited. However, following the presented in Figure 7 dependence, one can see that it is closer to the direct transition [51] (see Figure 7b) with Edg = 1.86 eV.
The complex shape of the absorption edge is clearly seen (Figure 7b). A significant difference in the spectral distribution of the absorption coefficient for TlGaSe2 [52] and TlGaSn2Se6 crystals is observed at energies below the beginning of inter-band transitions, in the spectral range of about ~790 nm (1.57 eV). However, due to a great number of intrinsic defects with energy levels below the conduction band gap, a clear separation of the direct and indirect transitions is not possible. Moreover, the huge anharmonic phonon contribution that is typical for chalcogenide crystals may also be superimposed. Similar absorption spectra were obtained in [53] for TlGaSe2 with Fe admixtures. The authors suggest that these absorption bands are associated with the substitution of the “host” metal atoms by Fe atoms. It was suggested in Ref. [54] that the substitution is possible because the ionic radii are nearly equal, 0.63 Å for Fe3+ and 0.61 Å for Ga3+. Therefore, it may be assumed that the ~790 nm peak (~1.57 eV) in the TlGaSn2Se6 crystals is related to the statistical substitution of Ga3+ (0.61 Å) ions with Sn4+ ions (0.69 Å), which agrees with the data reported in [19,28].

3.1.4. Nonlinear Optical Properties

We have performed studies of the third order nonlinear optical properties of the TlGaSn2Se6 crystal, focusing on the third harmonic generation (THG) of a fundamental 1064 nm Nd:YAG pulse laser (pulse duration about 7 nm, frequency repetition about 10 Hz) operating in reflected geometry (see Figure 8), with energy ranging up to 200 J/m2. For investigations of the influence of photoinduced radiation on the THG intensity, we have used the continuous wave (cw) laser (532 nm). The Nd:YAG laser (1064 nm ) interacting with crystals surface generated its THG signal, which was spectrally separated by a UV optical filter at 355 nm form fundamental ones before the Hamamatsu photomultiplier.
Beginning from 170 J/m2 (Figure 9), a sharp increase of the THG intensity appears. As a reference compound, we have chosen a Cu2ZnSiSe4 crystal that was calibrated earlier [55,56,57]. Figure 9 indicates that such technique allows evaluating the efficiency of the THG intensity determined from angle dependent maxima with respect to the reference sample.
The nonlinear THG optical effects may exist both in centrosymmetric, as well as acentric media. It is principally different to the second harmonic generation (SHG), which requires the existence of non-centrosymmetry. However, using external photopolarization, it is possible to additionally operate by the THG efficiency. We explore changes in the THG intensity under influence of the external laser operating with polarized light (green 532 cw lasers). We chose wavelengths of the photoinduced light that are higher than the energy gap value. Following the absorption coefficients measured at 355 nm for the titled crystal corresponding to the THG signal, we have evaluated that the light penetration depth is about 100 nm. The observed laser stimulated changes in the THG are determined alterations of dipole moments, and particularly by vectoral difference between the excited and ground state moments (Equation (2)). To explain such effects, we present the oversimplified expression describing the microscopic hyperpolarizabilities that are responsible for the THG effects as follows:
γ i j k l ≅ μ → i μ → j Δ μ → k Δ μ → l E g 4
where Δµi are transition dipole moments vectorial differences between the excited and ground state dipole moments for appropriate states.
It is well known that the THG intensity is determined by third rank hyperpolarizabilities. Since we observe significant light induced alterations of the THG, it indicates the existence of effective changes in appropriate materials constants (caused by light induced photopolarization).
Additional source of the effect is caused by space redistribution of the free carriers, which give additional contribution to the changes in the THG. In Figure 10 are presented principal results devoted to the laser induced changes of the THG. The red colour corresponds to the values of the THG immediately after irradiation (up to 1 s). The blue colour corresponds to the effect after stabilisation, which rated up to 200–400 s. The stabilisation of the laser operated changes is caused by the redistribution of the photoinduced layers due to the photopolarization, photothermal effects, and free carriers. The Cu2ZnSiSe4 crystals are used only as a reference. It is clear that, at the beginning, the changes for the reference crystals are absent and only after few minutes corresponding changes were observed. It is crucial that such behaviour is contrasted with the reference Cu2ZnSiSe4 crystals. This fact may confirm the principal difference of the laser induced THG for different chalcogenides.
Contrary to other quaternary chalcogenides, the TlInGe2S6 crystal photoinduced THG intensity signal decreases during photo-inducing treatment (see Figure 10). This behavior provides a unique opportunity to change the intensity of the THG by additional laser irradiation. This TlInGe2Se6 crystal property gives the possibility to use the titled compound in the construction of advanced optoelectronic devices.
Due to the extremely high degree of the phonon anharmonic components for such a kind of materials [58], which is described by the third order polar tensors, one can expect a possibility to occur of the additional contributions to the hyperpolarizabilities.
The principal manifestation of this effect is presented in the Figure 10. Following these results, one can see that for the Cu2ZnSiSe4 crystals the effect is absent immediately after illumination and is observed only after relaxation over 20–30 s. However, for the studied crystals, this effect is appeared immediately after the illumination. This peculiarity may be explained by different photothermal formed space gradients of free carriers and temperature, which also contribute in such a case. So, for the studied crystals, a rare opportunity occurs to modulate the intensities of the THG signal that allow us to propose the title compound to be considered a new type of nonlinear optical laser modulators [59]. It would be interesting in the future to perform the same studies for the Li-containing chalcogenide crystals revealing high laser stability and nonlinear optical (NLO) efficiencies [60].

4. Conclusions

Crystal structure of new quaternary selenide TlInSn2Se6 has been determined using powder XRD method. Its crystal structure is isomorphous with a TlInGe2Se6 structure type (non-centrosymmetric space group R3). For the TlInSn2Se6 alloy, we have recorded the X-ray photoelectron core-level and valence-band spectra. The present XPS data indicate that all of the spectral peculiarities of the survey spectra, except for the carbon and oxygen 1s core levels and Auger KLL lines, are well assigned to the core-levels or Auger lines of atoms, which compose the TlGaSn2Se6 alloy. The relative intensities of the XPS C 1s and O 1s core-level spectra of the pristine TlGaSn2Se6 alloy surface are relatively small, and their origin is a result of adsorption from the air of hydrocarbons and oxygen-containing species. The influence of middle-energy Ar+ ions on the XPS spectra of the TlInSn2Se6 alloy surface has been investigated. The present XPS data indicate that the alloy surface is rather rigid with respect to the influence on it the X-ray irradiation. An evaluation of the ionicity degree of the chemical Tl–Se bonds in the TlGaSn2Se6 compound indicates that it is very close to that in the related quaternary selenide TlInGe2Se6. Furthermore, a comparison on a common energy scale of the X-ray emission Se Kβ2 band, retrieving information on the energy distribution of the Se 4p states, with the X-ray photoelectron valence-band spectrum reveals that the main contributions of the Se 4p states occurs in the upper portion of the valence band of TlInSn2Se6, with their substantive contributions in other portions of the band as well. Optical band gap energy for directly allowed transitions was determined from the spectral distribution of the absorption coefficient (Edg = 1.86 eV). The position of the deep admixture level is estimated as ~1.57 eV. New quaternary selenide TlInSn2Se6 proved to be a very good nonlinear material that is sensitive to photoinduction operations (see Figure 10), making it to be interesting from the point of view of potential applications.

Author Contributions

Oleh V. Parasyuk, Oksana V. Tsisar, Lyudmyla V. Piskach—crystal preparation, Volodymyr S. Babizhetskyy, Volodymyr O. Levytskyy—XRD measurements and structure determination, Oleg Y. Khyzhun—X-ray photoelectron core-level and valence-band spectra studies, Jaroslaw Jedryka, Artur Maciag—THG measurements, Galyna L. Myronchuk—optical band determination, Iwan V. Kityk, Michal Piasecki—idea and article writing.

Conflicts of Interest

The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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Crystals 07 00341 g001 550
Figure 1. Observed (dots) and calculated (solid line) and their difference plot (bottom) of the XRD patterns of TlGaSn2Se6. Peak positions are marked by short vertical bars.
Figure 1. Observed (dots) and calculated (solid line) and their difference plot (bottom) of the XRD patterns of TlGaSn2Se6. Peak positions are marked by short vertical bars.
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Crystals 07 00341 g002 550
Figure 2. General crystallochemistry of TlGaSn2Se6. The unit cell and packing of metal atom coordination polyhedra are emphasized.
Figure 2. General crystallochemistry of TlGaSn2Se6. The unit cell and packing of metal atom coordination polyhedra are emphasized.
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Crystals 07 00341 g003 550
Figure 3. Survey X-ray photoelectron spectroscopy (XPS) spectra recorded for (1) pristine and (2) Ar+ ion-irradiated surfaces of the TlGaSn2Se6 alloy.
Figure 3. Survey X-ray photoelectron spectroscopy (XPS) spectra recorded for (1) pristine and (2) Ar+ ion-irradiated surfaces of the TlGaSn2Se6 alloy.
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Crystals 07 00341 g004a 550Crystals 07 00341 g004b 550
Figure 4. XPS core-level spectra recorded for: (1) pristine and (2) Ar+ ion-irradiated surfaces of the TlGaSn2Se6 alloy: (a) Ga 2p; (b) Sn 3d; (c) Tl 4d; (d) Se 3p and Tl 4f (with some nearby core-level spectra and Auger lines); and, (e) Se 3d.
Figure 4. XPS core-level spectra recorded for: (1) pristine and (2) Ar+ ion-irradiated surfaces of the TlGaSn2Se6 alloy: (a) Ga 2p; (b) Sn 3d; (c) Tl 4d; (d) Se 3p and Tl 4f (with some nearby core-level spectra and Auger lines); and, (e) Se 3d.
Crystals 07 00341 g004aCrystals 07 00341 g004b
Crystals 07 00341 g005 550
Figure 5. XPS valence band spectra recorded for (1) pristine and (2) Ar+ ion-irradiated surfaces of the TlGaSn2Se6 alloy.
Figure 5. XPS valence band spectra recorded for (1) pristine and (2) Ar+ ion-irradiated surfaces of the TlGaSn2Se6 alloy.
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Figure 6. Comparison (on a common energy scale) of the XPS valence-band spectrum and the X-ray emission Se Kβ2 band of the TlGaSn2Se6 alloy.
Figure 6. Comparison (on a common energy scale) of the XPS valence-band spectrum and the X-ray emission Se Kβ2 band of the TlGaSn2Se6 alloy.
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Figure 7. (a) Typical absorption spectral edge obtained at 300 K; (b) assumed direct renormalized absorption spectrum versus photon energy for TlGaSn2Se6.
Figure 7. (a) Typical absorption spectral edge obtained at 300 K; (b) assumed direct renormalized absorption spectrum versus photon energy for TlGaSn2Se6.
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Figure 8. Principal set-up for measurements of the reflected photoinduced third harmonic generation (THG).
Figure 8. Principal set-up for measurements of the reflected photoinduced third harmonic generation (THG).
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Figure 9. Third harmonic generation intensity for TlGaSn2Se6 (Note: Cu2ZnSiSe6 is used for reference).
Figure 9. Third harmonic generation intensity for TlGaSn2Se6 (Note: Cu2ZnSiSe6 is used for reference).
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Figure 10. Laser stimulated changes and relaxation of the THG versus the photoinduced treatment by 532 nm cw laser for the studied crystals.
Figure 10. Laser stimulated changes and relaxation of the THG versus the photoinduced treatment by 532 nm cw laser for the studied crystals.
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Table
Table 1. Details of the refinement of the crystal structure of TlGaSn2Se6.
Table 1. Details of the refinement of the crystal structure of TlGaSn2Se6.
Chemical formulaTlGaSn2Se6
Space groupR3 (No. 146)
Structure typeTlInGe2Se6
Pearson symbol and ZhR30, 3
Unit cell parameters
a, Å10.3289(2)
c, Å9.4340(4)
V, Å3871.64(6)
Calculated density, g cm−35.6301(4)
DiffractometerDRON 4-13
Radiation, λCu Kα, 1.54185 Å
Mode of refinementFull with fixed elements per cycle
2θ limits, step; (sinθ/λ)max8.80–98.82, 0.02; 0.493
DetectorNaI(Tl) scintillation counter
Scanning time/step, 2θ and sec0.02, 20
Number of reflections199
Number of parameters (all/free)29/5
Scale factor0.31792(1)
Goodness-of-fit1.99
Rb(I), RP, RPw4.8%, 4.1%, 5.4%
Table
Table 2. Refined atomic coordinates and isotropic displacement parameters of TlGaSn2Se6 structure.
Table 2. Refined atomic coordinates and isotropic displacement parameters of TlGaSn2Se6 structure.
AtomSiteXyzBiso, Å2
Tl3 a000.0000 (5)1.25 (2)
M9 b0.1944 (4)0.2355 (5)0.3882 (4)0.87 (2)
Se19 b−0.0456 (5)0.2085 (5)0.2874 (4)1.11 (2)
Se29 b0.3919 (6)0.4804 (5)0.3159 (5)0.90 (2)
M = 1/3Ga + 2/3Sn.
Table
Table 3. Selected inter-atomic distances (in Å) in the TlGaSn2Se6 structure.
Table 3. Selected inter-atomic distances (in Å) in the TlGaSn2Se6 structure.
AtomsDistanceAtomsDistance
Tl: – 3 Se13.636 (5)M: – Se22.422 (7)
3 Se13.810 (5)Se22.468 (6)
3 Se23.835 (6)Se12.518 (6)
3 Se24.022 (6)Se22.537 (6)
Se1: – M2.518 (6)Se2: – M2.422 (7)
M2.537 (6)M2.468 (6)
Tl3.636 (5)Tl3.835 (6)
Tl3.810 (5)Tl4.022 (6)
M = 1/3Ga + 2/3Sn.
Table
Table 4. Binding energies (in eV *) of constituent element core levels of pristine and bombarded with Ar+ ions of the TlGaSn2Se6 crystalline surfaces.
Table 4. Binding energies (in eV *) of constituent element core levels of pristine and bombarded with Ar+ ions of the TlGaSn2Se6 crystalline surfaces.
Core-LevelTlGaSn2Se6/Pristine SurfaceTlGaSn2Se6/Ar+ Ion-Bombarded Surface
Tl 5d5/2113.1713.26
Ga 3d19.6119.66
Sn 4d25.8125.57
Se 3d54.2554.22
Tl 4f7/2118.44118.35
Tl 4f5/2122.84122.77
Se 3p3/2160.47160.51
Se 3p1/2165.61165.77
Tl 4d5/2 **385.6385.5
Tl 4d3/2 **406.3406.2
Sn 3d5/2486.35486.10
Sn 3d3/2494.76494.52
Ga 2p3/2 **1117.81117.7
Ga 2p1/2 **1144.71144.6
* Uncertainty of the measurements is ±0.05 eV. ** Uncertainty of the measurements is ±0.1 eV.

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Parasyuk, O.V.; Babizhetskyy, V.S.; Khyzhun, O.Y.; Levytskyy, V.O.; Kityk, I.V.; Myronchuk, G.L.; Tsisar, O.V.; Piskach, L.V.; Jedryka, J.; Maciag, A.; et al. Novel Quaternary TlGaSn2Se6 Single Crystal as Promising Material for Laser Operated Infrared Nonlinear Optical Modulators. Crystals 2017, 7, 341. https://doi.org/10.3390/cryst7110341

AMA Style

Parasyuk OV, Babizhetskyy VS, Khyzhun OY, Levytskyy VO, Kityk IV, Myronchuk GL, Tsisar OV, Piskach LV, Jedryka J, Maciag A, et al. Novel Quaternary TlGaSn2Se6 Single Crystal as Promising Material for Laser Operated Infrared Nonlinear Optical Modulators. Crystals. 2017; 7(11):341. https://doi.org/10.3390/cryst7110341

Chicago/Turabian Style

Parasyuk, Oleh V., Volodymyr S. Babizhetskyy, Oleg Y. Khyzhun, Volodymyr O. Levytskyy, Iwan V. Kityk, Galyna L. Myronchuk, Oksana V. Tsisar, Lyudmyla V. Piskach, Jaroslaw Jedryka, Artur Maciag, and et al. 2017. "Novel Quaternary TlGaSn2Se6 Single Crystal as Promising Material for Laser Operated Infrared Nonlinear Optical Modulators" Crystals 7, no. 11: 341. https://doi.org/10.3390/cryst7110341

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