HUNGARIAN JOURNAL OF INDUSTRY AND CHEMISTRY Vol. 45(2) pp. 13–18 (2017) hjic.mk.uni-pannon.hu DOI: 10.1515/hjic-2017-0014 INVESTIGATIONS OF THE TlInP2Se6–In4(P2Se6)3 SYSTEM AND ITS OPTICAL PROPERTIES VALERIA TOVT, 1 IGOR BARCHIY, 1 * MICHAL PIASECKI, 2 IWAN KITYK, 3 AND ANATOLII FEDORCHUK 4 1 Department of Chemistry, Uzhgorod National University, Pidgirna St. 46, 88000 Uzhgorod, UKRAINE 2 Institute of Physics, Jan Dlugosz University, Armii Krajowej 13/15, 42-200 Częstochowa, POLAND 3 Faculty of Electrical Engineering, Częstochowa University of Technology, Dabrowskiego 69, 42201 Częstochowa, POLAND 4 Department of Inorganic and Organic Chemistry, Lviv National University of Veterinary Medicine and Biotechnologies, Pekarska St. 50, 79010 Lviv, UKRAINE The equilibrium phases were investigated and the corresponding phase diagram constructed for the TlInP2Se6– In4(P2Se6)3 system from physical and chemical analyses, namely differential thermal analysis (DTA), X-ray diffraction (XRD), and microstructural analysis (MSA). It was established that this system belongs to the eutectic type and is characterized by the formation of boundary solid phases containing complex compounds. Single crystals of the compounds TlInP2Se6 and In4(P2Se6)3 were grown using the Bridgman method. Both crystals were found to exhibit diffuse reflection spectra and photoinduced dependence of birefringence at various IR wavelengths generated by CO2 laser irradiation. Birefringence properties were investigated using the Senarmont method. Keywords: phase diagram, solid solution, crystal structure, optical properties, direct-gap semicon- ductor, indirect-gap semiconductor, photoinduced birefringence 1. Introduction Compounds with the formula M2P2Se6 possess promis- ing magneto-electric, piezoelectric, electro-optical, and thermoelectric properties that indicate their suitability as functional materials in optoelectronics [1-2]. Due to their crystal structure, they exhibit anisotropy in terms of their physical properties. In a multilevel structure of M2P2Se6 compounds, metal cations and pairs of phos- phorous atoms occupy the octahedral positions between planes of selenium atoms. This structure is characterized by its layered arrangement of atoms, which contributes to the formation of a dipole moment between the layers of cationic and anionic groups. The replacement of the metal cation М 2+ by other metal cations (М + , М 3+ or М 4+ ) leads to the deformation of the structure [3-4], changes the magnitude of the dipole moment and, con- sequently, its physical properties. The Tl2Se–In2Se3–“P2Se4” ternary system is com- posed of binary Tl2Se–In2Se3, Tl2Se–“P2Se4” and In2Se3–“P2Se4” systems. The Tl2Se–In2Se3 system is characterized by the formation of two intermediate ter- nary compounds: TlInSe2 melts congruently at 1023 K and TlIn5Se8 is formed according to the peritectic reac- tion L + In2Se3  TlIn5Se8 at 1029 K [5-6]. In the sys- *Correspondence: i_barchiy@ukr.net tem Tl2Se–“P2Se4” with a ratio of 2 to 1, interoperable components form the compound Tl4P2Se6 which pos- sesses a congruent nature of melting at 758 K [7]. The In2Se3–“P2Se4” system is characterized by the formation of the compound In4(P2Se6)3 in a syntectic reaction of L1 + L2  In4(P2Se6)3 at 880 K [8]. In the Tl2Se– In2Se3–“P2Se4” system at the intersection of incisions, the phases Tl4P2Se6–In4(P2Se6)3 and TlInSe2–“P2Se4” form the complex compound TlInP2Se6 [9]. 2. Experimental Ternary Tl4P2Se6 and In4(P2Se6)3 compounds were pre- pared by melting stoichiometric quantities of binary Tl2Se with elementary indium, phosphorous and seleni- um under a vacuum of 0.13 Pa in quartz ampoules using a single temperature method. In all syntheses, compo- nents were used that possess a purity greater than 99.999 %. The maximum temperatures of synthesis were 993 and 893 K for In4(P2Se6)3 and TlInP2Se6, re- spectively. The rate of heating up to the maximum tem- perature was 50 K h -1 . The melts were maintained at the maximum temperature for 72 hours. Cooling was per- formed at a rate of 50 K h -1 down to an annealing tem- perature of 573 K. The linearity of the heating and cool- ing processes was achieved by a RIF-101 temperature controller. The homogenization process occurred over 120 hours. Identification of the complex compounds and alloys was conducted by differential thermal analy- TOVT, BARCHIY, PIASECKI, KITYK, AND FEDORCHUK Hungarian Journal of Industry and Chemistry 14 sis (DTA) (PRA-01, chrome-alumina thermocouple 5 K), X-ray diffraction (XRD) (DRON-3 diffractometer, CuKα radiation, Ni filter) and microstructural analysis (MSA) (metallurgical microscope Lomo Metam R-1). Crystal structural calculations were conducted using the software package WinCSD [10]. Optical properties were investigated using an SF-18 spectrophotometer within the wavelength range of 400 – 750 nm. A CO2 laser was used for photoinduced electrons in samples employing 200 ns pulses with a pulse repetition fre- quency of about 10 Hz, a fundamental frequency of 10.6 μm and a frequency doubling of 5.3 μm beams. The birefringence was measured using a Er:glass cw laser at 1540 nm by application of the Senarmont method. 3. Results and Analysis 3.1. Phase diagram of the TlInP2Se6– In4(P2Se6)3 system The TlInP2Se6–In4(P2Se6)3 system is a quasi-binary sec- tion of the Tl2Se–In2Se3–“P2Se4” ternary system (Figs.1 and 2). It belongs to the eutectic type (V-type diagram by Rozeboom). The complex compounds TlInP2Se6 and In4(P2Se6)3 melt congruently at 875 K and 963 K, re- spectively. TlInP2Se6 is characterized by two polymor- phic transformations ltTlInP2Se6  mtTlInP2Se6 at 680 K and mtTlInP2Se6  htTlInP2Se6 at 711 K. The prefixes lt–, mt– and ht– represent low–, medium–, and high–temperature modifications, respectively. In4(P2Se6)3 is also characterized by two polymorphic transformations ltIn4(P2Se6)3  mtIn4(P2Se6)3 at 665 K and mtIn4(P2Se6)3  htIn4(P2Se6)3 at 903 K. When the temperature rises above 791 K, an invariant eutectic process is observed L  htTlInP2Se6 + mtIn4(P2Se6)3 (in the presence of 15 mol% In4(P2Se6)3). The system is described by the sequence of the ef- ficient peritectic processes htTlInP2Se6 + mtIn4(P2Se6)3  mtTlInP2Se6 (714 K) and mtTlInP2Se6 + mtIn4(P2Se6)3  ltTlInP2Se6 (689 K) based on the pol- ymorphic transformation of TlInP2Se6. The polymor- phism of In4(P2Se6)3 produces metatectic htIn4(P2Se6)3  L + mtIn4(P2Se6)3 (884 K) and eutectic mtIn4(P2Se6)3  ltTlInP2Se6 + ltIn4(P2Se6)3 (652 K) processes. Re- gions of homogeneity in solid solutions, based on the batched complex selenides during annealing at a tem- perature of 573 K, do not exceed 10 mol%. 3.2. Crystal structure of the compounds In4(P2Se6)3 and TlInP2Se6 The crystal structures of the compounds TlInP2Se6 and In4(P2Se6)3 were solved using the Rietveld method. As an initial model for TlInP2Se6 [2], the parameters of In4(P2Se6)3 were used [8]. Analysis of the crystalline structures of the investigated compounds (Table 1) showed that it is possible to define the structural group of the anionic group [P2Se6] 4– , which is formed by two single tetrahedra (Fig.3). Cationic atoms occupy posi- tions between the anionic groups and none are located between the layers. Figure 1. Results of the XRD analysis of the TlInP2Se6–In4(P2Se6)3 system. (I rel – Intensity, 2 theta - Angle of reflection) Figure 2. Phase diagram of the TlInP2Se6–In4(P2Se6)3 system. (1–L, 2–L+htIn4(P2Se6)3, 3–htIn4(P2Se6)3, 4–htTlInP2Se6, 5–L+mtIn4(P2Se6)3, 6–htIn4(P2Se6)3+mtIn4(P2Se6)3, 7–htTlInP2Se6, 8–htTlInP2Se6+mtIn4(P2Se6)3, 9–mtIn4(P2Se6)3, 10–htTlInP2Se6+mtTlInP2Se6, 11–mtTlInP2Se6, 12–mtTlInP2Se6+mtIn4(P2Se6)3, 13–mtTlInP2Se6+ltTlInP2Se6, 14–ltTlInP2Se6+mtIn4(P2Se6)3, 15–mtIn4(P2Se6)3+ltIn4(P2Se6)3, 16–ltTlInP2Se6, 17–ltTlInP2Se6+ltIn4(P2Se6)3, 18–ltIn4(P2Se6)3). Table 1. Crystal data of TlInP2Se6 and In4(P2Se6)3 compounds. Compound Crystal system Space group Lattice constant In4(P2Se6)3 [8] trigonal R3 h (146) a = 6.362(3), c = 19.929(6) Å In4(P2Se6)3 trigonal R3 h (146) a = 6.3808(8), c = 20.014(4) Å TlInP2Se6 [2] triclinic P-1 (2) a = 6.4310, b = 7.5002, c = 12.124 Å, TlInP2Se6 triclinic P-1 (2) α = 100.553, β = 93.735, γ = 113.451 INVESTIGATIONS OF THE TLINP2SE6–IN4(P2SE6)3 SYSTEM 45(2) pp. 13–18 (2017) 15 The structure of In4(P2Se6)3 can be derived from the structure of Sn2P2Se6 [11]. It is composed of multi- ple substitutions of the isovalent cations according to 2M 2+  M 4+ . The crystal structure of the compound In4(P2Se6)3 can be presented based on the composition of the anionic group [P2Se6] 4– (Fig.4), in which the indi- um atoms occupy the space between the anionic groups. The second coordination environment (SCE) [12] is of cuboctahedron form. Indium cations are surrounded by a triangular environment of anionic atoms of the group [P2Se6] 4– and within the frames of its environment bonding exists with six atoms of selenium while the coordination form is octahedral (Fig.5). The structural and chemical properties of the Ме І Ме ІІІ Р2Se6 compositions are related to the important role concerning the dimension of the cation on its loca- tion between the layers of the anionic [P2Se6] 4– groups. Crystallographic analysis showed that smaller cations occupy a position in the plane perpendicular to the main axis. Atoms located in a second coordination environ- ment of anionic groups in the structure of TlInP2Se6 compounds can be presented as a strongly distorted hexagonal-equivalent cuboctahedron (Fig.6). The atoms of metallic cations, located in the cavi- ties between the atoms of the anionic groups, are within an asymmetric environment (Fig.7). In 3+ cations move toward tetrahedral cavities on the boundary between tetrahedral and octahedral cavities, and Tl + cations move in the direction of the octahedral cavities. Moreover the In 3+ cations are located in the same plane together with the centres of the anionic [P2Se6] 4– groups (Fig.8) and some Tl + cations are shifted relative to the plane. Therefore, this arrangement is a source of the interesting electro-physical and optical properties of materials based on compounds of this type. 3.3. Optical response of single crystals of TlInP2Se6 and In4(P2Se6)3 The most important parameter of the energy spectra of semiconductors is the width of the band gap, Eg, which is defined by the difference in energy between the bot- tom of the conduction band, EC, and the top of the va- lence band, EV. All semiconductors can be divided into two groups. In the first group, the minimum of the con- duction band and the maximum of the valence band occupy the same point in the Brillouin zone, i.e. at an identical location in the space of quasi-moments. In this case, the optical transitions of electrons from the va- lence band to the conduction band (with the absorption of a quantum of light) and from the conduction band to the valence band (with the emission of a quantum of light) occur so that the electrons practically do not change their quasi-moments. Such transitions are char- acteristic of direct-gap semiconductors. For the second group, the absolute minimum of the conduction band and the absolute maximum of the valence band are at different points in the Brillouin zone, and optical inter- Figure 3. Structure of the anionic group [P2Se6] 4–. Figure 4. Arrangement of the polyhedra anionic group [P2Se6] 4– in In4(P2Se6)3. Figure 5. Coordination environment of the indium atoms in the structure of In4(P2Se6)3. Figure 6. Second (SCE) and nearest (NCE) coordina- tion environments of atoms in the [P2Se6] 4– anionic groups in the structure of TlInP2Se6. TOVT, BARCHIY, PIASECKI, KITYK, AND FEDORCHUK Hungarian Journal of Industry and Chemistry 16 band transitions must be accompanied by a large change in the electron quasi-moment. These are characteristic of indirect-gap semiconductors. Since the photon mo- ment is negligibly small compared with the electron quasi-moment, the latter case is possible only when the electron interacts with the phonon. According to the phase diagram, the single crystals of TlInP2Se6 and In4(P2Se6)3 were grown using the Bridgman method in two vertical zone furnaces. Exper- imental studies of optical spectra in the absorption re- gion yielded information on the energy spectrum of electrons near the edges of the conduction band and band gap. Studies concerning the dependence of diffuse reflection on wavelength (R = f(λ)) have shown that the compound TlInP2Se6 refers to indirect-gap semiconduc- tors. On the graph there are two rectilinear sections, one of which (for small wavelengths, , and large values of E) characterizes the interband transitions of electrons with phonon emission, and the other (for large  and small E) describes the processes of phonon absorption (Fig.9). The intersection of the first section with the wave- length axis, , yields the value of Eg + Ephonon ( = 560 nm, E = 2.21 eV), and the intersection of the second characterizes Eg – Ephonon ( = 605 nm and E = 2.05 eV). The length of the segment between the points of inter- section of both straight lines with the wavelength axis, , is equal to the doubled energy of the phonons, 2Ephonon (0.16 eV), interacting with the electron. The middle of this segment corresponds to the photon ener- gy equal to the width of the band gap of the indirect-gap semiconductor, Eg. Experimental calculations in terms of the compound TlInP2Se6 have shown that Eg = 2.13 eV and Ephonon = 0.08 eV. The compound In4(P2Se6)3 refers to direct-gap semiconductors, which characterizes the interband tran- sitions of electrons in terms of photon absorption (Fig.10). The intersection of the line with the wave- length axis,  ( = 651 nm), yields the value of Eg = 1.91 eV. The crystals of In4(P2Se6)3 and TlInP2Se6 were il- luminated by 10.6 μm and (its second harmonic) fre- quency doubling of 5.3 μm beams. Each channel of the beam was split by 200-ns CO2 laser pulses with a pulse repetition frequency of about 10 Hz. The angle between these two laser beams was changed from 18º to 22º. Figs.11 and 12 present these dependences. Treatment with a 10.6 μm beam achieved a smaller maximum bire- fringence (about 1.5510 -2 ) in comparison to the 5.2 μm beam. This indicates a different photoinduced anisotro- py for the In4(P2Se6)3 and TlInP2Se6 crystals. Because a) b) . Figure 7. Coordination environments of the thallium (a) and indium (b) atoms in the structure of TlInP2Se6. Figure 8. Arrangement of the polyhedra anionic group [P2Se6] 4– in the structure of TlInP2Se6. Figure 9. Dependence of the diffuse reflection R on the wavelength  for the compound TlInP2Se6. Figure 10. Dependence of the diffuse reflection R on the wavelength  for the compound In4(P2Se6)3. INVESTIGATIONS OF THE TLINP2SE6–IN4(P2SE6)3 SYSTEM 45(2) pp. 13–18 (2017) 17 these crystals contain chalcogenide anions that contrib- ute to the anharmonicity of the phonon, they play a cru- cial role in terms of the second harmonic generation [13-14]. The maximum changes in the birefringence achieved were less than 210 -2 and 6.310 -2 for CO2 laser wavelengths of 10.6 μm and 5.3 μm, respectively. 4. Conclusion Differential thermal analysis, X-ray diffraction and mi- crostructural analysis were used to construct a phase diagram for the TlInP2Se6–In4(P2Se6)3 system, which can be characterized by a eutectic-type interaction. The invariant eutectic process L  htTlInP2Se6 + mtIn4(P2Se6)3 (15 mol% In4(P2Se6)3) occurs at 791 K. Two polymorphic transformations were identified for TlInP2Se6 at 680 K and 711 K and for In4(P2Se6)3 at 665 K and 903 K. New compounds were not detected in the binary system. The regions of solid phases of the complex compounds TlInP2Se6 and In4(P2Se6)3 do not exceed 10 mol%. Single crystals of both test compounds were achieved by the Bridgman method. Investigations concerning the dependence of the diffuse reflection spectrum showed that the compound TlInP2Se6 is char- acteristic of indirect-gap semiconductors (Eg = 2.13 eV, Ephonon = 0.08 eV), while the compound In4(P2Se6)3 is characteristic of direct-gap semiconductors (Eg = 1.91 eV, Ephonon = 0.08 eV). The dependence of the birefrin- gence was photoinduced by wavelengths of 5.3 μm and 10.6 μm, which is indicative of different photoinduced anisotropy. Acknowledgement We are grateful for the financial support of this work by the Ministry of Education and Science of Ukraine under the project DB874P_0117U000380. 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