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Phonon spectra of two novel nonlinear optical crystals, PbGa2GeSe6 and PbGa2GeS6, are studied experimentally by Raman and infrared reflection spectroscopy and theoretically by density functional theory (DFT) lattice dynamics calculations. The experimental peak frequencies are compared with calculated vibration frequencies, and a very reasonable agreement is established. A comparative analysis of results for PbGa2GeSe6 and PbGa2GeS6 with earlier reported data on relevant binary and ternary metal chalcogenides clearly indicates that the similarity of their phonon spectra stems from the presence of GeS4 and GeSe4 tetrahedra as structural building blocks. [ABSTRACT FROM AUTHOR]

Two new quaternary selenides of the α-TlSe structure type have been synthesized and characterized. Single crystal X-ray diffraction analysis has revealed that Tl2Ga2SnSe6 crystallizes with space group I4/mmc, a = 8.095(1), c = 6.402(1) Å, with a refined composition of Tl1−xGa1−ySnySe2 (x = y = 0.345(5)), Z = 4, R1 = 0.028; wR2 = 0.066. The crystal structure of the isostructural compound Tl2Ga2GeSe6 has been determined by means of powder X-ray diffraction: space group I4/mmc, Z = 4, a = 8.0770(4), c = 6.2572(5) Å, refined composition Tl1−xGa1−yGeySe2, x = 0.343(5), y = 0.35(2), (RB(I) = 0.084; RP = 0.041; RPw = 0.058). According to their optical absorption spectra all compounds are semiconductors with relatively narrow direct band gaps of 2.15(3) and 2.05(5) eV for the Ge and Sn phase, respectively. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Photoinduced changes of piezoelectric coefficients for novel chalcogenide AgGaGe3-xSixSe8 (x = 0.15, 0.3, 0.6, 0.9) single crystals are discovered. The measurements were performed during illumination by cw 532 nm laser (above energy band gap) with power about 400 mW. The relaxation after switching off of the laser beam was studied. Additional temperature dependences of piezoelectric diagonal tensor components were recorded to separate the thermal effect with respect to pure electronic contribution to the photoinduced piezoelectricity. In addition to the photoinduced piezoelectric effect FTIR spectra are studied in order to explore the influence of the thermal contribution. The photoinduced changes with taking into account of temperature contribution have achieved magnitude equal to approximately 50-60% for all the studied samples and the changes were found to be completely reversible. The possible mechanisms for the observed effects are discussed and additional quantum chemical DFT simulations for the principal structural fragments were performed. [ABSTRACT FROM AUTHOR]

The dielectric function of orthorhombic Cu2CdGeS4 and Cu2CdSiS4 single crystals (space group Pmn21) is measured experimentally in the spectral range of 0.7-5.2 eV, and the results are interpreted via comparison with ab initio calculations. Polarized Raman-scattering measurements of both semiconductors are reported. All observed vibrational modes are assigned to specific lattice eigenmodes of the wurtz-stannite structure. Good agreement between theoretical and experimental mode frequencies is established. The differences in the experimental Raman spectra of the two compounds are well correlated with the effect of exchanging Si and Ge predicted by the theory. The strongest lines in the Raman-scattering spectra of both semiconductors are identified as being due to the "breathing" motion of GeS4 and SiS4 octahedra. [ABSTRACT FROM AUTHOR]

Tl 2 HgGeSe 4 crystal was successfully, for the first time, synthesized by the Bridgman-Stockbarger technology, and its electronic structure and peculiarities of optical constants were investigated using both experimental and theoretical techniques. The present X-ray photoelectron spectroscopy measurements show that the Tl 2 HgGeSe 4 crystal reveals small moisture sensitivity at ambient conditions and that the essential covalent constituent of the chemical bonding characterizes it. The latter suggestion was supported theoretically by ab initio calculations. The present experiments feature that the Tl 2 HgGeSe 4 crystal is a high-resistance semiconductor with a specific electrical conductivity of σ ∼ 10 -8 Ω -1 cm -1 (at 300 K). The crystal is characterized by p-type electroconductivity with an indirect energy band gap of 1.28 eV at room temperature. It was established that a good agreement with the experiments could be obtained when performing first-principles calculations using the modified Becke-Johnson functional as refined by Tran-Blaha with additional involvement in the calculating procedure of the Hubbard amendment parameter U and the impact of spin-orbit coupling (TB-mBJ + U + SO model). Under such a theoretical model, we have determined that the energy band gap of the Tl 2 HgGeSe 4 crystal is equal to 1.114 eV, and this band gap is indirect in nature. The optical constants of Tl 2 HgGeSe 4 are calculated based on the TB-mBJ + U + SO model.