The phonon structure of the silver sulfide Ag 2 S was investigated, experimentally using Raman spectroscopy, and theoretically using the density-functional perturbation theory for the first time. Seven Raman-active modes were observed and identified at 23,39,42,44, 62, 65, and 243 cm À1 . Symmetry assignments of all the vibrational modes were derived from considerations of point group symmetry. The phonon band structure and the relative Raman intensities were also investigated by ab initio calculations and compared with the experimental data. The temperature, laser power, and illumination time dependencies of frequency, linewidth, and intensity of the Raman-active modes are discussed. In the Raman spectra at higher frequencies 1300-1700 cm À1 , additional broad Raman modes observed in all samples at higher laser powers 8-10 mW were ascribed to luminescence from b-Ag 2 S. The phonon and Raman spectra of the b-Ag 2 S provide a useful insight into the b-Ag 2 S ! a-Ag 2 S phase transition. Finally, calculated infrared vibrational mode frequencies were compared with measured infrared mode frequencies.

Electronic band structure, together with formation energy of native point defects has been studied for β‐Ag2S superionic material. The density‐functional full‐potential calculations have been carried out for perfect crystal and defect structure. In latter case a 96‐atom supercell has been composed and account of structural relaxation around a vacancy has been taken up to second nearest neighbor. Formation energies of point defects have been obtained and compared. According to comparison, cation vacancies are the dominant intrinsic defects in β‐Ag2S and transition into a state with high ionic conductivity is probably related with their abrupt concentration increase with increasing temperature. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The percolation threshold (PT) temperatures Tc1,c2 of two non‐equivalent Ag+1,2 ions in Ag2S and Ag2Se lattices were determined from the temperature dependence of low‐frequency (25‐106Hz) impedance spectroscopy measurements. At PT temperatures Tc1≈200 K and Tc2≈ 334 K, an abrupt increase of the dielectric function ε1(ω) of Ag2Se crystal from a very large negative value, ε1 ≈‐109, to a very large positive value ε1≈ 106 was observed, which is assumed to result from the dipole relaxation. The strong peaks of the dielectric losses ε2(ω), tgδ(ω) and impedance Z in Ag2Se were also determined at Tc1,c2. Two significant peaks in ε1(ω), ε2(ω)and tgδ(ω) were observed at Tc1≈ 312‐320 K. and Tc2≈ 380‐430 K in the temperature dependences of these parameters. The measured parameters decreased significantly with increasing frequency, confirming the long relaxation times. The drastic decrease at T > 430 K of ε1from a very large positive value 109to a very large negative one ‐108 is in accordance with infrared (IR) ellipsometry measurements; this indicates that electrons of Ag+‐ ions are ionized because of the high dielectric function screening of lattice sites before the known β→α structural phase transition from monoclinic to a body‐centered cubic (bcc) structure. In addition, the activation energies of Ag+1,2 ions were estimated for Ag2S and Ag2Se crystals. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The unpolarized Stocks component of the Raman spectra of the layered ternary thallium dichalcogenide, TlInS2 was studied with the aid of a Raman confocal microscope system in the low‐frequency region of 35‐150 cm‐1 over the temperature range that embraced the region of the successive phase transitions in this crystal. The observed spectra were deconvoluted into Lorentzian peaks to single‐out the contribution of each Raman mode. The temperature dependence of the Raman frequency and broadening associated with each mode was then obtained. The irregular temperature behaviour of most modes was disclosed in the proximity of phase transitions. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependent infrared (IR) and optical spectroscopic ellipsometry methods are used to investigate peculiarities of the transitions from the ionic conductivity to the superionic conductivity states in Ag2S and Ag2Se compounds. The structural phase transitions (SPTs), known as β→α transition in these semiconductors, are investigated also with spectroscopic ellipsometry methods. The electronic band structure of Ag2S at the SPT changes sharply from the monoclinic to the bcc phase, which occurs at T = 453 K in the super ionic conductivity state, where all the interband transitions in optic region, corresponding to the monoclinic structure of β‐phase, disappear. The α‐phase of Ag2S is characterised with new lines in the dielectric functions ε1 and ε2, which arise at 5‐6.5 eV. The interband transitions do not change in the SPT from the orthorhombic to the bcc in Ag2Se, which occurs at T > 406 K. In IR ellipsometry for the degenerated Ag2Se, the free carriers' plasma energy is displaced abruptly from ħω=0.075 eV at T=300 K to ħω=0.125 eV at T=338 K. The same effect is seen in the IR ellipsometry for Ag2S, where negative ε1 appears at T=434‐440 K. These facts indicate a drastic increase of the free carriers concentrations in both of the semiconductors at temperatures before the β→α SPT. In Ag2S the second abrupt shift of the free carriers plasma energy from 0.035eV to 0.1 eV occurs at T=453 K, corresponding to β→ α SPT. The second shift is due to the electronic band structure change of Ag2S. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman spectra of layered ternary thallium chalcogenide TlInS2were studied with the aid of 3D confocal Raman system over the temperature range 77–300K in the frequency region of 120–400 cm–1. The observed lines in the obtained Raman spectra were denconvoluted into Lorentzian peaks and temperature dependence of each peak's parameters (peak position and half width at half maximum) were obtained. An irregular behaviour of the temperature dependence of Loretzian parameters is reported. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

This work reports the results of DFT‐based calculations of phonon spectra of TlGaTe2. The dispersion of phonon bands was calculated along the directions of Brillouin zone (BZ) that include symmetry points. The calculated phonon frequencies at the centre of BZ were compared with those obtained by Raman spectroscopy with the aid of a confocal laser microscopy system. A fairly good agreement between the calculated and experimental data was found. Complimentary, molar heat capacity at constant volume and Debye temperature were calculated in the range 5÷500 K on the base of the obtained phonon density of states. The obtained temperature dependencies were compared with available experimental data.The results of comparison were satisfactory. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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