First‐principles lattice dynamics and Raman scattering in ionic conductor β‐Ag₂S

Abstract

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.

Isosymmetric phase transitions, ultrahigh ductility, and topological nodal lines in α−Ag2S

“…S6 and S7 in SM [22]), which shows a direct band gap of 0.6 eV at the point. This value is in agreement with available first-principles results [39][40][41][42] but smaller than the experimental data (1.03 ± 0.1 eV) [6]. This is expected since the DFT calculations typically underestimate the band gap of semiconductors.…”

Impedance spectroscopy study of phase transitions to ionic and superionic conductivity states in Ag2S and Ag2Se

“…Second, some of the electrons ionized in the dielectric anomaly process would be captured back by free Ag + -ions to form neutral Ag atoms (Ag + +e→Ag). In any case, the sign change of ε 1 at T > 434 K confirms the Ag + ion-electron ionization in Ag 2 S. Ab-initio calculations [10] of the defect formation energies for Ag 1,2 atoms of Ag 2 Se yielded E a (Ag 1 ) = 0.43 eV and E a (Ag 2 ) = 0.5 eV. However, no localized electronic state was observed in the energy band structure of Ag 2 Se after removing an Ag atom because of the overlapped c-v bands.…”

“…The β-phase of Ag 2 Se is semimetal with overlapping c-v bands [10,14]. The plasma frequency ω p , which we determined via IR ellipsometry at T ≈ 300 K to be ε 1 (ω p ) = 0 (ћω p ≈ 0.07eV), yields n ≈ 3×10 18 cm -3 for the free carrier concentration.…”

Effect of Y doping on high-pressure behavior of Ag2S nanocrystals

“…At room temperature and ambient pressure, Ag 2 S has a monoclinic crystal structure called a-Ag 2 S, and known as the mineral acanthite at temperatures below $450 K. 29 Upon increasing the temperature, a structural phase transition occurs at 453 K from the ordered monoclinic a-structure to the disordered body-centered cubic b-structure with the mineral named argentite. 30,31 The high-temperature face-centered cubic phase, g-Ag 2 S, is stable from a temperature of $860 K until its melting point. 32,33 Less attention has been paid to the high-pressureinduced polymorph transformation in Ag 2 S materials.…”