Temperature dependent spectroscopic ellipsometry of Ag₂Se and Ag₂S with phase transitions from ionic to superionic conductivity state

Abstract

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)

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

“…Owing to the large anion defect formation energy within the ionic β phase, the IC is predominantly due to the motion of Ag + cation. Temperature‐dependent infrared (IR) and optical spectroscopic ellipsometry methods have been used to investigate the peculiarities of the transitions from the IC to the superionic conductivity states in Ag 2 S and Ag 2 Se compounds . The structural β → α phase transitions have also been investigated with spectroscopic ellipsometry methods.…”

Thermoelectric Properties of PbSe Nanocomposites from Solution-Processed Building Blocks

“…Meanwhile, all PbSe-Ag nanocomposites are characterized with higher σ than pure PbSe, with a maximum σ appeared around 400 K, but decrease with further increase in temperature. We attributed this maximum σ to the increase of free carriers during the typical Ag 2 Se phase transition ∼ 407 K (Figure S7), from orthorhombic to cubic phase. − Also, the higher σ found in all PbSe-Ag nanocomposites is attributed to the charge carrier donating effect of Ag, which was also later confirmed using Hall measurement. In contrast to pristine PbSe, PbSe-Ag nanocomposites showed a positive Seebeck coefficient ( p -type behavior) for the entire measurement temperature.…”

Ab initio Study of Atomic Structure and Electronic Properties of Different Phases of Polymorphic Ag2S

“…That makes the material of great interest from both fundamental science and technological applications perspectives. In the past, experimental studies have addressed the optical properties of bulk Ag 2 S materials [ 16,17 ] and colloidal quantum dots. [ 18,19 ] A theoretical study of the optical properties of different bulk Ag 2 S phases will be an important step forward in revealing relationships between the structure and optical properties of this material.…”