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

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 e1(ω) of Ag2Se crystal from a very large negative value, e1 ≈-109, to a very large positive value e1≈ 106 was observed, which is assumed to result from the dipole relaxation. The strong peaks of the dielectric losses e2(ω), tgδ(ω) and impedance Z in Ag2Se were also determined at Tc1,c2. Two significant peaks in e1(ω), e2(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 e1from 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)