Structures, phase transitions and electrical properties of Ag5Te2-ySeyCl (y=0-0.7)

Herein we report the structural, thermal and physical properties of Ag5Te2-ySeyCl (y = 0-0.7). Polymorphic Ag5Te2Cl (y = 0) is an ion conductor with two reversible phase transitions at 241 and 334 K. While the monoclinic room temperature beta- and the low temperature gamma-phase show conductivities below sigma = 10(-3) Omega(-1) cm(-1), the tetragonal high temperature alpha-phase is a promising conductor for electronic devices with conductivities up to 10(-1) Omega(-1) cm(-1) in addition to high moisture and photo stability. The alpha-beta-phase transition temperature of ion conducting Ag5Te2Cl can be lowered by partial substitution of tellurium by selenium. From differential thermal analysis (DTA) and differential scanning calorimetry (DSC) a linear decrease of the alpha-beta-phase transition temperature with increasing compositional parameter y to a minimum transition temperature of 239 K was observed for Ag5Te1.3Se0.7Cl. Phases up to a maximum selenium content of y = 0.7 were stabilized by preservation of the beta- and alpha-Ag5Te2Cl structure types. Ag5Te2-ySeyCl (y = 0-0.9) phases were characterised by powder X-ray diffraction (XRD) at room temperature. The stabilization of the alpha-Ag5Te2Cl structure type in Ag5Te2-ySeyCl around room temperature results in an increase of conductivity of more than 2 orders of magnitude compared to the ternary phase. Conductivity of up to sigma = 5.1 x 10(-2) Omega(-1) cm(-1) (314 K, alpha-Ag5Te1.6Se0.4Cl) was observed by impedance spectroscopy. In the range from room temperature down to 239 K any transition temperature can be chosen by simply varying the composition. (C) 2004 Elsevier SAS. All rights reserved.