Structures, phase transitions and electrical properties of Ag ₅Te _2-ySe _yCl (y = 0-0.7)

Herein we report the structural, thermal and physical properties of Ag ₅Te _2-ySe _yCl (y=0-0.7). Polymorphic Ag ₅Te ₂Cl (y=0) is an ion conductor with two reversible phase transitions at 241 and 334 K. While the monoclinic room temperature β- and the low temperature γ-phase show conductivities below σ=10-3Ω-1cm-1, the tetragonal high temperature α-phase is a promising conductor for electronic devices with conductivities up to 10-1Ω-1cm-1 in addition to high moisture and photo stability. The α-β-phase transition temperature of ion conducting Ag ₅Te ₂Cl 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 α-β-phase transition temperature with increasing compositional parameter y to a minimum transition temperature of 239 K was observed for Ag ₅Te _1.3Se _0.7Cl. Phases up to a maximum selenium content of y=0.7 were stabilized by preservation of the β- and α-Ag ₅Te ₂Cl structure types. Ag ₅Te _2-ySe _yCl (y=0-0.9) phases were characterised by powder X-ray diffraction (XRD) at room temperature. The stabilization of the α-Ag ₅Te ₂Cl structure type in Ag ₅Te _2-ySe _yCl 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 σ=5.1×10-2Ω-1cm-1 (314 K, α-Ag ₅Te _1.6Se _0.4Cl) 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.