Effects of partial anion substitution on the thermoelectric properties of silver(I) chalcogenide halides in the system Ag5Q2X with Q=Te, Se and S and X=Br and Cl

A selection of mixed conducting silver chalcogenide halides of the general formula Ag(5)Q(2)X with Q=sulfur, selenium and tellurium and X=chlorine and bromine has been investigated due to their thermoelectric properties. Recently, the ternary counterpart Ag5Te2Cl showed a defined d(10)-d(10) interaction in the disordered cation substructure at elevated temperatures where Ag5Te2Cl is present in its high temperature alpha-phase. A significant drop of the thermal diffusivity has been observed during the beta-alpha phase transition reducing the values from 0.12 close to 0.08 mm(2) s(-1). At the same transition the thermopower reacts on the increasing silver mobility and jumps towards less negative values. Thermal conductivities, thermopower and thermal diffusivity of selected compounds with various grades of anion substitution in Ag(5)Q(2)X were determined around the silver-order/disorder beta-alpha phase transition. A formation of attractive interactions could be observed for selenium substituted phases while no effect was detected for bromide and sulfide samples. Depending on the grade and type of substitution the thermopower changes significantly at and after the beta-alpha phase transition. Thermal conductivities are low reaching values around 0.2-0.3 W m(-1) K-1 at 299 K. Partial anion exchange can substantially tune the thermoelectric properties in Ag(5)Q(2)X phases. (C) 2011 Published by Elsevier Inc.