Effect of chloride substitution in the fast ion conductor Ag3SBr

Substitution of the larger iodide anion for bromide in the solid electrolyte Ag3SBr has previously been shown to enhance ionic conductivity in the superionic phase and to reduce the superionic-covalent phase transition temperature. In this investigation, we find it is possible to substitute appreciable quantities of the smaller chloride anion for bromide in Ag3SBr. As composition parameter x increases in nominally prepared Ag3SBr1-xClx samples, greater concentrations of silver halide are removed from the silver sulfide halide lattice. However, x-ray diffraction measurements confirm that the silver sulfide halide lattice persists up to the x=0.50 composition. In contrast to the case of iodide substitution, the effect of replacing bromide with chloride is to raise the transition temperature separating the superionic and covalent phases of the silver sulfide halide component. The overall magnitude of ionic conductivity does not change appreciably as a function of composition near the superionic-covalent phase transition temperature. But the presence of increasing quantities of low conductivity silver halide in these samples suggests that the ionic conductivity of the silver sulfide halide component is enhanced by chloride substitution. With increasing composition parameter, x-ray diffraction and specific heat measurements also confirm the presence of free Ag2S which leads to an electronic contribution to the conductivity above room temperature. (C) 1999 American Institute of Physics. [S0021-8979(99)06711-0].