Anhydrous proton conductivity in hydrogen alkali thiogermanates

Abstract

Ionic conductivities have been investigated for mixed hydrogen alkali thiogermanates possessing the adamantane-like Ge 4S 10 4− structural complex anion. Anhydrous compounds of the form H x M 4− x Ge 4S 10 (M=K and Rb) and H x Cs 4− x Ge 4S 10· yH 2S for 0≤ x≤4 and 0≤ y≤1 were synthesized using a novel preparation method involving reacting the respective alkali thiogermanate glass or adamantane-like crystalline phase with liquid hydrogen sulfide. The reaction products were characterized by mass, thermogravimetric analysis, and Raman and infrared spectroscopies. Conductivity values in the range of 10 −10 to 10 −5 S/cm were determined from a.c. impedance measurements of the protonated phases below a typical thermal decomposition temperature of ∼175 °C. The decrease in ionic conductivity above the decomposition temperature is consistent with the loss of mobile protons as hydrogen sulfide. Higher proton mobility in the mixed hydrogen alkali thiogermanates vs. the anhydrous thiogermanic acid H 4Ge 4S 10 may be partially attributed to lower decomposition temperatures and increased hydrogen bonding.