Anhydrous proton conductivity in hydrogen alkali thiogermanates

Ionic conductivities have been investigated for mixed hydrogen alkali thiogermanates possessing the adamantane-like Ge4S104− structural complex anion. Anhydrous compounds of the form HxM4−xGe4S10 (M=K and Rb) and HxCs4−xGe4S10·yH2S 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 H4Ge4S10 may be partially attributed to lower decomposition temperatures and increased hydrogen bonding.