Proton conductivity in hydrous and anhydrous thiogermanic acids

Abstract

Ionic conductivities have been investigated for hydrous and anhydrous H 4Ge 4S 10 and H 2Ge 4S 9 thiogermanic acids. The hydrous phases, denoted as H 4Ge 4S 10· xH 2O (7> x>9), are superprotonic conductors. Proton conductivity and thermal stability are dependent on the Ge 4S 10 4− cage structure. Two distinct structural units, adamantane-like and a unique more complex cage unit, have been produced in both the hydrous and anhydrous phases. The composition and structure of these phases are discussed in terms of Raman, infrared, mass and thermogravimetric data. Ionic conductivity values of 10 −10 to 10 −3 S/cm were determined from a.c. impedance measurements of the hydrous thiogermanic acids in the temperature range of −55 to 125 °C. Conductivity values of 10 −10 to 10 −7 S/cm were recorded for the corresponding anhydrous phases between 180 and 500 °C. The fast ionic conductivity of the hydrous phases is consistent with loosely bound water, whereas the conductivity of the anhydrous phases is correlated to the presence of thiol (S–H) groups. At higher temperatures these groups are observed to thermally decompose to hydrogen sulfide.