Synthesis and Characterization of Amorphous Hydrated Alkali Thio-Hydroxogermanates
By Poling, Steven A.; Nelson, Carly R. & Martin, Steve W.
Published in Chemistry of Materials
2005
Abstract
The synthesis, structure, and nonhumidified proton conductivity of the hydrated alkali thio-hydroxogermanates, denoted as MxGeSx(OH)4-x·yH2O (1 ≤ x ≤ 4, 0 < y < 8) for M = Na, K, Rb, and Cs, are reported. These materials are generally X-ray amorphous when produced by a low-temperature (∼75 °C) aqueous solution evaporation-precipitation route. Raman and IR spectroscopies indicate mixed chalcogenide germanium central anions with distinct asymmetric Ge-O and symmetric Ge-S stretching modes observable around 820-754 and 500-325 cm-1, respectively. These thio-oxoanions possess a combination of thermally stable hydroxyl groups and hydrophilic alkali associated with the nonbridging sulfurs. Alternating current impendence measurements performed under anhydrous conditions on low-pressure sealed pellets reveal fast ionic conductivity, 10-3-10-2 S/cm, for typical temperatures between 100 and 275 °C. The observed falloff in conductivity at higher temperatures is consistent with the appearance of endothermic transitions in differential scanning calorimetry measurements of hermetically sealed samples, presumably from the “boiling� or sublimation of a crystalline water sublattice. Corresponding onset temperatures were observed between 150 and 275 °C and dependent on the alkali and composition. Under dry atmosphere conditions, thermogravimetric analysis mass loss measurements indicate continuous mass loss above the preparation temperature of ∼75 °C.