Synthesis and Characterization of Amorphous Hydrated Alkali Thio-Hydroxogermanates

The synthesis, structure, and nonhumidified proton conductivity of the hydrated alkali thiohydroxogermanates, 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. Disciplines Inorganic Chemistry | Materials Science and Engineering Comments Reprinted with permission from Chemistry of Materials 17 (2005): 1728–1735, doi:10.1021/cm0488581. Copyright 2005 American Chemical Society. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/mse_pubs/51 Synthesis and Characterization of Amorphous Hydrated Alkali Thio-Hydroxogermanates Steven A. Poling, Carly R. Nelson, and Steve W. Martin* Department of Materials Science and Engineering, 2220 HooVer Hall, Iowa State UniVersity of Science and Technology, Ames, Iowa 50011 ReceiVed July 13, 2004. ReVised Manuscript ReceiVed January 24, 2005 The synthesis, structure, and nonhumidified proton conductivity of the hydrated alkali thiohydroxogermanates, denoted as MxGeSx(OH)4-x‚yH2O (1 e x e 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 lowpressure 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.