Synthesis and Characterization of Hydrated Rubidium Thio-hydroxosilicogermanates Using Mechanochemical Hydrothermal Synthesis

The synthesis, structure, and proton conductivity of the hydrated thio-hydroxosilicogermanates RbzGexSi1-xSz(OH)4-z·yH2O (2 ≤ z ≤ 3; 0 < y < 1.7) are reported. X-ray diffraction indicates that the z = 2 materials are amorphous and show no unreacted starting materials, which is consistent with the mechanochemical hydrothermal evaporation−precipitation synthesis method. Infrared spectroscopy shows a fully reacted system where both silicon and germanium central anions have vibrational modes at 950, 780, 667, and 450 cm-1 associated with asymmetric modes for Si−O, Ge−O, Si−S, and Ge−S chemical bonds, respectively. O−H stretching and bending modes are present at 3370 and 1655 cm-1, respectively, with the intensities of both modes decreasing with the addition of SiO2. Raman spectroscopy reveals symmetric stretching modes of the Ge−S unit at 420 cm-1. A new peak appears at 460 cm-1 with the substitution of Si4+ for Ge4+. There are weak peaks above 460 cm-1 that can be attributed to H2O and O−H libration modes. Thermogravimetric analysis shows that water loss begins above the synthesis temperature of 75 °C. All water is lost by 300 °C which is consistent with the onset of the loss of conductivity. Alternating current impedance spectroscopy measurements performed on low-pressure sealed pellets show conductivity values ranging from 10-5 to 10-2.5 S/cm from 100 to 260 °C, respectively. Maximum conductivity values of 10-4.5 and 10-3.5 S/ cm at 120 °C are obtained for 2RbSH + 0.9GeO2 + 0.1SiO2 + 0.8H2O and 3RbSH + 0.9GeO2 + 0.1SiO2 + 1.1H2O, respectively. a.c. impedance experiments under humidified conditions resulted in large increases in the proton conductivity at low temperatures. For x = 0.9 and 0.8 for the 2RbSH + xGeO2 + (1−x)SiO2 +yH2O samples, the humidified d.c. conductivity was 10-4 S/cm at 60 °C, up from 10-8 S/cm for samples measured under dry nitrogen. The effects of water and SiO2 content on the proton conductivity are also discussed. Disciplines Ceramic Materials | Inorganic Chemistry | Materials Chemistry | Materials Science and Engineering Comments Reprinted with permission from Chemistry of Materials 18 (2006): 6436–6441, doi:10.1021/cm0618796. Copyright 2006 American Chemical Society. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/mse_pubs/53 Synthesis and Characterization of Hydrated Rubidium Thio-hydroxosilicogermanates Using Mechanochemical Hydrothermal Synthesis Carly R. Nelson, Sarah Olson, Steven A. Poling, and Steve W. Martin* Department of Materials Science and Engineering, 2220 HooVer Hall, Iowa State UniVersity of Science and Technology, Ames, Iowa 50011 ReceiVed August 9, 2006 The synthesis, structure, and proton conductivity of the hydrated thio-hydroxosilicogermanates RbzGexSi1-xSz(OH)4-z‚yH2O (2 e z e 3; 0 < y < 1.7) are reported. X-ray diffraction indicates that the z ) 2 materials are amorphous and show no unreacted starting materials, which is consistent with the mechanochemical hydrothermal evaporation-precipitation synthesis method. Infrared spectroscopy shows a fully reacted system where both silicon and germanium central anions have vibrational modes at ∼950, ∼780, ∼667, and ∼450 cm-1 associated with asymmetric modes for Si-O, Ge-O, Si-S, and Ge-S chemical bonds, respectively. O-H stretching and bending modes are present at ∼3370 and 1655 cm-1, respectively, with the intensities of both modes decreasing with the addition of SiO2. Raman spectroscopy reveals symmetric stretching modes of the Ge-S unit at ∼420 cm-1. A new peak appears at ∼460 cm-1 with the substitution of Si4+ for Ge4+. There are weak peaks above ∼460 cm-1 that can be attributed to H2O and O-H libration modes. Thermogravimetric analysis shows that water loss begins above the synthesis temperature of ∼75 °C. All water is lost by ∼300 °C which is consistent with the onset of the loss of conductivity. Alternating current impedance spectroscopy measurements performed on low-pressure sealed pellets show conductivity values ranging from 10-5 to 10-2.5 S/cm from ∼100 to 260 °C, respectively. Maximum conductivity values of 10-4.5 and 10-3.5 S/cm at 120 °C are obtained for 2RbSH + 0.9GeO2 + 0.1SiO2 + 0.8H2O and 3RbSH + 0.9GeO2 + 0.1SiO2 + 1.1H2O, respectively. a.c. impedance experiments under humidified conditions resulted in large increases in the proton conductivity at low temperatures. For x ) 0.9 and 0.8 for the 2RbSH + xGeO2 + (1-x)SiO2 + yH2O samples, the humidified d.c. conductivity was ∼10-4 S/cm at 60 °C, up from ∼10-8 S/cm for samples measured under dry nitrogen. The effects of water and SiO2 content on the proton conductivity are also discussed.