Structure of Proton-Conducting Alkali Thio-Hydroxogermanates

Abstract

Using a combination of neutron diffraction, infrared spectroscopy, and first-principles calculations, we have investigated the structure of hydrated and dehydrated proton conducting alkali thio-hydroxogermanates of general formula M2GeS2(OH)2·yH2O (M = K, Rb, and Cs). The results show that the structure of hydrated and dry materials are basically the same, which confirms previous indications that the main effect of heating these materials is just a loss of water. We suggest that in the hydrated state the structure of these materials is built of dimers of thio-hydroxogermanate anions, with the water molecules acting as bridges between such dimers. In the dehydrated structure, the thio-hydroxogermanate anions instead form an extended network through the formation of interdimer hydrogen bonds through the −OH groups in the structure. The alkali ions are suggested to act as "space-fillers" in voids formed by the thio-hydroxogermanate anion dimers, in both the hydrated and the dehydrated state. Disciplines Inorganic Chemistry | Materials Chemistry | Materials Science and Engineering CommentsReprinted with permission from Chemistry of Materials 20 (2008) Using a combination of neutron diffraction, infrared spectroscopy, and first-principles calculations, we have investigated the structure of hydrated and dehydrated proton conducting alkali thio-hydroxogermanates of general formula M 2 GeS 2 (OH) 2 · yH 2 O (M ) K, Rb, and Cs). The results show that the structure of hydrated and dry materials are basically the same, which confirms previous indications that the main effect of heating these materials is just a loss of water. We suggest that in the hydrated state the structure of these materials is built of dimers of thio-hydroxogermanate anions, with the water molecules acting as bridges between such dimers. In the dehydrated structure, the thio-hydroxogermanate anions instead form an extended network through the formation of interdimer hydrogen bonds through the -OH groups in the structure. The alkali ions are suggested to act as "space-fillers" in voids formed by the thiohydroxogermanate anion dimers, in both the hydrated and the dehydrated state.

Alkali-ion concentration dependence of the structure of proton-conducting alkali thio-hydroxogermanates investigated with neutron diffraction

“…On the basis of the previous report on K 2 GeS 2 (OD) 2 •yD 2 O, the peak at ≈1 Å is assigned to O-D correlations, the peak at ≈1.5 Å is assigned to D-D correlations of water molecules, the peak at ≈1.8 Å is assigned to Ge-O correlations, the peak at ≈2.2 Å is assigned to Ge-S correlations, and the broad 3.3 Å peak is assigned to O-S correlations and different correlations involving the alkali ions. [7] Following further the assignment in our previous report on K 2 GeS 2 (OD) 2 •yD 2 O, [7] the two shoulders of the broad peak centered at ≈3.3 Å can be assigned to Na/K-O correlations (r ≈ 2.8 Å) and Na/K-S correlations (r 3.2 Å), respectively. Depending on the coordination number and type of alkali ion, these correlations are expected at slightly different distances in the respective pair-correlation functions.…”

“…The smooth oscillations for the x = 1 − 3 materials are in agreement with an amorphous structure, as observed in previous investigations using X-ray diffraction (x = 1 − 3) and neutron diffraction (x = 2) techniques. [1,7] However, for x = 4 we observe for both the Na and K based materials that the structure factor contains a number of Bragg peaks for Q 5 Å −1 , superimposed on a smoothly varying diffraction pattern. The presence of Bragg peaks reveals a crystalline phase in addition to an amorphous phase relating to a disordered network of alkali thio-hydroxogermanate anions.…”

“…The results from these studies suggest that the materials are amorphous and built up of dimers of thio-hydroxogermanate anions, with water molecules acting as bridges between such dimers and with the alkali ions located in "voids" formed by the network of thio-hydroxogermanate anion dimers. [6,7] The structure of the dehydrated equivalents is build up of a percolating network of dimers of thiohydroxogermanate anions, which are linked to each other via hydrogen bonds and where the alkali ions, as for the hydrated materials, are located in "voids" formed by the thiohydroxogermanate anion dimers. [7] However, little is known about how the structure changes as a function of alkali-ion concentration.…”

“…[6,7] The structure of the dehydrated equivalents is build up of a percolating network of dimers of thiohydroxogermanate anions, which are linked to each other via hydrogen bonds and where the alkali ions, as for the hydrated materials, are located in "voids" formed by the thiohydroxogermanate anion dimers. [7] However, little is known about how the structure changes as a function of alkali-ion concentration. Such information is important for the understanding and further development of these materials, for example in relation to higher proton conductivity.…”

Using Neutron Spin−Echo To Investigate Proton Dynamics in Proton-Conducting Perovskites

“…26 Furthermore, we show that NSE, which indeed is the only neutron scattering technique that gives access to the long time-scales needed to accurately investigate the comparatively slow proton diffusion in hydrated perovskites, can be powerfully combined with kinetic modeling based on first-principles calculations, since both techniques cover the same time-and length-scales.…”