Hydrated alkali thio-hydroxogermanates, MxGeSx(OH)4_x-yH2O, where M = Na, K, Rb or Cs; 1 < x < 4; y * 1, represent a novel class of amorphous proton­conducting materials, which were first synthesized by Poling et al. [16, 82] at Iowa State University. The conductivities of these materials typically reach a maximum of the order of 10~2 Scm-1 in the intermediate-temperature range of 100-300 °C [16], which competes with even the best perovskite-type oxides. Karlsson et al. [85] reported a structural investigation of these materials using a combination of neutron diffraction and first-principles calculations. A key result from the experiment was that the neutron structure-factors of the hydrated and dehydrated materials (Fig. 9.13) are overall similar to each other, indicating that there are no dramatic structural changes such as phase transitions or structure degradation as the materials are dehydrated. In order to gain understanding for what such a structure may look like, the authors proceeded their analysis by generating a candidate three-dimen­sional structure of the Cs-based compound, by taking the orthorhombic crystal structure ofNa2GeS2(OH)25H2O as the starting point in the calculations, replacing the Na ions with Cs ions and reducing the number of water molecules from five to one in order to agree with the real composition. Snapshots of the generated structures of dehydrated and hydrated materials are shown in Fig. 9.14, whereas Fig. 9.15 shows a close up of the local configuration of the hydrated material.

In the hydrated state (Figs. 9.14a and 9.15), the calculations suggest a structure built of thio-hydroxogermanate anion dimers connected to each other via hydrogen bonds to water molecules located between the dimers. In the dehydrated state (Fig. 9.14b), the calculations suggest instead that the thio-hydroxogermanate anions form an extended network through the creation of inter-dimer hydrogen bonds, whereas the alkali ions are found to act as “space fillers” in “voids” formed by the thio-hydroxogermanate anion dimers in both the hydrated and dehydrated state. These generated structures are justified by comparing the experimental and calcu­lated pair-correlation functions, which are shown in Fig. 9.16. It can be appreciated that the experimental and calculated pair-correlation functions are overall similar

which suggests that the real structures are at least reasonably-well described by the structural models for both the hydrated and dehydrated materials. However, to obtain a complete picture of the structure and elucidate how the structure depends on the type and concentration of alkali ion, further investigations, such as reverse Monte-Carlo simulations of diffraction data, are necessary.