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

The proton-conducting hydrated alkali thio-hydroxogermanate's MxGeSx(OH)4 − x·yH2O (M = Na and K; x = 1 − 4, y ≈ 0.5 − 2) were investigated by means of neutron diffraction with the aim to elucidate how the structure changes as a function of alkali-ion concentration, x, type of alkali ion, M, and water content, y. For x = 1 − 3 we find that the materials are amorphous and composed of thio-hydroxogermanate anions, water molecules, and charge balancing alkali ions, whereas for x = 4 we find that the materials contain also a crystalline phase, suggesting that it is difficult to prepare purely amorphous materials for the highest alkali-ion concentration, for both the Na and K based materials. For x = 1 − 3, the structure is reflected by an intermediate-range ordering, with a characteristic length-scale ranging from approximately 6 to 9 Å, which is dependent on both x and M and which may be related to the separation distance between dimers of thio-hydroxogermanate anions. As x increases, the intermediate-range ordering shortens, possibly as the result of an increasing level of hydration water that may act as a dielectric medium that reduces the repulsive interaction between the negatively charged thio-hydroxogermanate anions and/or between the positively charged alkali ions. A comparison of the structural results to the reported conductivities of the same materials indicates a non-trivial relationship, which depends on both the type and concentration of alkali ions, as well as on the level of hydration water.