Water Binding Geometries in Complex Oxides as Determined by Rotational Equilibrium.

Water binding geometry can have a significant effect on the stability of materials and their performance. The experimental determination of H₂O orientation in crystals is challenging due to the small x-ray scattering cross section of low-Z elements such as hydrogen. We have shown previously that rotational equilibrium constraints can successfully overcome experimental limitations for identifying water orientations in ionic crystals. In order to avoid singularities in energy the crystal should be charge neutral. However, this can be unexpectedly challenging in the presence of partial occupancies, a situation that is often encountered in real materials. Here we explore the effect how charge neutrality violation affects predicted H₂O orientations. Pb-uranyl-oxide, curite (H10, O32, Pb2.894, U8), is chosen as an example where the structure could not be completely resolved from x-ray experiments. We tested several crystallographic approaches to select charges with and without charge neutrality constraint and found that ~80% of the 10000 calculated initial configurations, for 20 different charge models converged to a single final H₂O configuration. This strongly suggests that the rotational equilibrium method can be used successfully in cases where charge neutrality violation occur.