Structure and properties of ammonium-doped water ice

The ammonium anion is comparatively soluble in ice and may also increase the solubility of associated cations that would otherwise not be easily accommodated in the crystal structure of ice. In previous work significant differences in the absolute unit-cell dimensions and in the thermal expansivity of water ice crystals grown from a solution of ammonium carbamate have been observed. The goal here is to evaluate the reproducibility of this observation in ice grown from other ammonium-bearing solutions and to test the hypothesis that ammonium can catalyse the transition from disordered to ordered ice in the same way that, for example, potassium hydroxide is known to do. The work has implications for the properties of ice crystallising from subsurface oceans inside icy moons, where ammonium-bearing solutions have been thought for many decades to be present.