Superionic water-ammonia mixtures

Abstract:

The interior of the Giant Planets Uranus and Neptune contains large amounts of water, ammonia and methane (referred to as planetary ices). Many observable properties of these planets, such as luminosity, gravitational moments and magnetic fields, are thought to be determined by the physical and chemical properties of matter within this ice layer. Hence, the phase diagrams, equations of state and structural properties of these materials and their respective mixtures are of great interest.
Especially the phase diagrams of water and ammonia gained much attention since Cavazzoni et al. [1] proposed superionic phases for these materials, which are characterized by highly mobile hydrogen ions in a lattice of oxygen and nitrogen ions, respectively. For water, the influence of such a phase on the properties of the Giant Planets as well as on exoplanets has been discussed widely. [2,3] Nevertheless, it is an open question how the properties of such a water layer change when another compound, e.g., ammonia is introduced. Considering a 1:1 mixture, we have performed ab initio simulations based on density functional theory using the VASP code [4] heating up structures which we had found from evolutionary random structure search calculations with XtalOpt [5]. We propose possible superionic water-ammonia structures present up to several Mbar. Moreover, we investigate the equation of state and transport properties of this mixture such as diffusion coefficients in order to compare with the pure compounds. These results are essential to construct new interior models for Neptune-like planets.

[1] C. Cavazzoni et al., Science 283, 44 (1999).
[2] R. Redmer et al., Icarus 211, 798 (2011).
[3] L. Zeng and D. Sasselov, ApJ 784, 96 (2014).
[4] G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993).
[5] D. C. Lonie and E. Zurek, Comput. Phys. Commun. 182, 372 (2011).