Phase diagram of ammonia as determined in this work; each symbol represents an equilibrium simulation run. The gray and black curves indicate the isentropes of Neptune and Uranus as given by Redmer et al. 14 The background color only serves as guide to the eye since heating and cooling simulations do not yield sharp boundaries.
Pair distribution functions (a) g HH, (b) g NN, and (c) g NH at 5000 K for different densities.
Pair distribution functions for averaged ion positions, (a) the rotationally disordered solid (1000 K, 1.5 g/cm3) and (b) the superionic phase (2000 K, 3.0 g/cm3). The solid curves represent the simulation results, while the dotted black lines indicate the positions of the nitrogen ions on an ideal fcc lattice.
Diffusion coefficients of (a) hydrogen and (b) nitrogen for temperatures between 500 K and 10 000 K. The colors of the isotherms are the same in (a) and (b). The dashed lines in (a) indicate first-order phase transitions, while the dotted line serves as guide to the eye, since the diffusion coefficient does not exist in this range.
Thermal EOS p(ϱ, T) divided by the density ϱ for different isotherms between 500 K and 10 000 K. The dashed lines indicate first-order phase transitions.
Caloric EOS u(ϱ, T) for different isotherms from 500 K to 10 000 K. The presented data already include the vibrational correction using the power spectra S vv (ν, ϱ, T) from the simulations, see Sec. II B for further details. Again, the dashed lines indicate first-order phase transitions.
Vibrational correction for the caloric EOS. The solid lines represent the vibrational correction using the set of experimental frequencies by Tassaing et al., 40 whereas the circles illustrate the vibrational correction based on the ab initio spectra from the simulations.
Hugoniot curve derived from FT-DFT-MD simulations (solid lines), including different vibrational corrections (vc) for the caloric EOS, compared to experimental data 17,22,23 (symbols). The symbols and line colors are the same in (a) and (b).
Vibrational spectrum obtained from a simulation at 230 K and 0.6933 g/cm3 (black curve) in comparison with experimental data by Tassaing et al. 40 (dashed lines with full circles) over the characteristic vibrational temperature T vib and the wavenumber , respectively. The blue colored lines indicate degenerate vibrations, while the orange ones illustrate non-degenerate vibrations.
EOS data of ammonia obtained from FT-DFT-MD simulations.
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