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Equation of state and phase diagram of ammonia at high pressures from ab initio simulations
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Image of FIG. 1.
FIG. 1.

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 The background color only serves as guide to the eye since heating and cooling simulations do not yield sharp boundaries.

Image of FIG. 2.
FIG. 2.

Phase diagram based on measurements as well as simulations performed by Ninet (colored areas with solid black phase boundaries) in comparison to the present work (colored symbols as in Fig. 1 ) and experimental data by Ojwang (blue and orange dashed lines).

Image of FIG. 3.
FIG. 3.

Pair distribution functions (a) , (b) , and (c) at 5000 K for different densities.

Image of FIG. 4.
FIG. 4.

Pair distribution functions for averaged ion positions, (a) the rotationally disordered solid (1000 K, 1.5 g/cm) and (b) the superionic phase (2000 K, 3.0 g/cm). The solid curves represent the simulation results, while the dotted black lines indicate the positions of the nitrogen ions on an ideal fcc lattice.

Image of FIG. 5.
FIG. 5.

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.

Image of FIG. 6.
FIG. 6.

Thermal EOS (ϱ, ) divided by the density ϱ for different isotherms between 500 K and 10 000 K. The dashed lines indicate first-order phase transitions.

Image of FIG. 7.
FIG. 7.

Caloric EOS (ϱ, ) for different isotherms from 500 K to 10 000 K. The presented data already include the vibrational correction using the power spectra (ν, ϱ, ) from the simulations, see Sec. II B for further details. Again, the dashed lines indicate first-order phase transitions.

Image of FIG. 8.
FIG. 8.

Vibrational correction for the caloric EOS. The solid lines represent the vibrational correction using the set of experimental frequencies by Tassaing , whereas the circles illustrate the vibrational correction based on the spectra from the simulations.

Image of FIG. 9.
FIG. 9.

Hugoniot curve derived from FT-DFT-MD simulations (solid lines), including different vibrational corrections (vc) for the caloric EOS, compared to experimental data (symbols). The symbols and line colors are the same in (a) and (b).

Image of FIG. 10.
FIG. 10.

Vibrational spectrum obtained from a simulation at 230 K and 0.6933 g/cm (black curve) in comparison with experimental data by Tassaing (dashed lines with full circles) over the characteristic vibrational temperature and the wavenumber , respectively. The blue colored lines indicate degenerate vibrations, while the orange ones illustrate non-degenerate vibrations.


Generic image for table
Table I.

EOS data of ammonia obtained from FT-DFT-MD simulations.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Equation of state and phase diagram of ammonia at high pressures from ab initio simulations