Ionic Phases of Ammonia-Rich Hydrate at High Densities

Mixtures of ammonia and water are major components of the “hot ice” mantle regions of icy planets. The ammonia-rich ammonia hemihydrate (AHH) plays a pivotal role as it precipitates from water-rich mixtures under pressure. It has been predicted to form ionic high-pressure structures, with fully disintegrated water molecules. Utilizing Raman spectroscopy measurements up to 123 GPa and first-principles calculations, we report the spontaneous ionization of AHH under compression. Spectroscopic measurements reveal that molecular AHH begins to transform into an ionic state at 26 GPa and then above $\sim 69\mathrm{GPa}$ transforms into the fully ionic $P\overline{3}m1$ phase, AHH-III, characterized as ammonium oxide $({\mathrm{NH}}_4^{+}{)}_2{\mathrm{O}}^{2-}$.

Figure 1

Representative lattice modes and vibrational modes of Raman spectra for AHH at room temperature and different pressures. Different colors depict different phases. The shown spectra are from the different samples (S2 and S3). Additional details are provided in the Supplemental Material [31]. The scales for the intensity at low and high frequency are as indicated for each rescaled spectrum.

Figure 2

Raman vibrational frequencies of AHH at room temperature from experimental results. Different colored symbols depict different runs (see legend). See Supplemental Material [31] for more sample details, including full width at half maximum as error bars for DMA high-frequency vibrons. Density-functional perturbation theory (DFPT) calculations are shown as black lines and gray regions as a function of pressure. Calculated frequencies for AHH-II* are shown in the DMA region with thickness related to the vibron intensity. The diagonal-dashed lines indicate the region where the sample signal may be covered by the Raman of second-order diamond.

Figure 3

(a)–(d) Raman spectra (lower) and renormalized fitted peak intensities compared with theoretical DFPT intensities (top), at four representative pressures. (e),(f) Crystal structure of AHH-$P\overline{3}m1$ at 100 GPa with Raman-active vibron displacement vectors for the (e) lower- and (f) higher-frequency N—H stretch modes. Red (blue, white) spheres denote O (N, H) atoms, respectively, and hydrogen bonds from ${\mathrm{NH}}_4^{+}$ to ${\mathrm{O}}^{2-}$ are shown as dashed lines.

Figure 4

Top: enthalpy change along a transition from AHH-II via AHH-II* to AHH-III, involving successive proton transfers from ${\mathrm{H}}_2\mathrm{O}$ via ${\mathrm{OH}}^{-}$ to ${\mathrm{O}}^{2-}$, at three different pressures. Bottom: the proposed phase diagram of AHH based upon this Letter and low-pressure data from Wilson et al. [27] (dashed lines below 10 GPa and unfilled circles, where red, green, black unfilled circles represent DMA, AHH-II, mixed phases, respectively). For our experimental data, different colors depict different phases, and different filled symbols represent different runs.