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Melting and dissociation of ammonia at high pressure and high temperature
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View: Figures


Image of FIG. 1.
FIG. 1.

Phase diagram of NH3. The Uranus and Neptune isentropes are from Ref. 4. Filled triangles and diamonds represent synchrotron XRD and Raman measurements of the melting line in this study, respectively. The open triangles and diamonds indicate the observed solid-solid phase transitions in our synchrotron XRD and Raman studies, respectively (fitted with a dotted blue line). The open circles indicate the temperature prior to the appearance of flash. The chemical decomposition band, which is associated with flash, appearance of N2 and H2 in bulk quantities, and polymorphic modifications of NH3 on quenching, is indicated by the gray region. The error bars in our measured T reflect the uncertainties (200 K). The uncertainty in P is ∼2 GPa. Our results for the melting line (the dashed-dotted orange line) are a fit of the Kechin melt equation:35 T m (P) = T 0(1 + P/a) b e −cP , where a = 3.051 GPa, b = 1.466, c = 0.039 GPa−1, T 0 = 200 K, and P is in units of GPa. The solid squares and the open-rectangles dashed line show the previously reported melting points and the (IV,V)-III transition, respectively (Ref. 10). Inset: The phases proposed by Cavazzoni et al. 2 (solid, molecular, ionic, and superionic) are indicated by different colors.

Image of FIG. 2.
FIG. 2.

Representative Raman spectral changes of NH3 at (a) 5 GPa, (b) 15 GPa, and (c) 50 GPa as a function of temperature. The arrows indicate the N2 vibrons. (d) Raman spectra of fluid phase at different pressures (the temperature was measured just above the melting line, see Fig. 1). In (a), temperatures were too low to measure with radiometry. The sample at 50 GPa is from a previous lower pressure heating run, which is why a weak N2 vibron signal appears prior to the heating run shown in (c). In (e), we show the detailed vibron spectra through melting. The statistical data analysis shows that at 1600 K, the spectrum can be best fit by a superposition of narrow bands corresponding to solid and a broad band of fluid, while the spectrum at 1720 K can be equally well fit by a single broad band corresponding to fluid.

Image of FIG. 3.
FIG. 3.

Raman spectra of the products of laser-heated ammonia at different pressures. The Raman spectrum of nitrogen exhibits two major vibron bands characteristic of pure bulk N2 (v 1, v 2). The inset shows a fine splitting of the ν2 nitrogen vibron. The NH3 sample at 30 GPa was heated until the appearance of N2 (blue curve). When this sample was cold-compressed to 50 GPa (red curve), the lower-frequency (v 2) peak, which is related to the N2 molecules centered on the 6c(D2d) site, split into two peaks (inset). This splitting is associated with the distortion of the Pm3n (or formation of a low-symmetry) structure of pure N2.31

Image of FIG. 4.
FIG. 4.

Raman spectra illustrating reversible and irreversible phenomena in laser-heated ammonia-V. In (a), the sample was heated until the flash appeared. On quenching it is found that there is a split in the N–H band. In (b), the sample was heated until N2 formed (middle trace) but without the appearance of the flash. The NH3 Raman spectra before and after heating are similar. These experiments are reproducible.

Image of FIG. 5.
FIG. 5.

Representative synchrotron x-ray diffraction (λ = 0.3344 Å) patterns, background-corrected, of NH3 collected at 24 GPa as a function of temperature. Green and red tick marks indicate the allowed reflections for structure with lattice parameter a = 3.793 Å (Ir) and a = 4.244 Å (NH3-III), respectively, while gray tick marks indicate the allowed reflections for P212121 structure (NH3-IV). The strong peaks of Ir, indicated by green tick marks, are masked. Remnants of Bragg peaks of solid NH3 remain even at the highest temperature because of large axial temperature gradients (see Sec. II). Panel (b) shows the appearance of a diffuse halo corresponding to fluid NH3 above the melting temperature. The diffuse halo is very weak and cannot be seen in the scale of the main figure (a). Thus, we plotted the difference in intensity between the patterns measured at different temperatures and that measured at 300 K, to subtract the incoherent and background scattering;16 at 2110 K, a diffuse scattering halo associated with liquid NH3 is clearly seen. Narrow Bragg peaks corresponding to solid phases are omitted for clarity. Dashed red lines in (b) are the baselines shifted vertically for clarity.

Image of FIG. 6.
FIG. 6.

(a)–(d) Raman and IR spectra of NH3 (at 300 K and 51 and 60 GPa) before and after heating (quenched); Raman spectra are offset for clarity. Photos of sample holes in the Ir coupler (e) and the flash that appears at high laser power (f) are in the center. The lower right coupler hole was heated at 51 GPa until the appearance of flash, and quenched. The sample was then compressed to 60 GPa and the process was repeated for the central hole (the N2 peak in the initial spectra is due to a previous heating cycle).

Image of FIG. 7.
FIG. 7.

Gibbs free energy of ammonia in comparison to that of ½N2 + 3/2H2 mixture at 700 K (blue thick lines).

Image of FIG. 8.
FIG. 8.

Calculated enthalpy difference for ½N2 + 3/2H2 – NH3 and entropy of ½N2 + 3/2H2 and of NH3 at 7.3 GPa.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Melting and dissociation of ammonia at high pressure and high temperature