Melting curve and chemical stability of ammonia at high pressure: Combined x-ray diffraction and Raman study

The melting curve and stability of ammonia (${\mathrm{NH}}_3$) is investigated up to 40 GPa and 3500 K by x-ray diffraction and Raman spectroscopy in the laser-heated diamond anvil cell. The ${\mathrm{NH}}_3$ samples were directly heated by the $10.6\mu \mathrm{m}$ radiation of a ${\mathrm{CO}}_2$ laser to reduce the risks of chemical reactions. Melting was unambiguously detected by the appearance of the liquid diffraction signal upon temperature increase. The melting temperature of ${\mathrm{NH}}_3$ is found to steadily increase with pressure up to 40 GPa, and the previously reported turnover is not observed. As a result, the melting line of ${\mathrm{NH}}_3$ is expected to cross the isentropes of Neptune and Uranus in the pressure range 55–65 GPa, implying the possible presence of superionic solid ${\mathrm{NH}}_3$ in these planets. Our x-ray and Raman measurements confirm the appearance of ${\mathrm{N}}_2$ and ${\mathrm{H}}_2$ upon heating the liquid phase from 6 to 40 GPa. But while the equilibrium $2{\mathrm{NH}}_3\rightleftharpoons {\mathrm{N}}_2+3{\mathrm{H}}_2$ balances towards the dissociated elements at low pressure and high temperature, ammonia is found to the more stable species in the range 20–40 GPa, 300–3000 K.

Figure 1

Phase diagram of ammonia for $P>5\mathrm{GPa}$, adapted from Ref. [13]. The green region corresponds to the molecular fluid. The yellow and gray regions correspond to the molecular solid phases composed of ${\mathrm{NH}}_3$ units. The blue region represents the stability domain of the ionic phase reported by Refs. [13, 14], and the red region, that of the superionic phase.

Figure 2

Schematic of the experimental setup at the ID27 beamline of the ESRF. The sample is heated from one side by a ${\mathrm{CO}}_2$ laser ($\lambda =10.6\mu \mathrm{m}$). A polarizer is used to fine-tune the laser power. A ZnSe lens produces a heating spot of about $40\mu \mathrm{m}$, which is 10 times as large as the width of the x-ray beam. The temperature is measured by pyrometry using reflective objectives. The MCC is interposed between the sample and the detector to filter out a large part of the Compton scattering from the diamond anvils.

Figure 3

Examples of measured thermal emission spectra from ammonia samples. The blue circles are experimental data, and the red line are fits to either the Wien (top) or the Planck (middle) distributions. The green dots show the two-color temperature as defined in Ref. [28]. The best-fit temperature in each case is indicated. The temperatures reported in this study are those obtained from the Planck distribution, and the error bars are taken from the two-color distribution.

Figure 4

X-ray diffraction patterns collected at several temperatures below and above melting at (a) 25 GPa and (b), (c) 39 GPa. In (a), the blue and orange colored areas emphasize the signal from liquid ammonia. Diffraction peaks from solid ammonia III and V still remain present above the melting temperature due to the thermal gradients, and their positions are shown by the vertical bars. In (b), the diffraction patterns are obtained by subtracting the image plate at 990 K and partial masking of the solid peaks. The black dotted line is a Gaussian fit of the liquid signal at $T=2360\mathrm{K}$ and 39 GPa, and the yellow surface corresponds to the liquid signal. The inset shows the diffraction images of ${\mathrm{NH}}_3$ at 39 GPa for $T=2240\mathrm{K}$ and $T=2360\mathrm{K}$. The dotted yellow arcs delimit the region in which the halo of the liquid scattering is located. The red color indicates regions that have been masked before integration.

Figure 5

P-T phase diagram of ammonia. Red squares are the melting points from this work. The red circles correspond to the melting points of Ref. [24]. The red solid line is a fit of the Simon-Glatzel law to the melting points of Ninet and Datchi [24]. The orange dashed line is the melting curve of Ojwang et al. [26] inferred from their Raman study. The black triangles are the x-ray diffraction melting points obtained by the same authors. Orange circles, yellow triangles, blue squares, and purple lozenges were obtained from the ab initio molecular dynamics simulations of Bethkenhagen et al. [17], and respectively represent the dissociated fluid, the molecular fluid ${\mathrm{NH}}_3$, the molecular solid ${\mathrm{NH}}_3$, and the superionic solid ${\mathrm{NH}}_3$. The isentropes of Neptune and Uranus are taken from Redmer et al. [38].

Figure 6

Diffraction pattern of ${\mathrm{NH}}_3$ at 39 GPa and 2360 K, compared to the simulated patterns of ${\mathrm{NH}}_3$-III, ${\mathrm{NH}}_3$-V, and $\varepsilon -{\mathrm{N}}_2$. The cell parameters obtained by a Le Bail refinement are $a=4.0057\left(82\right)\AA$ for ${\mathrm{NH}}_3$-III; $a=2.822\left(21\right)\AA , b=4.898\left(12\right)\AA$, and $c=4.574\left(13\right)\AA$ for ${\mathrm{NH}}_3$-V; $a=7.0124\left(89\right)\AA$ and $c=10.105\left(21\right)\AA$ for $\varepsilon -{\mathrm{N}}_2$.

Figure 7

(a) Evolution of the low-frequency Raman spectra of ${\mathrm{NH}}_3$ with temperature at 31 GPa. From the bottom to the top spectrum, the power of the ${\mathrm{CO}}_2$ laser is gradually increased to ramp up the temperature. The lattice modes of ${\mathrm{NH}}_3$-IV are visible at 300 K and gradually disappear with $T$ due to the transition to phase III. The temperatures between 300 and 820 K could not be measured due to insufficient thermal emission. (b) Comparison between the N-H stretch Raman band in the solid ($T=1470\mathrm{K}$) and liquid ($T=2400\mathrm{K}$) phases at 35 GPa. The black lines are the experimental data, and the blue and red lines are fits using Gaussian peaks for the solid and liquid, respectively. The dotted lines show the decomposition into Gaussian peaks, offset for visibility. The fit residuals are shown in the top plot.

Figure 8

Evolution with temperature of the Raman spectra of ${\mathrm{NH}}_3$ at (a) 6 GPa and (b) 31 GPa. In both cases, the purple curves show the initial spectrum from a freshly loaded sample before heating. The N-H stretching band, composed of ${\nu }_1, {\nu }_3$, and overtones of ${\nu }_4$ modes, occurs in the frequency window $3100–3500{\mathrm{cm}}^{-1}$. The band between 2250 and $2750{\mathrm{cm}}^{-1}$ is the second-order Raman band of diamond. Upon heating the ${\mathrm{N}}_2$ and ${\mathrm{H}}_2$ vibron peaks appear at about $2340{\mathrm{cm}}^{-1}$ and $4260{\mathrm{cm}}^{-1}$, respectively. Additional hot vibrational bands of ${\mathrm{N}}_2$ and ${\mathrm{H}}_2$ appear at higher temperatures as indicated by the arrows. In (a), the temperatures indicated in parentheses are estimated from the laser power. After a stepwise return to 300 K, the principal ${\mathrm{N}}_2$ and ${\mathrm{H}}_2$ vibrons remain as seen in the blue spectrum in (a) and (b). The inset of (a) shows a photograph of the sample after temperature quench. The darker region is where the sample was heated and contains inclusions of ${\mathrm{N}}_2$ and ${\mathrm{H}}_2$ solids.