$({\mathbf{N}}_2{)}_6{\mathrm{Ne}}_7$: A High Pressure van der Waals Insertion Compound

The binary phase diagram of ${\mathrm{N}}_2\text{−}\mathrm{Ne}$ mixtures has been measured at 296 K by visual observation and Raman spectroscopy. The topology of the phase diagram points to the existence of the stoichiometric compound $({\mathrm{N}}_2{)}_6{\mathrm{Ne}}_7$. Its structure has been solved by single-crystal synchrotron x-ray diffraction. The ${\mathrm{N}}_2$ molecules form a guest lattice that hosts the Ne atoms. This insertion compound can be viewed as a clathrate with the centers of the ${\mathrm{N}}_2$ molecules forming distorted dodecahedron cages, each enclosing 14 Ne atoms. Remarkably, the $({\mathrm{N}}_2{)}_6{\mathrm{Ne}}_7$ compound is somehow the first clathrate organized by the quadrupolar interaction.

Figure 1

(a) Binary phase diagram of the ${\mathrm{N}}_2\text{−}\mathrm{Ne}$ mixture at 296 K plotted as pressure vs Ne concentration. The solid $S_1$ stands for the rich ${\mathrm{N}}_2$ solid phase, $S_2$ for the rich Ne solid phase, and $F$ for the fluid phase. On the vertical axis, we indicate the domains of existence for the pure constituents. The mutual solubilities of ${\mathrm{N}}_2$ and Ne in the solid phase are less than 2%. The red dots form the measured liquidus. The van der Waals compound is found at $x_{\mathrm{Ne}}=0.54$. (b) Evolution with pressure of the Raman frequency of the ${\mathrm{N}}_2$ vibrons in a ${\mathrm{N}}_2\text{−}\mathrm{Ne}$ mixture at $x_{\mathrm{Ne}}=0.54$. The red circles are obtained in $({\mathrm{N}}_2{)}_6{\mathrm{Ne}}_7$. The empty red circles are obtained in the fluid. Below 11 GPa in the solid phase, the two vibrons are not resolved by our Raman setup. The error bars are smaller than the size of the circles. Empty black circles are the data for pure ${\mathrm{N}}_2$ from Schneider et al. [23]. The vertical dashed lines indicate the melting pressures for pure ${\mathrm{N}}_2$ and $({\mathrm{N}}_2{)}_6{\mathrm{Ne}}_7$.

Figure 2

(a) Composite image, obtained by adding 120 image plates, showing the indexed Bragg peaks of the single crystal of $({\mathrm{N}}_2{)}_6{\mathrm{Ne}}_7$ at 8 GPa. For clarity, only a part of the full image is presented, and the observed peaks are circled. (b) Photograph of a single crystal of $({\mathrm{N}}_2{)}_6{\mathrm{Ne}}_7$ surrounded by the solid-solid phase separation between pure solid ${\mathrm{N}}_2$ and $({\mathrm{N}}_2{)}_6{\mathrm{Ne}}_7$, grown from the ${\mathrm{N}}_2\text{−}\mathrm{Ne}$ mixture with initial concentration $x_{\mathrm{Ne}}=0.45$. The hexagonal faceting of the macroscopic crystal reveals the underlying hexagonal structure of the $({\mathrm{N}}_2{)}_6{\mathrm{Ne}}_7$ compound.

Figure 3

Crystal structure of the $({\mathrm{N}}_2{)}_6{\mathrm{Ne}}_7$ compound. Neon and nitrogen are represented by black and blue spheres, respectively. The black dashed lines join the centers of adjacent ${\mathrm{N}}_2$ molecules. (a) Unit cell represented along the $c$ axis ([001] direction). (b) Details of the Ne structure and of their nitrogen surroundings. In [b1], only ${\mathrm{N}}_2\text{−}{\mathrm{N}}_2$ distances ranging from 3.05 to 3.16 Å at 8.0 GPa are shown. The view is slightly rotated from a pure $b$-axis view to show the 3D perspective. In [b2] and [b3], ${\mathrm{N}}_2\text{−}{\mathrm{N}}_2$ distances ranging from 3.05 to 3.68 Å at 8.0 GPa are shown. The red planes show the surfaces supported by the centers of the ${\mathrm{N}}_2$ molecules. They form a cagelike structure that can be seen as a distortion from a dodecahedron, due to the presence of the top and bottom Ne atoms which open the cage. For clarity, only the centers of the molecules are represented (blue spheres marked with a black star). The view in [b2] is along the $c$ axis, whereas [b3] is slightly rotated from a pure $b$-axis view.